1bc421551SDag-Erling Smørgrav<!DOCTYPE html> 2bc421551SDag-Erling Smørgrav<html lang="en"> 3bc421551SDag-Erling Smørgrav<head> 4bc421551SDag-Erling Smørgrav <title>Theory and pragmatics of the tz code and data</title> 5bc421551SDag-Erling Smørgrav <meta charset="UTF-8"> 6bc421551SDag-Erling Smørgrav <style> 7bc421551SDag-Erling Smørgrav pre {margin-left: 2em; white-space: pre-wrap;} 8bc421551SDag-Erling Smørgrav </style> 9bc421551SDag-Erling Smørgrav</head> 10bc421551SDag-Erling Smørgrav 11bc421551SDag-Erling Smørgrav<body> 12bc421551SDag-Erling Smørgrav<h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1> 13bc421551SDag-Erling Smørgrav <h3>Outline</h3> 14bc421551SDag-Erling Smørgrav <nav> 15bc421551SDag-Erling Smørgrav <ul> 16bc421551SDag-Erling Smørgrav <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code> 17bc421551SDag-Erling Smørgrav database</a></li> 18bc421551SDag-Erling Smørgrav <li><a href="#naming">Timezone identifiers</a></li> 19bc421551SDag-Erling Smørgrav <li><a href="#abbreviations">Time zone abbreviations</a></li> 20bc421551SDag-Erling Smørgrav <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code> 21bc421551SDag-Erling Smørgrav database</a></li> 22bc421551SDag-Erling Smørgrav <li><a href="#functions">Time and date functions</a></li> 23bc421551SDag-Erling Smørgrav <li><a href="#stability">Interface stability</a></li> 24bc421551SDag-Erling Smørgrav <li><a href="#leapsec">Leap seconds</a></li> 25bc421551SDag-Erling Smørgrav <li><a href="#calendar">Calendrical issues</a></li> 2675411d15SDag-Erling Smørgrav <li><a href="#planets">Time and time zones off earth</a></li> 27bc421551SDag-Erling Smørgrav </ul> 28bc421551SDag-Erling Smørgrav </nav> 29bc421551SDag-Erling Smørgrav 30bc421551SDag-Erling Smørgrav<section> 31bc421551SDag-Erling Smørgrav <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2> 32bc421551SDag-Erling Smørgrav<p> 33bc421551SDag-Erling SmørgravThe <a 34bc421551SDag-Erling Smørgravhref="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code> 35bc421551SDag-Erling Smørgravdatabase</a> attempts to record the history and predicted future of 36bc421551SDag-Erling Smørgravcivil time scales. 37bc421551SDag-Erling SmørgravIt organizes <a href="tz-link.html">time zone and daylight saving time 38bc421551SDag-Erling Smørgravdata</a> by partitioning the world into <a 39bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones"><dfn>timezones</dfn></a> 40bc421551SDag-Erling Smørgravwhose clocks all agree about timestamps that occur after the <a 41bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a> 42bc421551SDag-Erling Smørgrav(1970-01-01 00:00:00 <a 43bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr 44bc421551SDag-Erling Smørgravtitle="Coordinated Universal Time">UTC</abbr></a>). 45bc421551SDag-Erling SmørgravAlthough 1970 is a somewhat-arbitrary cutoff, there are significant 46bc421551SDag-Erling Smørgravchallenges to moving the cutoff earlier even by a decade or two, due 47bc421551SDag-Erling Smørgravto the wide variety of local practices before computer timekeeping 48bc421551SDag-Erling Smørgravbecame prevalent. 49bc421551SDag-Erling SmørgravMost timezones correspond to a notable location and the database 50bc421551SDag-Erling Smørgravrecords all known clock transitions for that location; 51bc421551SDag-Erling Smørgravsome timezones correspond instead to a fixed <abbr>UTC</abbr> offset. 52bc421551SDag-Erling Smørgrav</p> 53bc421551SDag-Erling Smørgrav 54bc421551SDag-Erling Smørgrav<p> 55bc421551SDag-Erling SmørgravEach timezone typically corresponds to a geographical region that is 56bc421551SDag-Erling Smørgravsmaller than a traditional time zone, because clocks in a timezone 57bc421551SDag-Erling Smørgravall agree after 1970 whereas a traditional time zone merely 58bc421551SDag-Erling Smørgravspecifies current standard time. For example, applications that deal 59bc421551SDag-Erling Smørgravwith current and future timestamps in the traditional North 60bc421551SDag-Erling SmørgravAmerican mountain time zone can choose from the timezones 61bc421551SDag-Erling Smørgrav<code>America/Denver</code> which observes US-style daylight saving 62bc421551SDag-Erling Smørgravtime (<abbr>DST</abbr>), 63bc421551SDag-Erling Smørgravand <code>America/Phoenix</code> which does not observe <abbr>DST</abbr>. 64bc421551SDag-Erling SmørgravApplications that also deal with past timestamps in the mountain time 65bc421551SDag-Erling Smørgravzone can choose from over a dozen timezones, such as 66bc421551SDag-Erling Smørgrav<code>America/Boise</code>, <code>America/Edmonton</code>, and 67bc421551SDag-Erling Smørgrav<code>America/Hermosillo</code>, each of which currently uses mountain 68bc421551SDag-Erling Smørgravtime but differs from other timezones for some timestamps after 1970. 69bc421551SDag-Erling Smørgrav</p> 70bc421551SDag-Erling Smørgrav 71bc421551SDag-Erling Smørgrav<p> 72bc421551SDag-Erling SmørgravClock transitions before 1970 are recorded for location-based timezones, 73bc421551SDag-Erling Smørgravbecause most systems support timestamps before 1970 and could 74bc421551SDag-Erling Smørgravmisbehave if data entries were omitted for pre-1970 transitions. 75bc421551SDag-Erling SmørgravHowever, the database is not designed for and does not suffice for 76bc421551SDag-Erling Smørgravapplications requiring accurate handling of all past times everywhere, 77bc421551SDag-Erling Smørgravas it would take far too much effort and guesswork to record all 78bc421551SDag-Erling Smørgravdetails of pre-1970 civil timekeeping. 79bc421551SDag-Erling SmørgravAlthough some information outside the scope of the database is 80bc421551SDag-Erling Smørgravcollected in a file <code>backzone</code> that is distributed along 81bc421551SDag-Erling Smørgravwith the database proper, this file is less reliable and does not 82bc421551SDag-Erling Smørgravnecessarily follow database guidelines. 83bc421551SDag-Erling Smørgrav</p> 84bc421551SDag-Erling Smørgrav 85bc421551SDag-Erling Smørgrav<p> 86bc421551SDag-Erling SmørgravAs described below, reference source code for using the 87bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> database is also available. 88bc421551SDag-Erling SmørgravThe <code><abbr>tz</abbr></code> code is upwards compatible with <a 89bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international 90bc421551SDag-Erling Smørgravstandard for <a 91bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems. 92a979394aSDag-Erling SmørgravAs of this writing, the current edition of POSIX is POSIX.1-2024, 93a979394aSDag-Erling Smørgravwhich has been published but not yet in HTML form. 94a979394aSDag-Erling SmørgravUnlike its predecessor POSIX.1-2017 (<a 95bc421551SDag-Erling Smørgravhref="https://pubs.opengroup.org/onlinepubs/9699919799/"> The Open 96bc421551SDag-Erling SmørgravGroup Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018 97a979394aSDag-Erling SmørgravEdition), POSIX.1-2024 requires support for the 98a979394aSDag-Erling Smørgrav<code><abbr>tz</abbr></code> database, which has a 99a979394aSDag-Erling Smørgravmodel for describing civil time that is more complex than the 100a979394aSDag-Erling Smørgravstandard and daylight saving times required by POSIX.1-2017. 101bc421551SDag-Erling SmørgravA <code><abbr>tz</abbr></code> timezone corresponds to a ruleset that can 102bc421551SDag-Erling Smørgravhave more than two changes per year, these changes need not merely 103bc421551SDag-Erling Smørgravflip back and forth between two alternatives, and the rules themselves 104bc421551SDag-Erling Smørgravcan change at times. 105bc421551SDag-Erling SmørgravWhether and when a timezone changes its clock, 106bc421551SDag-Erling Smørgravand even the timezone's notional base offset from <abbr>UTC</abbr>, 107bc421551SDag-Erling Smørgravare variable. 108bc421551SDag-Erling SmørgravIt does not always make sense to talk about a timezone's 109bc421551SDag-Erling Smørgrav"base offset", which is not necessarily a single number. 110bc421551SDag-Erling Smørgrav</p> 111bc421551SDag-Erling Smørgrav 112bc421551SDag-Erling Smørgrav</section> 113bc421551SDag-Erling Smørgrav 114bc421551SDag-Erling Smørgrav<section> 115bc421551SDag-Erling Smørgrav <h2 id="naming">Timezone identifiers</h2> 116bc421551SDag-Erling Smørgrav<p> 117bc421551SDag-Erling SmørgravEach timezone has a name that uniquely identifies the timezone. 118bc421551SDag-Erling SmørgravInexperienced users are not expected to select these names unaided. 119bc421551SDag-Erling SmørgravDistributors should provide documentation and/or a simple selection 120bc421551SDag-Erling Smørgravinterface that explains each name via a map or via descriptive text like 121bc421551SDag-Erling Smørgrav"Czech Republic" instead of the timezone name "<code>Europe/Prague</code>". 122bc421551SDag-Erling SmørgravIf geolocation information is available, a selection interface can 123bc421551SDag-Erling Smørgravlocate the user on a timezone map or prioritize names that are 124bc421551SDag-Erling Smørgravgeographically close. For an example selection interface, see the 125bc421551SDag-Erling Smørgrav<code>tzselect</code> program in the <code><abbr>tz</abbr></code> code. 126*967a49a2SDag-Erling SmørgravUnicode's <a href="https://cldr.unicode.org">Common Locale Data 127*967a49a2SDag-Erling SmørgravRepository (<abbr>CLDR</abbr>)</a> 128*967a49a2SDag-Erling Smørgravcontains data that may be useful for other selection 129bc421551SDag-Erling Smørgravinterfaces; it maps timezone names like <code>Europe/Prague</code> to 130bc421551SDag-Erling Smørgravlocale-dependent strings like "Prague", "Praha", "Прага", and "布拉格". 131bc421551SDag-Erling Smørgrav</p> 132bc421551SDag-Erling Smørgrav 133bc421551SDag-Erling Smørgrav<p> 134bc421551SDag-Erling SmørgravThe naming conventions attempt to strike a balance 135bc421551SDag-Erling Smørgravamong the following goals: 136bc421551SDag-Erling Smørgrav</p> 137bc421551SDag-Erling Smørgrav 138bc421551SDag-Erling Smørgrav<ul> 139bc421551SDag-Erling Smørgrav <li> 140bc421551SDag-Erling Smørgrav Uniquely identify every timezone where clocks have agreed since 1970. 141bc421551SDag-Erling Smørgrav This is essential for the intended use: static clocks keeping local 142bc421551SDag-Erling Smørgrav civil time. 143bc421551SDag-Erling Smørgrav </li> 144bc421551SDag-Erling Smørgrav <li> 145bc421551SDag-Erling Smørgrav Indicate to experts where the timezone's clocks typically are. 146bc421551SDag-Erling Smørgrav </li> 147bc421551SDag-Erling Smørgrav <li> 148bc421551SDag-Erling Smørgrav Be robust in the presence of political changes. 149bc421551SDag-Erling Smørgrav For example, names are typically not tied to countries, to avoid 150bc421551SDag-Erling Smørgrav incompatibilities when countries change their name (e.g., 151bc421551SDag-Erling Smørgrav Swaziland→Eswatini) or when locations change countries (e.g., Hong 152bc421551SDag-Erling Smørgrav Kong from UK colony to China). 153bc421551SDag-Erling Smørgrav There is no requirement that every country or national 154bc421551SDag-Erling Smørgrav capital must have a timezone name. 155bc421551SDag-Erling Smørgrav </li> 156bc421551SDag-Erling Smørgrav <li> 157bc421551SDag-Erling Smørgrav Be portable to a wide variety of implementations. 158bc421551SDag-Erling Smørgrav </li> 159bc421551SDag-Erling Smørgrav <li> 160bc421551SDag-Erling Smørgrav Use a consistent naming conventions over the entire world. 161bc421551SDag-Erling Smørgrav </li> 162bc421551SDag-Erling Smørgrav</ul> 163bc421551SDag-Erling Smørgrav 164bc421551SDag-Erling Smørgrav<p> 165a979394aSDag-Erling SmørgravNames normally have the format 166bc421551SDag-Erling Smørgrav<var>AREA</var><code>/</code><var>LOCATION</var>, where 167bc421551SDag-Erling Smørgrav<var>AREA</var> is a continent or ocean, and 168bc421551SDag-Erling Smørgrav<var>LOCATION</var> is a specific location within the area. 169bc421551SDag-Erling SmørgravNorth and South America share the same area, '<code>America</code>'. 170bc421551SDag-Erling SmørgravTypical names are '<code>Africa/Cairo</code>', 171bc421551SDag-Erling Smørgrav'<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'. 172bc421551SDag-Erling SmørgravSome names are further qualified to help avoid confusion; for example, 173bc421551SDag-Erling Smørgrav'<code>America/Indiana/Petersburg</code>' distinguishes Petersburg, 174bc421551SDag-Erling SmørgravIndiana from other Petersburgs in America. 175bc421551SDag-Erling Smørgrav</p> 176bc421551SDag-Erling Smørgrav 177bc421551SDag-Erling Smørgrav<p> 178bc421551SDag-Erling SmørgravHere are the general guidelines used for 179bc421551SDag-Erling Smørgravchoosing timezone names, 180bc421551SDag-Erling Smørgravin decreasing order of importance: 181bc421551SDag-Erling Smørgrav</p> 182bc421551SDag-Erling Smørgrav 183bc421551SDag-Erling Smørgrav<ul> 184bc421551SDag-Erling Smørgrav <li> 185bc421551SDag-Erling Smørgrav Use only valid POSIX file name components (i.e., the parts of 186bc421551SDag-Erling Smørgrav names other than '<code>/</code>'). 187bc421551SDag-Erling Smørgrav Do not use the file name components '<code>.</code>' and 188bc421551SDag-Erling Smørgrav '<code>..</code>'. 189bc421551SDag-Erling Smørgrav Within a file name component, use only <a 190bc421551SDag-Erling Smørgrav href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters, 191bc421551SDag-Erling Smørgrav '<code>.</code>', '<code>-</code>' and '<code>_</code>'. 192bc421551SDag-Erling Smørgrav Do not use digits, as that might create an ambiguity with <a 193a979394aSDag-Erling Smørgrav href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX's proleptic 194bc421551SDag-Erling Smørgrav <code>TZ</code> strings</a>. 195bc421551SDag-Erling Smørgrav A file name component must not exceed 14 characters or start with 196bc421551SDag-Erling Smørgrav '<code>-</code>'. 197bc421551SDag-Erling Smørgrav E.g., prefer <code>America/Noronha</code> to 198bc421551SDag-Erling Smørgrav <code>America/Fernando_de_Noronha</code>. 199bc421551SDag-Erling Smørgrav Exceptions: see the discussion of legacy names below. 200bc421551SDag-Erling Smørgrav </li> 201bc421551SDag-Erling Smørgrav <li> 202bc421551SDag-Erling Smørgrav A name must not be empty, or contain '<code>//</code>', or 203bc421551SDag-Erling Smørgrav start or end with '<code>/</code>'. 204*967a49a2SDag-Erling Smørgrav Also, a name must not be '<code>Etc/Unknown</code>', as 205*967a49a2SDag-Erling Smørgrav <abbr>CLDR</abbr> uses that string for an unknown or invalid timezone. 206bc421551SDag-Erling Smørgrav </li> 207bc421551SDag-Erling Smørgrav <li> 208bc421551SDag-Erling Smørgrav Do not use names that differ only in case. 209bc421551SDag-Erling Smørgrav Although the reference implementation is case-sensitive, some 210bc421551SDag-Erling Smørgrav other implementations are not, and they would mishandle names 211bc421551SDag-Erling Smørgrav differing only in case. 212bc421551SDag-Erling Smørgrav </li> 213bc421551SDag-Erling Smørgrav <li> 214bc421551SDag-Erling Smørgrav If one name <var>A</var> is an initial prefix of another 215bc421551SDag-Erling Smørgrav name <var>AB</var> (ignoring case), then <var>B</var> must not 216bc421551SDag-Erling Smørgrav start with '<code>/</code>', as a regular file cannot have the 217bc421551SDag-Erling Smørgrav same name as a directory in POSIX. 218bc421551SDag-Erling Smørgrav For example, <code>America/New_York</code> precludes 219bc421551SDag-Erling Smørgrav <code>America/New_York/Bronx</code>. 220bc421551SDag-Erling Smørgrav </li> 221bc421551SDag-Erling Smørgrav <li> 222bc421551SDag-Erling Smørgrav Uninhabited regions like the North Pole and Bouvet Island 223bc421551SDag-Erling Smørgrav do not need locations, since local time is not defined there. 224bc421551SDag-Erling Smørgrav </li> 225bc421551SDag-Erling Smørgrav <li> 226*967a49a2SDag-Erling Smørgrav If all clocks in a region have agreed since 1970, 227*967a49a2SDag-Erling Smørgrav give them just one name even if some of the clocks disagreed before 1970, 228*967a49a2SDag-Erling Smørgrav or reside in different countries or in notable or faraway locations. 229bc421551SDag-Erling Smørgrav Otherwise these tables would become annoyingly large. 230*967a49a2SDag-Erling Smørgrav For example, do not create a name <code>Indian/Crozet</code> 231*967a49a2SDag-Erling Smørgrav as a near-duplicate or alias of <code>Asia/Dubai</code> 232*967a49a2SDag-Erling Smørgrav merely because they are different countries or territories, 233*967a49a2SDag-Erling Smørgrav or their clocks disagreed before 1970, or the 234*967a49a2SDag-Erling Smørgrav <a href="https://en.wikipedia.org/wiki/Crozet_Islands">Crozet Islands</a> 235*967a49a2SDag-Erling Smørgrav are notable in their own right, 236*967a49a2SDag-Erling Smørgrav or the Crozet Islands are not adjacent to other locations 237*967a49a2SDag-Erling Smørgrav that use <code>Asia/Dubai</code>. 238bc421551SDag-Erling Smørgrav </li> 239bc421551SDag-Erling Smørgrav <li> 240bc421551SDag-Erling Smørgrav If boundaries between regions are fluid, such as during a war or 241bc421551SDag-Erling Smørgrav insurrection, do not bother to create a new timezone merely 242bc421551SDag-Erling Smørgrav because of yet another boundary change. This helps prevent table 243bc421551SDag-Erling Smørgrav bloat and simplifies maintenance. 244bc421551SDag-Erling Smørgrav </li> 245bc421551SDag-Erling Smørgrav <li> 246bc421551SDag-Erling Smørgrav If a name is ambiguous, use a less ambiguous alternative; 247bc421551SDag-Erling Smørgrav e.g., many cities are named San José and Georgetown, so 248bc421551SDag-Erling Smørgrav prefer <code>America/Costa_Rica</code> to 249bc421551SDag-Erling Smørgrav <code>America/San_Jose</code> and <code>America/Guyana</code> 250bc421551SDag-Erling Smørgrav to <code>America/Georgetown</code>. 251bc421551SDag-Erling Smørgrav </li> 252bc421551SDag-Erling Smørgrav <li> 253bc421551SDag-Erling Smørgrav Keep locations compact. 254bc421551SDag-Erling Smørgrav Use cities or small islands, not countries or regions, so that any 255bc421551SDag-Erling Smørgrav future changes do not split individual locations into different 256bc421551SDag-Erling Smørgrav timezones. 257bc421551SDag-Erling Smørgrav E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>, 258bc421551SDag-Erling Smørgrav since 259bc421551SDag-Erling Smørgrav <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France 260bc421551SDag-Erling Smørgrav has had multiple time zones</a>. 261bc421551SDag-Erling Smørgrav </li> 262bc421551SDag-Erling Smørgrav <li> 263bc421551SDag-Erling Smørgrav Use mainstream English spelling, e.g., prefer 264bc421551SDag-Erling Smørgrav <code>Europe/Rome</code> to <code>Europa/Roma</code>, and 265bc421551SDag-Erling Smørgrav prefer <code>Europe/Athens</code> to the Greek 266bc421551SDag-Erling Smørgrav <code>Ευρώπη/Αθήνα</code> or the Romanized 267bc421551SDag-Erling Smørgrav <code>Evrópi/Athína</code>. 268bc421551SDag-Erling Smørgrav The POSIX file name restrictions encourage this guideline. 269bc421551SDag-Erling Smørgrav </li> 270bc421551SDag-Erling Smørgrav <li> 271bc421551SDag-Erling Smørgrav Use the most populous among locations in a region, 272bc421551SDag-Erling Smørgrav e.g., prefer <code>Asia/Shanghai</code> to 273bc421551SDag-Erling Smørgrav <code>Asia/Beijing</code>. 274bc421551SDag-Erling Smørgrav Among locations with similar populations, pick the best-known 275bc421551SDag-Erling Smørgrav location, e.g., prefer <code>Europe/Rome</code> to 276bc421551SDag-Erling Smørgrav <code>Europe/Milan</code>. 277bc421551SDag-Erling Smørgrav </li> 278bc421551SDag-Erling Smørgrav <li> 279bc421551SDag-Erling Smørgrav Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to 280bc421551SDag-Erling Smørgrav <code>Atlantic/Canaries</code>. 281bc421551SDag-Erling Smørgrav </li> 282bc421551SDag-Erling Smørgrav <li> 283bc421551SDag-Erling Smørgrav Omit common suffixes like '<code>_Islands</code>' and 284bc421551SDag-Erling Smørgrav '<code>_City</code>', unless that would lead to ambiguity. 285bc421551SDag-Erling Smørgrav E.g., prefer <code>America/Cayman</code> to 286bc421551SDag-Erling Smørgrav <code>America/Cayman_Islands</code> and 287bc421551SDag-Erling Smørgrav <code>America/Guatemala</code> to 288bc421551SDag-Erling Smørgrav <code>America/Guatemala_City</code>, but prefer 289bc421551SDag-Erling Smørgrav <code>America/Mexico_City</code> to 290bc421551SDag-Erling Smørgrav <code>America/Mexico</code> 291bc421551SDag-Erling Smørgrav because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the 292bc421551SDag-Erling Smørgrav country of Mexico has several time zones</a>. 293bc421551SDag-Erling Smørgrav </li> 294bc421551SDag-Erling Smørgrav <li> 295bc421551SDag-Erling Smørgrav Use '<code>_</code>' to represent a space. 296bc421551SDag-Erling Smørgrav </li> 297bc421551SDag-Erling Smørgrav <li> 298bc421551SDag-Erling Smørgrav Omit '<code>.</code>' from abbreviations in names. 299bc421551SDag-Erling Smørgrav E.g., prefer <code>Atlantic/St_Helena</code> to 300bc421551SDag-Erling Smørgrav <code>Atlantic/St._Helena</code>. 301bc421551SDag-Erling Smørgrav </li> 302bc421551SDag-Erling Smørgrav <li> 303bc421551SDag-Erling Smørgrav Do not change established names if they only marginally violate 304bc421551SDag-Erling Smørgrav the above guidelines. 305bc421551SDag-Erling Smørgrav For example, do not change the existing name <code>Europe/Rome</code> to 306bc421551SDag-Erling Smørgrav <code>Europe/Milan</code> merely because Milan's population has grown 307bc421551SDag-Erling Smørgrav to be somewhat greater than Rome's. 308bc421551SDag-Erling Smørgrav </li> 309bc421551SDag-Erling Smørgrav <li> 310bc421551SDag-Erling Smørgrav If a name is changed, put its old spelling in the 311bc421551SDag-Erling Smørgrav '<code>backward</code>' file as a link to the new spelling. 312bc421551SDag-Erling Smørgrav This means old spellings will continue to work. 313bc421551SDag-Erling Smørgrav Ordinarily a name change should occur only in the rare case when 314bc421551SDag-Erling Smørgrav a location's consensus English-language spelling changes; for example, 315bc421551SDag-Erling Smørgrav in 2008 <code>Asia/Calcutta</code> was renamed to <code>Asia/Kolkata</code> 316bc421551SDag-Erling Smørgrav due to long-time widespread use of the new city name instead of the old. 317bc421551SDag-Erling Smørgrav </li> 318bc421551SDag-Erling Smørgrav</ul> 319bc421551SDag-Erling Smørgrav 320bc421551SDag-Erling Smørgrav<p> 321bc421551SDag-Erling SmørgravGuidelines have evolved with time, and names following old versions of 322bc421551SDag-Erling Smørgravthese guidelines might not follow the current version. When guidelines 323bc421551SDag-Erling Smørgravhave changed, old names continue to be supported. Guideline changes 324bc421551SDag-Erling Smørgravhave included the following: 325bc421551SDag-Erling Smørgrav</p> 326bc421551SDag-Erling Smørgrav 327bc421551SDag-Erling Smørgrav<ul> 328bc421551SDag-Erling Smørgrav<li> 329bc421551SDag-Erling SmørgravOlder versions of this package used a different naming scheme. 330bc421551SDag-Erling SmørgravSee the file '<code>backward</code>' for most of these older names 331bc421551SDag-Erling Smørgrav(e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>'). 332bc421551SDag-Erling SmørgravThe other old-fashioned names still supported are 333bc421551SDag-Erling Smørgrav'<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and 334bc421551SDag-Erling Smørgrav'<code>EET</code>' (see the file '<code>europe</code>'). 335bc421551SDag-Erling Smørgrav</li> 336bc421551SDag-Erling Smørgrav 337bc421551SDag-Erling Smørgrav<li> 338bc421551SDag-Erling SmørgravOlder versions of this package defined legacy names that are 339bc421551SDag-Erling Smørgravincompatible with the first guideline of location names, but which are 340bc421551SDag-Erling Smørgravstill supported. 341bc421551SDag-Erling SmørgravThese legacy names are mostly defined in the file 342bc421551SDag-Erling Smørgrav'<code>etcetera</code>'. 343bc421551SDag-Erling SmørgravAlso, the file '<code>backward</code>' defines the legacy names 344bc421551SDag-Erling Smørgrav'<code>Etc/GMT0</code>', '<code>Etc/GMT-0</code>', '<code>Etc/GMT+0</code>', 345bc421551SDag-Erling Smørgrav'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', 346bc421551SDag-Erling Smørgravand the file '<code>northamerica</code>' defines the legacy names 347bc421551SDag-Erling Smørgrav'<code>EST5EDT</code>', '<code>CST6CDT</code>', 348bc421551SDag-Erling Smørgrav'<code>MST7MDT</code>', and '<code>PST8PDT</code>'. 349bc421551SDag-Erling Smørgrav</li> 350bc421551SDag-Erling Smørgrav 351bc421551SDag-Erling Smørgrav<li> 352bc421551SDag-Erling SmørgravOlder versions of these guidelines said that 353bc421551SDag-Erling Smørgravthere should typically be at least one name for each <a 354bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr 355bc421551SDag-Erling Smørgravtitle="International Organization for Standardization">ISO</abbr> 356bc421551SDag-Erling Smørgrav3166-1</a> officially assigned two-letter code for an inhabited 357bc421551SDag-Erling Smørgravcountry or territory. 358bc421551SDag-Erling SmørgravThis old guideline has been dropped, as it was not needed to handle 359bc421551SDag-Erling Smørgravtimestamps correctly and it increased maintenance burden. 360bc421551SDag-Erling Smørgrav</li> 361bc421551SDag-Erling Smørgrav</ul> 362bc421551SDag-Erling Smørgrav 363bc421551SDag-Erling Smørgrav<p> 364bc421551SDag-Erling SmørgravThe file <code>zone1970.tab</code> lists geographical locations used 365bc421551SDag-Erling Smørgravto name timezones. 366bc421551SDag-Erling SmørgravIt is intended to be an exhaustive list of names for geographic 367bc421551SDag-Erling Smørgravregions as described above; this is a subset of the timezones in the data. 368bc421551SDag-Erling SmørgravAlthough a <code>zone1970.tab</code> location's 369bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Longitude">longitude</a> 370bc421551SDag-Erling Smørgravcorresponds to 371bc421551SDag-Erling Smørgravits <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean 372bc421551SDag-Erling Smørgravtime (<abbr>LMT</abbr>)</a> offset with one hour for every 15° 373bc421551SDag-Erling Smørgraveast longitude, this relationship is not exact. 374bc421551SDag-Erling SmørgravThe backward-compatibility file <code>zone.tab</code> is similar 375bc421551SDag-Erling Smørgravbut conforms to the older-version guidelines related to <abbr>ISO</abbr> 3166-1; 376bc421551SDag-Erling Smørgravit lists only one country code per entry and unlike <code>zone1970.tab</code> 377bc421551SDag-Erling Smørgravit can list names defined in <code>backward</code>. 37846c59934SDag-Erling SmørgravApplications that process only timestamps from now on can instead use the file 37946c59934SDag-Erling Smørgrav<code>zonenow.tab</code>, which partitions the world more coarsely, 38046c59934SDag-Erling Smørgravinto regions where clocks agree now and in the predicted future; 38146c59934SDag-Erling Smørgravthis file is smaller and simpler than <code>zone1970.tab</code> 38246c59934SDag-Erling Smørgravand <code>zone.tab</code>. 383bc421551SDag-Erling Smørgrav</p> 384bc421551SDag-Erling Smørgrav 385bc421551SDag-Erling Smørgrav<p> 386bc421551SDag-Erling SmørgravThe database defines each timezone name to be a zone, or a link to a zone. 387bc421551SDag-Erling SmørgravThe source file <code>backward</code> defines links for backward 388bc421551SDag-Erling Smørgravcompatibility; it does not define zones. 389bc421551SDag-Erling SmørgravAlthough <code>backward</code> was originally designed to be optional, 390bc421551SDag-Erling Smørgravnowadays distributions typically use it 391bc421551SDag-Erling Smørgravand no great weight should be attached to whether a link 392bc421551SDag-Erling Smørgravis defined in <code>backward</code> or in some other file. 393bc421551SDag-Erling SmørgravThe source file <code>etcetera</code> defines names that may be useful 394a979394aSDag-Erling Smørgravon platforms that do not support proleptic <code>TZ</code> strings 395a979394aSDag-Erling Smørgravlike <code><+08>-8</code>; 396bc421551SDag-Erling Smørgravno other source file other than <code>backward</code> 397bc421551SDag-Erling Smørgravcontains links to its zones. 398bc421551SDag-Erling SmørgravOne of <code>etcetera</code>'s names is <code>Etc/UTC</code>, 399bc421551SDag-Erling Smørgravused by functions like <code>gmtime</code> to obtain leap 400bc421551SDag-Erling Smørgravsecond information on platforms that support leap seconds. 401bc421551SDag-Erling SmørgravAnother <code>etcetera</code> name, <code>GMT</code>, 402bc421551SDag-Erling Smørgravis used by older code releases. 403bc421551SDag-Erling Smørgrav</p> 404bc421551SDag-Erling Smørgrav</section> 405bc421551SDag-Erling Smørgrav 406bc421551SDag-Erling Smørgrav<section> 407bc421551SDag-Erling Smørgrav <h2 id="abbreviations">Time zone abbreviations</h2> 408bc421551SDag-Erling Smørgrav<p> 409bc421551SDag-Erling SmørgravWhen this package is installed, it generates time zone abbreviations 410bc421551SDag-Erling Smørgravlike '<code>EST</code>' to be compatible with human tradition and POSIX. 411bc421551SDag-Erling SmørgravHere are the general guidelines used for choosing time zone abbreviations, 412bc421551SDag-Erling Smørgravin decreasing order of importance: 413bc421551SDag-Erling Smørgrav</p> 414bc421551SDag-Erling Smørgrav 415bc421551SDag-Erling Smørgrav<ul> 416bc421551SDag-Erling Smørgrav <li> 417bc421551SDag-Erling Smørgrav Use three to six characters that are ASCII alphanumerics or 418bc421551SDag-Erling Smørgrav '<code>+</code>' or '<code>-</code>'. 419bc421551SDag-Erling Smørgrav Previous editions of this database also used characters like 420bc421551SDag-Erling Smørgrav space and '<code>?</code>', but these characters have a 421bc421551SDag-Erling Smørgrav special meaning to the 422bc421551SDag-Erling Smørgrav <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a> 423bc421551SDag-Erling Smørgrav and cause commands like 424bc421551SDag-Erling Smørgrav '<code><a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a> 425bc421551SDag-Erling Smørgrav `<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>' 426bc421551SDag-Erling Smørgrav to have unexpected effects. 427bc421551SDag-Erling Smørgrav Previous editions of this guideline required upper-case letters, but the 428bc421551SDag-Erling Smørgrav Congressman who introduced 429bc421551SDag-Erling Smørgrav <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro 430bc421551SDag-Erling Smørgrav Standard Time</a> preferred "ChST", so lower-case letters are now 431bc421551SDag-Erling Smørgrav allowed. 432bc421551SDag-Erling Smørgrav Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>', 433bc421551SDag-Erling Smørgrav '<code>+</code>', and alphanumeric characters from the portable 434bc421551SDag-Erling Smørgrav character set in the current locale. 435bc421551SDag-Erling Smørgrav In practice ASCII alphanumerics and '<code>+</code>' and 436bc421551SDag-Erling Smørgrav '<code>-</code>' are safe in all locales. 437bc421551SDag-Erling Smørgrav 438bc421551SDag-Erling Smørgrav <p> 439bc421551SDag-Erling Smørgrav In other words, in the C locale the POSIX extended regular 440bc421551SDag-Erling Smørgrav expression <code>[-+[:alnum:]]{3,6}</code> should match the 441bc421551SDag-Erling Smørgrav abbreviation. 442a979394aSDag-Erling Smørgrav This guarantees that all abbreviations could have been specified 443a979394aSDag-Erling Smørgrav explicitly by a POSIX proleptic <code>TZ</code> string. 444bc421551SDag-Erling Smørgrav </p> 445bc421551SDag-Erling Smørgrav </li> 446bc421551SDag-Erling Smørgrav <li> 447bc421551SDag-Erling Smørgrav Use abbreviations that are in common use among English-speakers, 448bc421551SDag-Erling Smørgrav e.g., 'EST' for Eastern Standard Time in North America. 449bc421551SDag-Erling Smørgrav We assume that applications translate them to other languages 450bc421551SDag-Erling Smørgrav as part of the normal localization process; for example, 451bc421551SDag-Erling Smørgrav a French application might translate 'EST' to 'HNE'. 452bc421551SDag-Erling Smørgrav 453bc421551SDag-Erling Smørgrav <p> 454bc421551SDag-Erling Smørgrav <small>These abbreviations (for standard/daylight/etc. time) are: 455bc421551SDag-Erling Smørgrav ACST/ACDT Australian Central, 456bc421551SDag-Erling Smørgrav AST/ADT/APT/AWT/ADDT Atlantic, 457bc421551SDag-Erling Smørgrav AEST/AEDT Australian Eastern, 458bc421551SDag-Erling Smørgrav AHST/AHDT Alaska-Hawaii, 459bc421551SDag-Erling Smørgrav AKST/AKDT Alaska, 460bc421551SDag-Erling Smørgrav AWST/AWDT Australian Western, 461bc421551SDag-Erling Smørgrav BST/BDT Bering, 462bc421551SDag-Erling Smørgrav CAT/CAST Central Africa, 463bc421551SDag-Erling Smørgrav CET/CEST/CEMT Central European, 464bc421551SDag-Erling Smørgrav ChST Chamorro, 46575411d15SDag-Erling Smørgrav CST/CDT/CWT/CPT Central [North America], 466bc421551SDag-Erling Smørgrav CST/CDT China, 467bc421551SDag-Erling Smørgrav GMT/BST/IST/BDST Greenwich, 468bc421551SDag-Erling Smørgrav EAT East Africa, 46975411d15SDag-Erling Smørgrav EST/EDT/EWT/EPT Eastern [North America], 470bc421551SDag-Erling Smørgrav EET/EEST Eastern European, 471bc421551SDag-Erling Smørgrav GST/GDT Guam, 472bc421551SDag-Erling Smørgrav HST/HDT/HWT/HPT Hawaii, 473bc421551SDag-Erling Smørgrav HKT/HKST/HKWT Hong Kong, 474bc421551SDag-Erling Smørgrav IST India, 475bc421551SDag-Erling Smørgrav IST/GMT Irish, 476bc421551SDag-Erling Smørgrav IST/IDT/IDDT Israel, 477bc421551SDag-Erling Smørgrav JST/JDT Japan, 478bc421551SDag-Erling Smørgrav KST/KDT Korea, 479bc421551SDag-Erling Smørgrav MET/MEST Middle European (a backward-compatibility alias for 480bc421551SDag-Erling Smørgrav Central European), 481bc421551SDag-Erling Smørgrav MSK/MSD Moscow, 48275411d15SDag-Erling Smørgrav MST/MDT/MWT/MPT Mountain, 483bc421551SDag-Erling Smørgrav NST/NDT/NWT/NPT/NDDT Newfoundland, 484bc421551SDag-Erling Smørgrav NST/NDT/NWT/NPT Nome, 485bc421551SDag-Erling Smørgrav NZMT/NZST New Zealand through 1945, 486bc421551SDag-Erling Smørgrav NZST/NZDT New Zealand 1946–present, 487bc421551SDag-Erling Smørgrav PKT/PKST Pakistan, 48875411d15SDag-Erling Smørgrav PST/PDT/PWT/PPT Pacific, 489bc421551SDag-Erling Smørgrav PST/PDT Philippine, 490bc421551SDag-Erling Smørgrav SAST South Africa, 491bc421551SDag-Erling Smørgrav SST Samoa, 492bc421551SDag-Erling Smørgrav UTC Universal, 493bc421551SDag-Erling Smørgrav WAT/WAST West Africa, 494bc421551SDag-Erling Smørgrav WET/WEST/WEMT Western European, 495bc421551SDag-Erling Smørgrav WIB Waktu Indonesia Barat, 496bc421551SDag-Erling Smørgrav WIT Waktu Indonesia Timur, 497bc421551SDag-Erling Smørgrav WITA Waktu Indonesia Tengah, 498bc421551SDag-Erling Smørgrav YST/YDT/YWT/YPT/YDDT Yukon</small>. 499bc421551SDag-Erling Smørgrav </p> 500bc421551SDag-Erling Smørgrav </li> 501bc421551SDag-Erling Smørgrav <li> 502bc421551SDag-Erling Smørgrav <p> 503bc421551SDag-Erling Smørgrav For times taken from a city's longitude, use the 504bc421551SDag-Erling Smørgrav traditional <var>x</var>MT notation. 505bc421551SDag-Erling Smørgrav The only abbreviation like this in current use is '<abbr>GMT</abbr>'. 506bc421551SDag-Erling Smørgrav The others are for timestamps before 1960, 507bc421551SDag-Erling Smørgrav except that Monrovia Mean Time persisted until 1972. 508bc421551SDag-Erling Smørgrav Typically, numeric abbreviations (e.g., '<code>-</code>004430' for 509bc421551SDag-Erling Smørgrav MMT) would cause trouble here, as the numeric strings would exceed 510bc421551SDag-Erling Smørgrav the POSIX length limit. 511bc421551SDag-Erling Smørgrav </p> 512bc421551SDag-Erling Smørgrav 513bc421551SDag-Erling Smørgrav <p> 514bc421551SDag-Erling Smørgrav <small>These abbreviations are: 515bc421551SDag-Erling Smørgrav AMT Asunción, Athens; 51675411d15SDag-Erling Smørgrav BMT Baghdad, Bangkok, Batavia, Bermuda, Bern, Bogotá, 517bc421551SDag-Erling Smørgrav Brussels, Bucharest; 518bc421551SDag-Erling Smørgrav CMT Calamarca, Caracas, Chisinau, Colón, Córdoba; 519bc421551SDag-Erling Smørgrav DMT Dublin/Dunsink; 520bc421551SDag-Erling Smørgrav EMT Easter; 521bc421551SDag-Erling Smørgrav FFMT Fort-de-France; 522bc421551SDag-Erling Smørgrav FMT Funchal; 523bc421551SDag-Erling Smørgrav GMT Greenwich; 524bc421551SDag-Erling Smørgrav HMT Havana, Helsinki, Horta, Howrah; 525bc421551SDag-Erling Smørgrav IMT Irkutsk, Istanbul; 526bc421551SDag-Erling Smørgrav JMT Jerusalem; 527bc421551SDag-Erling Smørgrav KMT Kaunas, Kyiv, Kingston; 52875411d15SDag-Erling Smørgrav LMT Lima, Lisbon, local; 529bc421551SDag-Erling Smørgrav MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo, 530bc421551SDag-Erling Smørgrav Moratuwa, Moscow; 531bc421551SDag-Erling Smørgrav PLMT Phù Liễn; 532bc421551SDag-Erling Smørgrav PMT Paramaribo, Paris, Perm, Pontianak, Prague; 533bc421551SDag-Erling Smørgrav PMMT Port Moresby; 53475411d15SDag-Erling Smørgrav PPMT Port-au-Prince; 535bc421551SDag-Erling Smørgrav QMT Quito; 536bc421551SDag-Erling Smørgrav RMT Rangoon, Riga, Rome; 537bc421551SDag-Erling Smørgrav SDMT Santo Domingo; 538bc421551SDag-Erling Smørgrav SJMT San José; 539bc421551SDag-Erling Smørgrav SMT Santiago, Simferopol, Singapore, Stanley; 540bc421551SDag-Erling Smørgrav TBMT Tbilisi; 541bc421551SDag-Erling Smørgrav TMT Tallinn, Tehran; 54275411d15SDag-Erling Smørgrav WMT Warsaw.</small> 543bc421551SDag-Erling Smørgrav </p> 544bc421551SDag-Erling Smørgrav 545bc421551SDag-Erling Smørgrav <p> 546bc421551SDag-Erling Smørgrav <small>A few abbreviations also follow the pattern that 547bc421551SDag-Erling Smørgrav <abbr>GMT</abbr>/<abbr>BST</abbr> established for time in the UK. 548bc421551SDag-Erling Smørgrav They are: 549bc421551SDag-Erling Smørgrav BMT/BST for Bermuda 1890–1930, 550bc421551SDag-Erling Smørgrav CMT/BST for Calamarca Mean Time and Bolivian Summer Time 551bc421551SDag-Erling Smørgrav 1890–1932, 552bc421551SDag-Erling Smørgrav DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time 553bc421551SDag-Erling Smørgrav 1880–1916, 554bc421551SDag-Erling Smørgrav MMT/MST/MDST for Moscow 1880–1919, and 555bc421551SDag-Erling Smørgrav RMT/LST for Riga Mean Time and Latvian Summer time 1880–1926. 556bc421551SDag-Erling Smørgrav </small> 557bc421551SDag-Erling Smørgrav </p> 558bc421551SDag-Erling Smørgrav </li> 559bc421551SDag-Erling Smørgrav <li> 560bc421551SDag-Erling Smørgrav Use '<abbr>LMT</abbr>' for local mean time of locations before the 561bc421551SDag-Erling Smørgrav introduction of standard time; see "<a href="#scope">Scope of the 562bc421551SDag-Erling Smørgrav <code><abbr>tz</abbr></code> database</a>". 563bc421551SDag-Erling Smørgrav </li> 564bc421551SDag-Erling Smørgrav <li> 565bc421551SDag-Erling Smørgrav If there is no common English abbreviation, use numeric offsets like 566bc421551SDag-Erling Smørgrav <code>-</code>05 and <code>+</code>0530 that are generated 567bc421551SDag-Erling Smørgrav by <code>zic</code>'s <code>%z</code> notation. 568bc421551SDag-Erling Smørgrav </li> 569bc421551SDag-Erling Smørgrav <li> 570bc421551SDag-Erling Smørgrav Use current abbreviations for older timestamps to avoid confusion. 571bc421551SDag-Erling Smørgrav For example, in 1910 a common English abbreviation for time 572bc421551SDag-Erling Smørgrav in central Europe was 'MEZ' (short for both "Middle European 573bc421551SDag-Erling Smørgrav Zone" and for "Mitteleuropäische Zeit" in German). 574bc421551SDag-Erling Smørgrav Nowadays 'CET' ("Central European Time") is more common in 575bc421551SDag-Erling Smørgrav English, and the database uses 'CET' even for circa-1910 576bc421551SDag-Erling Smørgrav timestamps as this is less confusing for modern users and avoids 577bc421551SDag-Erling Smørgrav the need for determining when 'CET' supplanted 'MEZ' in common 578bc421551SDag-Erling Smørgrav usage. 579bc421551SDag-Erling Smørgrav </li> 580bc421551SDag-Erling Smørgrav <li> 581bc421551SDag-Erling Smørgrav Use a consistent style in a timezone's history. 582bc421551SDag-Erling Smørgrav For example, if a history tends to use numeric 583bc421551SDag-Erling Smørgrav abbreviations and a particular entry could go either way, use a 584bc421551SDag-Erling Smørgrav numeric abbreviation. 585bc421551SDag-Erling Smørgrav </li> 586bc421551SDag-Erling Smørgrav <li> 587bc421551SDag-Erling Smørgrav Use 588bc421551SDag-Erling Smørgrav <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a> 589bc421551SDag-Erling Smørgrav (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for 590bc421551SDag-Erling Smørgrav locations while uninhabited. 591bc421551SDag-Erling Smørgrav The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in 592bc421551SDag-Erling Smørgrav some sense undefined; this notation is derived 593*967a49a2SDag-Erling Smørgrav from <a href="https://www.rfc-editor.org/rfc/rfc3339">Internet 594bc421551SDag-Erling Smørgrav <abbr title="Request For Comments">RFC</abbr> 3339</a>. 595a979394aSDag-Erling Smørgrav (The abbreviation 'Z' that 596*967a49a2SDag-Erling Smørgrav <a href="https://www.rfc-editor.org/rfc/rfc9557">Internet 597a979394aSDag-Erling Smørgrav <abbr>RFC</abbr> 9557</a> uses for this concept 598a979394aSDag-Erling Smørgrav would violate the POSIX requirement 599a979394aSDag-Erling Smørgrav of at least three characters in an abbreviation.) 600bc421551SDag-Erling Smørgrav </li> 601bc421551SDag-Erling Smørgrav</ul> 602bc421551SDag-Erling Smørgrav 603bc421551SDag-Erling Smørgrav<p> 604bc421551SDag-Erling SmørgravApplication writers should note that these abbreviations are ambiguous 605bc421551SDag-Erling Smørgravin practice: e.g., 'CST' means one thing in China and something else 606bc421551SDag-Erling Smørgravin North America, and 'IST' can refer to time in India, Ireland or 607bc421551SDag-Erling SmørgravIsrael. 608bc421551SDag-Erling SmørgravTo avoid ambiguity, use numeric <abbr>UT</abbr> offsets like 609bc421551SDag-Erling Smørgrav'<code>-</code>0600' instead of time zone abbreviations like 'CST'. 610bc421551SDag-Erling Smørgrav</p> 611bc421551SDag-Erling Smørgrav</section> 612bc421551SDag-Erling Smørgrav 613bc421551SDag-Erling Smørgrav<section> 614bc421551SDag-Erling Smørgrav <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2> 615bc421551SDag-Erling Smørgrav<p> 616bc421551SDag-Erling SmørgravThe <code><abbr>tz</abbr></code> database is not authoritative, and it 617bc421551SDag-Erling Smørgravsurely has errors. 618bc421551SDag-Erling SmørgravCorrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>. 619bc421551SDag-Erling SmørgravUsers requiring authoritative data should consult national standards 620bc421551SDag-Erling Smørgravbodies and the references cited in the database's comments. 621bc421551SDag-Erling Smørgrav</p> 622bc421551SDag-Erling Smørgrav 623bc421551SDag-Erling Smørgrav<p> 624bc421551SDag-Erling SmørgravErrors in the <code><abbr>tz</abbr></code> database arise from many sources: 625bc421551SDag-Erling Smørgrav</p> 626bc421551SDag-Erling Smørgrav 627bc421551SDag-Erling Smørgrav<ul> 628bc421551SDag-Erling Smørgrav <li> 629bc421551SDag-Erling Smørgrav The <code><abbr>tz</abbr></code> database predicts future 630bc421551SDag-Erling Smørgrav timestamps, and current predictions 631bc421551SDag-Erling Smørgrav will be incorrect after future governments change the rules. 632bc421551SDag-Erling Smørgrav For example, if today someone schedules a meeting for 13:00 next 633bc421551SDag-Erling Smørgrav October 1, Casablanca time, and tomorrow Morocco changes its 634bc421551SDag-Erling Smørgrav daylight saving rules, software can mess up after the rule change 635bc421551SDag-Erling Smørgrav if it blithely relies on conversions made before the change. 636bc421551SDag-Erling Smørgrav </li> 637bc421551SDag-Erling Smørgrav <li> 638bc421551SDag-Erling Smørgrav The pre-1970 entries in this database cover only a tiny sliver of how 639bc421551SDag-Erling Smørgrav clocks actually behaved; the vast majority of the necessary 640bc421551SDag-Erling Smørgrav information was lost or never recorded. 641bc421551SDag-Erling Smørgrav Thousands more timezones would be needed if 642bc421551SDag-Erling Smørgrav the <code><abbr>tz</abbr></code> database's scope were extended to 643bc421551SDag-Erling Smørgrav cover even just the known or guessed history of standard time; for 644bc421551SDag-Erling Smørgrav example, the current single entry for France would need to split 645bc421551SDag-Erling Smørgrav into dozens of entries, perhaps hundreds. 646bc421551SDag-Erling Smørgrav And in most of the world even this approach would be misleading 647bc421551SDag-Erling Smørgrav due to widespread disagreement or indifference about what times 648bc421551SDag-Erling Smørgrav should be observed. 649bc421551SDag-Erling Smørgrav In her 2015 book 650bc421551SDag-Erling Smørgrav <cite><a 651bc421551SDag-Erling Smørgrav href="https://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The 652bc421551SDag-Erling Smørgrav Global Transformation of Time, 1870–1950</a></cite>, 653bc421551SDag-Erling Smørgrav Vanessa Ogle writes 654bc421551SDag-Erling Smørgrav "Outside of Europe and North America there was no system of time 655bc421551SDag-Erling Smørgrav zones at all, often not even a stable landscape of mean times, 656bc421551SDag-Erling Smørgrav prior to the middle decades of the twentieth century". 657bc421551SDag-Erling Smørgrav See: Timothy Shenk, <a 658bc421551SDag-Erling Smørgravhref="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked: 659bc421551SDag-Erling Smørgrav A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17. 660bc421551SDag-Erling Smørgrav </li> 661bc421551SDag-Erling Smørgrav <li> 662bc421551SDag-Erling Smørgrav Most of the pre-1970 data entries come from unreliable sources, often 663bc421551SDag-Erling Smørgrav astrology books that lack citations and whose compilers evidently 664bc421551SDag-Erling Smørgrav invented entries when the true facts were unknown, without 665bc421551SDag-Erling Smørgrav reporting which entries were known and which were invented. 666bc421551SDag-Erling Smørgrav These books often contradict each other or give implausible entries, 667bc421551SDag-Erling Smørgrav and on the rare occasions when they are checked they are 668bc421551SDag-Erling Smørgrav typically found to be incorrect. 669bc421551SDag-Erling Smørgrav </li> 670bc421551SDag-Erling Smørgrav <li> 671bc421551SDag-Erling Smørgrav For the UK the <code><abbr>tz</abbr></code> database relies on 672bc421551SDag-Erling Smørgrav years of first-class work done by 673bc421551SDag-Erling Smørgrav Joseph Myers and others; see 674bc421551SDag-Erling Smørgrav "<a href="https://www.polyomino.org.uk/british-time/">History of 675bc421551SDag-Erling Smørgrav legal time in Britain</a>". 676bc421551SDag-Erling Smørgrav Other countries are not done nearly as well. 677bc421551SDag-Erling Smørgrav </li> 678bc421551SDag-Erling Smørgrav <li> 679bc421551SDag-Erling Smørgrav Sometimes, different people in the same city maintain clocks 680bc421551SDag-Erling Smørgrav that differ significantly. 681bc421551SDag-Erling Smørgrav Historically, railway time was used by railroad companies (which 682bc421551SDag-Erling Smørgrav did not always 683bc421551SDag-Erling Smørgrav agree with each other), church-clock time was used for birth 684bc421551SDag-Erling Smørgrav certificates, etc. 685bc421551SDag-Erling Smørgrav More recently, competing political groups might disagree about 686bc421551SDag-Erling Smørgrav clock settings. Often this is merely common practice, but 687bc421551SDag-Erling Smørgrav sometimes it is set by law. 688bc421551SDag-Erling Smørgrav For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France 689bc421551SDag-Erling Smørgrav was legally <abbr>UT</abbr> +00:09:21 outside train stations and 690bc421551SDag-Erling Smørgrav <abbr>UT</abbr> +00:04:21 inside. Other examples include 691bc421551SDag-Erling Smørgrav Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and 692bc421551SDag-Erling Smørgrav Ürümqi to this day. 693bc421551SDag-Erling Smørgrav </li> 694bc421551SDag-Erling Smørgrav <li> 695bc421551SDag-Erling Smørgrav Although a named location in the <code><abbr>tz</abbr></code> 696bc421551SDag-Erling Smørgrav database stands for the containing region, its pre-1970 data 697bc421551SDag-Erling Smørgrav entries are often accurate for only a small subset of that region. 698bc421551SDag-Erling Smørgrav For example, <code>Europe/London</code> stands for the United 699bc421551SDag-Erling Smørgrav Kingdom, but its pre-1847 times are valid only for locations that 700bc421551SDag-Erling Smørgrav have London's exact meridian, and its 1847 transition 701bc421551SDag-Erling Smørgrav to <abbr>GMT</abbr> is known to be valid only for the L&NW and 702bc421551SDag-Erling Smørgrav the Caledonian railways. 703bc421551SDag-Erling Smørgrav </li> 704bc421551SDag-Erling Smørgrav <li> 705bc421551SDag-Erling Smørgrav The <code><abbr>tz</abbr></code> database does not record the 706bc421551SDag-Erling Smørgrav earliest time for which a timezone's 707bc421551SDag-Erling Smørgrav data entries are thereafter valid for every location in the region. 708bc421551SDag-Erling Smørgrav For example, <code>Europe/London</code> is valid for all locations 709bc421551SDag-Erling Smørgrav in its region after <abbr>GMT</abbr> was made the standard time, 710bc421551SDag-Erling Smørgrav but the date of standardization (1880-08-02) is not in the 711bc421551SDag-Erling Smørgrav <code><abbr>tz</abbr></code> database, other than in commentary. 712bc421551SDag-Erling Smørgrav For many timezones the earliest time of 713bc421551SDag-Erling Smørgrav validity is unknown. 714bc421551SDag-Erling Smørgrav </li> 715bc421551SDag-Erling Smørgrav <li> 716bc421551SDag-Erling Smørgrav The <code><abbr>tz</abbr></code> database does not record a 717bc421551SDag-Erling Smørgrav region's boundaries, and in many cases the boundaries are not known. 718bc421551SDag-Erling Smørgrav For example, the timezone 719bc421551SDag-Erling Smørgrav <code>America/Kentucky/Louisville</code> represents a region 720bc421551SDag-Erling Smørgrav around the city of Louisville, the boundaries of which are 721bc421551SDag-Erling Smørgrav unclear. 722bc421551SDag-Erling Smørgrav </li> 723bc421551SDag-Erling Smørgrav <li> 724bc421551SDag-Erling Smørgrav Changes that are modeled as instantaneous transitions in the 725bc421551SDag-Erling Smørgrav <code><abbr>tz</abbr></code> 726bc421551SDag-Erling Smørgrav database were often spread out over hours, days, or even decades. 727bc421551SDag-Erling Smørgrav </li> 728bc421551SDag-Erling Smørgrav <li> 729bc421551SDag-Erling Smørgrav Even if the time is specified by law, locations sometimes 730bc421551SDag-Erling Smørgrav deliberately flout the law. 731bc421551SDag-Erling Smørgrav </li> 732bc421551SDag-Erling Smørgrav <li> 733bc421551SDag-Erling Smørgrav Early timekeeping practices, even assuming perfect clocks, were 734bc421551SDag-Erling Smørgrav often not specified to the accuracy that the 735bc421551SDag-Erling Smørgrav <code><abbr>tz</abbr></code> database requires. 736bc421551SDag-Erling Smørgrav </li> 737bc421551SDag-Erling Smørgrav <li> 738bc421551SDag-Erling Smørgrav The <code><abbr>tz</abbr></code> database cannot represent stopped clocks. 739bc421551SDag-Erling Smørgrav However, on 1911-03-11 at 00:00, some public-facing French clocks 740bc421551SDag-Erling Smørgrav were changed by stopping them for a few minutes to effect a transition. 741bc421551SDag-Erling Smørgrav The <code><abbr>tz</abbr></code> database models this via a 742bc421551SDag-Erling Smørgrav backward transition; the relevant French legislation does not 743bc421551SDag-Erling Smørgrav specify exactly how the transition was to occur. 744bc421551SDag-Erling Smørgrav </li> 745bc421551SDag-Erling Smørgrav <li> 746bc421551SDag-Erling Smørgrav Sometimes historical timekeeping was specified more precisely 747bc421551SDag-Erling Smørgrav than what the <code><abbr>tz</abbr></code> code can handle. 748bc421551SDag-Erling Smørgrav For example, from 1880 to 1916 clocks in Ireland observed Dublin Mean 749bc421551SDag-Erling Smørgrav Time (estimated to be <abbr>UT</abbr> 750bc421551SDag-Erling Smørgrav −00:25:21.1); although the <code><abbr>tz</abbr></code> 751bc421551SDag-Erling Smørgrav source data can represent the .1 second, TZif files and the code cannot. 752bc421551SDag-Erling Smørgrav In practice these old specifications were rarely if ever 753bc421551SDag-Erling Smørgrav implemented to subsecond precision. 754bc421551SDag-Erling Smørgrav </li> 755bc421551SDag-Erling Smørgrav <li> 756bc421551SDag-Erling Smørgrav Even when all the timestamp transitions recorded by the 757bc421551SDag-Erling Smørgrav <code><abbr>tz</abbr></code> database are correct, the 758bc421551SDag-Erling Smørgrav <code><abbr>tz</abbr></code> rules that generate them may not 759bc421551SDag-Erling Smørgrav faithfully reflect the historical rules. 760bc421551SDag-Erling Smørgrav For example, from 1922 until World War II the UK moved clocks 761bc421551SDag-Erling Smørgrav forward the day following the third Saturday in April unless that 762bc421551SDag-Erling Smørgrav was Easter, in which case it moved clocks forward the previous 763bc421551SDag-Erling Smørgrav Sunday. 764bc421551SDag-Erling Smørgrav Because the <code><abbr>tz</abbr></code> database has no 765bc421551SDag-Erling Smørgrav way to specify Easter, these exceptional years are entered as 766bc421551SDag-Erling Smørgrav separate <code><abbr>tz</abbr> Rule</code> lines, even though the 767bc421551SDag-Erling Smørgrav legal rules did not change. 768bc421551SDag-Erling Smørgrav When transitions are known but the historical rules behind them are not, 769bc421551SDag-Erling Smørgrav the database contains <code>Zone</code> and <code>Rule</code> 770bc421551SDag-Erling Smørgrav entries that are intended to represent only the generated 771bc421551SDag-Erling Smørgrav transitions, not any underlying historical rules; however, this 772bc421551SDag-Erling Smørgrav intent is recorded at best only in commentary. 773bc421551SDag-Erling Smørgrav </li> 774bc421551SDag-Erling Smørgrav <li> 775bc421551SDag-Erling Smørgrav The <code><abbr>tz</abbr></code> database models time 776bc421551SDag-Erling Smørgrav using the <a 777bc421551SDag-Erling Smørgrav href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic 778bc421551SDag-Erling Smørgrav Gregorian calendar</a> with days containing 24 equal-length hours 779bc421551SDag-Erling Smørgrav numbered 00 through 23, except when clock transitions occur. 780bc421551SDag-Erling Smørgrav Pre-standard time is modeled as local mean time. 781bc421551SDag-Erling Smørgrav However, historically many people used other calendars and other timescales. 782bc421551SDag-Erling Smørgrav For example, the Roman Empire used 783bc421551SDag-Erling Smørgrav the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian 784bc421551SDag-Erling Smørgrav calendar</a>, 785bc421551SDag-Erling Smørgrav and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman 786bc421551SDag-Erling Smørgrav timekeeping</a> had twelve varying-length daytime hours with a 787bc421551SDag-Erling Smørgrav non-hour-based system at night. 788bc421551SDag-Erling Smørgrav And even today, some local practices diverge from the Gregorian 789bc421551SDag-Erling Smørgrav calendar with 24-hour days. These divergences range from 790bc421551SDag-Erling Smørgrav relatively minor, such as Japanese bars giving times like "24:30" for the 791bc421551SDag-Erling Smørgrav wee hours of the morning, to more-significant differences such as <a 79246c59934SDag-Erling Smørgrav href="https://theworld.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the 793bc421551SDag-Erling Smørgrav east African practice of starting the day at dawn</a>, renumbering 794bc421551SDag-Erling Smørgrav the Western 06:00 to be 12:00. These practices are largely outside 795bc421551SDag-Erling Smørgrav the scope of the <code><abbr>tz</abbr></code> code and data, which 796bc421551SDag-Erling Smørgrav provide only limited support for date and time localization 797a979394aSDag-Erling Smørgrav such as that required by POSIX. 798bc421551SDag-Erling Smørgrav If <abbr>DST</abbr> is not used a different time zone 799bc421551SDag-Erling Smørgrav can often do the trick; for example, in Kenya a <code>TZ</code> setting 800bc421551SDag-Erling Smørgrav like <code><-03>3</code> or <code>America/Cayenne</code> starts 801bc421551SDag-Erling Smørgrav the day six hours later than <code>Africa/Nairobi</code> does. 802bc421551SDag-Erling Smørgrav </li> 803bc421551SDag-Erling Smørgrav <li> 804bc421551SDag-Erling Smørgrav Early clocks were less reliable, and data entries do not represent 805bc421551SDag-Erling Smørgrav clock error. 806bc421551SDag-Erling Smørgrav </li> 807bc421551SDag-Erling Smørgrav <li> 808bc421551SDag-Erling Smørgrav The <code><abbr>tz</abbr></code> database assumes Universal Time 809bc421551SDag-Erling Smørgrav (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not 810bc421551SDag-Erling Smørgrav standardized for older timestamps. 811bc421551SDag-Erling Smørgrav In the <code><abbr>tz</abbr></code> database commentary, 812bc421551SDag-Erling Smørgrav <abbr>UT</abbr> denotes a family of time standards that includes 813bc421551SDag-Erling Smørgrav Coordinated Universal Time (<abbr>UTC</abbr>) along with other 814bc421551SDag-Erling Smørgrav variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>, 815bc421551SDag-Erling Smørgrav with days starting at midnight. 816bc421551SDag-Erling Smørgrav Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern 817bc421551SDag-Erling Smørgrav timestamps, <abbr>UTC</abbr> was not defined until 1960, so 81875411d15SDag-Erling Smørgrav commentary uses the more general abbreviation <abbr>UT</abbr> for 819bc421551SDag-Erling Smørgrav timestamps that might predate 1960. 820bc421551SDag-Erling Smørgrav Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly, 821bc421551SDag-Erling Smørgrav and since pre-1972 <abbr>UTC</abbr> seconds varied in length, 822bc421551SDag-Erling Smørgrav interpretation of older timestamps can be problematic when 823bc421551SDag-Erling Smørgrav subsecond accuracy is needed. 824bc421551SDag-Erling Smørgrav </li> 825bc421551SDag-Erling Smørgrav <li> 826bc421551SDag-Erling Smørgrav Civil time was not based on atomic time before 1972, and we do not 827bc421551SDag-Erling Smørgrav know the history of 828bc421551SDag-Erling Smørgrav <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's 829bc421551SDag-Erling Smørgrav rotation</a> accurately enough to map <a 830bc421551SDag-Erling Smørgrav href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr 831bc421551SDag-Erling Smørgrav title="International System of Units">SI</abbr></a> seconds to 832bc421551SDag-Erling Smørgrav historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a> 833bc421551SDag-Erling Smørgrav to more than about one-hour accuracy. 834bc421551SDag-Erling Smørgrav See: Stephenson FR, Morrison LV, Hohenkerk CY. 835bc421551SDag-Erling Smørgrav <a href="https://dx.doi.org/10.1098/rspa.2016.0404">Measurement of 836bc421551SDag-Erling Smørgrav the Earth's rotation: 720 BC to AD 2015</a>. 837bc421551SDag-Erling Smørgrav <cite>Proc Royal Soc A</cite>. 2016;472:20160404. 838bc421551SDag-Erling Smørgrav Also see: Espenak F. <a 839bc421551SDag-Erling Smørgrav href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty 840bc421551SDag-Erling Smørgrav in Delta T (ΔT)</a>. 841bc421551SDag-Erling Smørgrav </li> 842bc421551SDag-Erling Smørgrav <li> 843bc421551SDag-Erling Smørgrav The relationship between POSIX time (that is, <abbr>UTC</abbr> but 844bc421551SDag-Erling Smørgrav ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap 84575411d15SDag-Erling Smørgrav seconds</a>) and <abbr>UTC</abbr> is not agreed upon. 84675411d15SDag-Erling Smørgrav This affects time stamps during the leap second era (1972–2035). 847bc421551SDag-Erling Smørgrav Although the POSIX 848bc421551SDag-Erling Smørgrav clock officially stops during an inserted leap second, at least one 849bc421551SDag-Erling Smørgrav proposed standard has it jumping back a second instead; and in 850bc421551SDag-Erling Smørgrav practice POSIX clocks more typically either progress glacially during 851bc421551SDag-Erling Smørgrav a leap second, or are slightly slowed while near a leap second. 852bc421551SDag-Erling Smørgrav </li> 853bc421551SDag-Erling Smørgrav <li> 854bc421551SDag-Erling Smørgrav The <code><abbr>tz</abbr></code> database does not represent how 855bc421551SDag-Erling Smørgrav uncertain its information is. 856bc421551SDag-Erling Smørgrav Ideally it would contain information about when data entries are 857bc421551SDag-Erling Smørgrav incomplete or dicey. 858bc421551SDag-Erling Smørgrav Partial temporal knowledge is a field of active research, though, 859bc421551SDag-Erling Smørgrav and it is not clear how to apply it here. 860bc421551SDag-Erling Smørgrav </li> 861bc421551SDag-Erling Smørgrav</ul> 862bc421551SDag-Erling Smørgrav 863bc421551SDag-Erling Smørgrav<p> 864bc421551SDag-Erling SmørgravIn short, many, perhaps most, of the <code><abbr>tz</abbr></code> 865bc421551SDag-Erling Smørgravdatabase's pre-1970 and future timestamps are either wrong or 866bc421551SDag-Erling Smørgravmisleading. 867bc421551SDag-Erling SmørgravAny attempt to pass the 868bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> database off as the definition of time 869bc421551SDag-Erling Smørgravshould be unacceptable to anybody who cares about the facts. 870bc421551SDag-Erling SmørgravIn particular, the <code><abbr>tz</abbr></code> database's 871bc421551SDag-Erling Smørgrav<abbr>LMT</abbr> offsets should not be considered meaningful, and 872bc421551SDag-Erling Smørgravshould not prompt creation of timezones 873bc421551SDag-Erling Smørgravmerely because two locations 874bc421551SDag-Erling Smørgravdiffer in <abbr>LMT</abbr> or transitioned to standard time at 875bc421551SDag-Erling Smørgravdifferent dates. 876bc421551SDag-Erling Smørgrav</p> 877bc421551SDag-Erling Smørgrav</section> 878bc421551SDag-Erling Smørgrav 879bc421551SDag-Erling Smørgrav<section> 880bc421551SDag-Erling Smørgrav <h2 id="functions">Time and date functions</h2> 881bc421551SDag-Erling Smørgrav<p> 882bc421551SDag-Erling SmørgravThe <code><abbr>tz</abbr></code> code contains time and date functions 883bc421551SDag-Erling Smørgravthat are upwards compatible with those of POSIX. 884bc421551SDag-Erling SmørgravCode compatible with this package is already 885bc421551SDag-Erling Smørgrav<a href="tz-link.html#tzdb">part of many platforms</a>, where the 886bc421551SDag-Erling Smørgravprimary use of this package is to update obsolete time-related files. 887bc421551SDag-Erling SmørgravTo do this, you may need to compile the time zone compiler 888a979394aSDag-Erling Smørgrav<code>zic</code> supplied with this package instead of using the 889a979394aSDag-Erling Smørgravsystem <code>zic</code>, since the format of <code>zic</code>'s 890bc421551SDag-Erling Smørgravinput is occasionally extended, and a platform may still be shipping 891bc421551SDag-Erling Smørgravan older <code>zic</code>. 892bc421551SDag-Erling Smørgrav</p> 893bc421551SDag-Erling Smørgrav 894a979394aSDag-Erling Smørgrav<p> 895a979394aSDag-Erling SmørgravIn POSIX, time display in a process is controlled by the 896a979394aSDag-Erling Smørgravenvironment variable <code>TZ</code>, which can have two forms: 897a979394aSDag-Erling Smørgrav</p> 898bc421551SDag-Erling Smørgrav<ul> 899bc421551SDag-Erling Smørgrav <li> 900a979394aSDag-Erling Smørgrav A <dfn>proleptic <code>TZ</code></dfn> value 901a979394aSDag-Erling Smørgrav like <code>CET-1CEST,M3.5.0,M10.5.0/3</code> uses a complex 902a979394aSDag-Erling Smørgrav notation that specifies a single standard time along with daylight 903a979394aSDag-Erling Smørgrav saving rules that apply to all years past, present, and future. 904a979394aSDag-Erling Smørgrav </li> 905a979394aSDag-Erling Smørgrav <li> 906a979394aSDag-Erling Smørgrav A <dfn>geographical <code>TZ</code></dfn> value 907a979394aSDag-Erling Smørgrav like <code>Europe/Berlin</code> names a location that stands for 908a979394aSDag-Erling Smørgrav civil time near that location, which can have more than 909a979394aSDag-Erling Smørgrav one standard time and more than one set of daylight saving rules, 910a979394aSDag-Erling Smørgrav to record timekeeping practice more accurately. 911a979394aSDag-Erling Smørgrav These names are defined by the <code><abbr>tz</abbr></code> database. 912a979394aSDag-Erling Smørgrav </li> 913a979394aSDag-Erling Smørgrav</ul> 914a979394aSDag-Erling Smørgrav 915a979394aSDag-Erling Smørgrav<h3 id="POSIX.1-2017">POSIX.1-2017 properties and limitations</h3> 916bc421551SDag-Erling Smørgrav<p> 917a979394aSDag-Erling SmørgravSome platforms support only the features required by POSIX.1-2017, 918a979394aSDag-Erling Smørgravand have not yet upgraded to POSIX.1-2024. 919a979394aSDag-Erling SmørgravCode intended to be portable to these platforms must deal 920a979394aSDag-Erling Smørgravwith problems that were fixed in later POSIX editions. 921a979394aSDag-Erling Smørgrav</p> 922a979394aSDag-Erling Smørgrav 923a979394aSDag-Erling Smørgrav<ul> 924a979394aSDag-Erling Smørgrav <li> 925a979394aSDag-Erling Smørgrav POSIX.1-2017 does not require support for geographical <code>TZ</code>, 926a979394aSDag-Erling Smørgrav and there is no convenient and efficient way to determine 927a979394aSDag-Erling Smørgrav the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary 928a979394aSDag-Erling Smørgrav timestamps, particularly for timezones 929a979394aSDag-Erling Smørgrav that do not fit into the POSIX model. 930a979394aSDag-Erling Smørgrav </li> 931a979394aSDag-Erling Smørgrav <li> 932a979394aSDag-Erling Smørgrav <p> 933a979394aSDag-Erling Smørgrav The proleptic <code>TZ</code> string, 934a979394aSDag-Erling Smørgrav which is all that POSIX.1-2017 requires, 935a979394aSDag-Erling Smørgrav has a format that is hard to describe and is error-prone in practice. 936a979394aSDag-Erling Smørgrav Also, proleptic <code>TZ</code> strings cannot deal with daylight 937bc421551SDag-Erling Smørgrav saving time rules not based on the Gregorian calendar (as in 93875411d15SDag-Erling Smørgrav Morocco), or with situations where more than two time zone 939bc421551SDag-Erling Smørgrav abbreviations or <abbr>UT</abbr> offsets are used in an area. 940bc421551SDag-Erling Smørgrav </p> 941bc421551SDag-Erling Smørgrav 942bc421551SDag-Erling Smørgrav <p> 943a979394aSDag-Erling Smørgrav A proleptic <code>TZ</code> string has the following format: 944bc421551SDag-Erling Smørgrav </p> 945bc421551SDag-Erling Smørgrav 946bc421551SDag-Erling Smørgrav <p> 947bc421551SDag-Erling Smørgrav <var>stdoffset</var>[<var>dst</var>[<var>offset</var>][<code>,</code><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]]] 948bc421551SDag-Erling Smørgrav </p> 949bc421551SDag-Erling Smørgrav 950bc421551SDag-Erling Smørgrav <p> 951bc421551SDag-Erling Smørgrav where: 952bc421551SDag-Erling Smørgrav </p> 953bc421551SDag-Erling Smørgrav 954bc421551SDag-Erling Smørgrav <dl> 955bc421551SDag-Erling Smørgrav <dt><var>std</var> and <var>dst</var></dt><dd> 956bc421551SDag-Erling Smørgrav are 3 or more characters specifying the standard 957bc421551SDag-Erling Smørgrav and daylight saving time (<abbr>DST</abbr>) zone abbreviations. 958bc421551SDag-Erling Smørgrav Starting with POSIX.1-2001, <var>std</var> and <var>dst</var> 959bc421551SDag-Erling Smørgrav may also be in a quoted form like '<code><+09></code>'; 960bc421551SDag-Erling Smørgrav this allows "<code>+</code>" and "<code>-</code>" in the names. 961bc421551SDag-Erling Smørgrav </dd> 962bc421551SDag-Erling Smørgrav <dt><var>offset</var></dt><dd> 963bc421551SDag-Erling Smørgrav is of the form 964bc421551SDag-Erling Smørgrav '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>' 965bc421551SDag-Erling Smørgrav and specifies the offset west of <abbr>UT</abbr>. 966bc421551SDag-Erling Smørgrav '<var>hh</var>' may be a single digit; 967bc421551SDag-Erling Smørgrav 0≤<var>hh</var>≤24. 968bc421551SDag-Erling Smørgrav The default <abbr>DST</abbr> offset is one hour ahead of 969bc421551SDag-Erling Smørgrav standard time. 970bc421551SDag-Erling Smørgrav </dd> 971bc421551SDag-Erling Smørgrav <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd> 972bc421551SDag-Erling Smørgrav specifies the beginning and end of <abbr>DST</abbr>. 973bc421551SDag-Erling Smørgrav If this is absent, the system supplies its own ruleset 97475411d15SDag-Erling Smørgrav for <abbr>DST</abbr>, typically current <abbr>US</abbr> 97575411d15SDag-Erling Smørgrav <abbr>DST</abbr> rules. 976bc421551SDag-Erling Smørgrav </dd> 977bc421551SDag-Erling Smørgrav <dt><var>time</var></dt><dd> 978bc421551SDag-Erling Smørgrav takes the form 979bc421551SDag-Erling Smørgrav '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]' 980bc421551SDag-Erling Smørgrav and defaults to 02:00. 981bc421551SDag-Erling Smørgrav This is the same format as the offset, except that a 982bc421551SDag-Erling Smørgrav leading '<code>+</code>' or '<code>-</code>' is not allowed. 983bc421551SDag-Erling Smørgrav </dd> 984bc421551SDag-Erling Smørgrav <dt><var>date</var></dt><dd> 985bc421551SDag-Erling Smørgrav takes one of the following forms: 986bc421551SDag-Erling Smørgrav <dl> 987bc421551SDag-Erling Smørgrav <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd> 988bc421551SDag-Erling Smørgrav origin-1 day number not counting February 29 989bc421551SDag-Erling Smørgrav </dd> 990bc421551SDag-Erling Smørgrav <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd> 991bc421551SDag-Erling Smørgrav origin-0 day number counting February 29 if present 992bc421551SDag-Erling Smørgrav </dd> 993bc421551SDag-Erling Smørgrav <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var> 994bc421551SDag-Erling Smørgrav (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5, 995bc421551SDag-Erling Smørgrav 1≤<var>m</var>≤12)</dt><dd> 996bc421551SDag-Erling Smørgrav for the <var>d</var>th day of week <var>n</var> of 997bc421551SDag-Erling Smørgrav month <var>m</var> of the year, where week 1 is the first 998bc421551SDag-Erling Smørgrav week in which day <var>d</var> appears, and 999bc421551SDag-Erling Smørgrav '<code>5</code>' stands for the last week in which 1000bc421551SDag-Erling Smørgrav day <var>d</var> appears (which may be either the 4th or 1001bc421551SDag-Erling Smørgrav 5th week). 1002bc421551SDag-Erling Smørgrav Typically, this is the only useful form; the <var>n</var> 1003bc421551SDag-Erling Smørgrav and <code>J</code><var>n</var> forms are rarely used. 1004bc421551SDag-Erling Smørgrav </dd> 1005bc421551SDag-Erling Smørgrav </dl> 1006bc421551SDag-Erling Smørgrav </dd> 1007bc421551SDag-Erling Smørgrav </dl> 1008bc421551SDag-Erling Smørgrav 1009bc421551SDag-Erling Smørgrav <p> 1010a979394aSDag-Erling Smørgrav Here is an example proleptic <code>TZ</code> string for New 1011bc421551SDag-Erling Smørgrav Zealand after 2007. 1012bc421551SDag-Erling Smørgrav It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead 1013bc421551SDag-Erling Smørgrav of <abbr>UT</abbr>, and that daylight saving time 1014bc421551SDag-Erling Smørgrav (<abbr>NZDT</abbr>) is observed from September's last Sunday at 1015bc421551SDag-Erling Smørgrav 02:00 until April's first Sunday at 03:00: 1016bc421551SDag-Erling Smørgrav </p> 1017bc421551SDag-Erling Smørgrav 1018bc421551SDag-Erling Smørgrav <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> 1019bc421551SDag-Erling Smørgrav 1020bc421551SDag-Erling Smørgrav <p> 1021a979394aSDag-Erling Smørgrav This proleptic <code>TZ</code> string is hard to remember, and 1022bc421551SDag-Erling Smørgrav mishandles some timestamps before 2008. 1023a979394aSDag-Erling Smørgrav With this package you can use a geographical <code>TZ</code> instead: 1024bc421551SDag-Erling Smørgrav </p> 1025bc421551SDag-Erling Smørgrav 1026bc421551SDag-Erling Smørgrav <pre><code>TZ='Pacific/Auckland'</code></pre> 1027bc421551SDag-Erling Smørgrav </li> 1028a979394aSDag-Erling Smørgrav</ul> 1029a979394aSDag-Erling Smørgrav 1030a979394aSDag-Erling Smørgrav<p> 1031a979394aSDag-Erling SmørgravPOSIX.1-2017 also has the limitations of POSIX.1-2024, 1032a979394aSDag-Erling Smørgravdiscussed in the next section. 1033a979394aSDag-Erling Smørgrav</p> 1034a979394aSDag-Erling Smørgrav 1035a979394aSDag-Erling Smørgrav<h3 id="POSIX.1-2024">POSIX.1-2024 properties and limitations</h3> 1036a979394aSDag-Erling Smørgrav<p> 1037a979394aSDag-Erling SmørgravPOSIX.1-2024 extends POSIX.1-2017 in the following significant ways: 1038a979394aSDag-Erling Smørgrav</p> 1039a979394aSDag-Erling Smørgrav<ul> 1040bc421551SDag-Erling Smørgrav <li> 1041a979394aSDag-Erling Smørgrav POSIX.1-2024 requires support for geographical <code>TZ</code>. 1042a979394aSDag-Erling Smørgrav Earlier POSIX editions require support only for proleptic <code>TZ</code>. 1043bc421551SDag-Erling Smørgrav </li> 1044bc421551SDag-Erling Smørgrav <li> 1045a979394aSDag-Erling Smørgrav POSIX.1-2024 requires <code>struct tm</code> 1046a979394aSDag-Erling Smørgrav to have a <abbr>UT</abbr> offset member <code>tm_gmtoff</code> 1047a979394aSDag-Erling Smørgrav and a time zone abbreviation member <code>tm_zone</code>. 1048a979394aSDag-Erling Smørgrav Earlier POSIX editions lack this requirement. 1049a979394aSDag-Erling Smørgrav </li> 1050a979394aSDag-Erling Smørgrav <li> 1051a979394aSDag-Erling Smørgrav DST transition times can range from −167:59:59 1052a979394aSDag-Erling Smørgrav to 167:59:59 instead of merely from 00:00:00 to 24:59:59. 1053a979394aSDag-Erling Smørgrav This allows for proleptic TZ strings 1054a979394aSDag-Erling Smørgrav like <code>"<-02>2<-01>,M3.5.0/-1,M10.5.0/0"</code> 1055a979394aSDag-Erling Smørgrav where the transition time −1:00 means 23:00 the previous day. 1056a979394aSDag-Erling Smørgrav </li> 1057a979394aSDag-Erling Smørgrav</ul> 1058a979394aSDag-Erling Smørgrav<p> 1059a979394aSDag-Erling SmørgravHowever POSIX.1-2024, like earlier POSIX editions, has some limitations: 1060a979394aSDag-Erling Smørgrav<ul> 1061a979394aSDag-Erling Smørgrav <li> 1062bc421551SDag-Erling Smørgrav The <code>TZ</code> environment variable is process-global, which 1063bc421551SDag-Erling Smørgrav makes it hard to write efficient, thread-safe applications that 1064bc421551SDag-Erling Smørgrav need access to multiple timezones. 1065bc421551SDag-Erling Smørgrav </li> 1066bc421551SDag-Erling Smørgrav <li> 1067bc421551SDag-Erling Smørgrav In POSIX, there is no tamper-proof way for a process to learn the 1068bc421551SDag-Erling Smørgrav system's best idea of local (wall clock) time. 1069bc421551SDag-Erling Smørgrav This is important for applications that an administrator wants 1070bc421551SDag-Erling Smørgrav used only at certain times – without regard to whether the 1071bc421551SDag-Erling Smørgrav user has fiddled the 1072bc421551SDag-Erling Smørgrav <code>TZ</code> environment variable. 1073bc421551SDag-Erling Smørgrav While an administrator can "do everything in <abbr>UT</abbr>" to 1074bc421551SDag-Erling Smørgrav get around the problem, doing so is inconvenient and precludes 1075bc421551SDag-Erling Smørgrav handling daylight saving time shifts – as might be required to 1076bc421551SDag-Erling Smørgrav limit phone calls to off-peak hours. 1077bc421551SDag-Erling Smørgrav </li> 1078bc421551SDag-Erling Smørgrav <li> 1079bc421551SDag-Erling Smørgrav POSIX requires that <code>time_t</code> clock counts exclude leap 1080bc421551SDag-Erling Smørgrav seconds. 1081bc421551SDag-Erling Smørgrav </li> 1082bc421551SDag-Erling Smørgrav <li> 1083a979394aSDag-Erling Smørgrav POSIX does not define the <abbr>DST</abbr> transitions 1084a979394aSDag-Erling Smørgrav for <code>TZ</code> values like 1085a979394aSDag-Erling Smørgrav "<code>EST5EDT</code>". 1086a979394aSDag-Erling Smørgrav Traditionally the current <abbr>US</abbr> <abbr>DST</abbr> rules 1087a979394aSDag-Erling Smørgrav were used to interpret such values, but this meant that the 1088a979394aSDag-Erling Smørgrav <abbr>US</abbr> <abbr>DST</abbr> rules were compiled into each 1089a979394aSDag-Erling Smørgrav time conversion package, and when 1090a979394aSDag-Erling Smørgrav <abbr>US</abbr> time conversion rules changed (as in the United 1091a979394aSDag-Erling Smørgrav States in 1987 and again in 2007), all packages that 1092a979394aSDag-Erling Smørgrav interpreted <code>TZ</code> values had to be updated 1093a979394aSDag-Erling Smørgrav to ensure proper results. 1094a979394aSDag-Erling Smørgrav </li> 1095a979394aSDag-Erling Smørgrav</ul> 1096a979394aSDag-Erling Smørgrav 1097a979394aSDag-Erling Smørgrav<h3 id="POSIX-extensions">Extensions to POSIX in the 1098a979394aSDag-Erling Smørgrav<code><abbr>tz</abbr></code> code</h3> 1099a979394aSDag-Erling Smørgrav<p> 1100a979394aSDag-Erling Smørgrav The <code><abbr>tz</abbr></code> code defines some properties 1101a979394aSDag-Erling Smørgrav left unspecified by POSIX, and attempts to support some 1102a979394aSDag-Erling Smørgrav extensions to POSIX. 1103a979394aSDag-Erling Smørgrav</p> 1104a979394aSDag-Erling Smørgrav 1105a979394aSDag-Erling Smørgrav<ul> 1106a979394aSDag-Erling Smørgrav <li> 1107bc421551SDag-Erling Smørgrav The <code><abbr>tz</abbr></code> code attempts to support all the 1108bc421551SDag-Erling Smørgrav <code>time_t</code> implementations allowed by POSIX. 1109bc421551SDag-Erling Smørgrav The <code>time_t</code> type represents a nonnegative count of seconds 1110bc421551SDag-Erling Smørgrav since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds. 1111bc421551SDag-Erling Smørgrav In practice, <code>time_t</code> is usually a signed 64- or 32-bit 1112bc421551SDag-Erling Smørgrav integer; 32-bit signed <code>time_t</code> values stop working after 1113bc421551SDag-Erling Smørgrav 2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these 1114bc421551SDag-Erling Smørgrav days typically use a signed 64-bit integer. 1115bc421551SDag-Erling Smørgrav Unsigned 32-bit integers are used on one or two platforms, and 36-bit 1116bc421551SDag-Erling Smørgrav and 40-bit integers are also used occasionally. 1117bc421551SDag-Erling Smørgrav Although earlier POSIX versions allowed <code>time_t</code> to be a 1118bc421551SDag-Erling Smørgrav floating-point type, this was not supported by any practical system, 1119a979394aSDag-Erling Smørgrav and POSIX.1-2013+ and the <code><abbr>tz</abbr></code> code both 1120bc421551SDag-Erling Smørgrav require <code>time_t</code> to be an integer type. 1121bc421551SDag-Erling Smørgrav </li> 1122bc421551SDag-Erling Smørgrav <li> 1123bc421551SDag-Erling Smørgrav <p> 1124a979394aSDag-Erling Smørgrav If the <code>TZ</code> environment variable uses the geographical format, 1125a979394aSDag-Erling Smørgrav it is used in generating 1126a979394aSDag-Erling Smørgrav the name of a file from which time-related information is read. 1127bc421551SDag-Erling Smørgrav The file's format is <dfn><abbr>TZif</abbr></dfn>, 1128bc421551SDag-Erling Smørgrav a timezone information format that contains binary data; see 1129*967a49a2SDag-Erling Smørgrav <a href="https://www.rfc-editor.org/rfc/9636">Internet 1130*967a49a2SDag-Erling Smørgrav <abbr>RFC</abbr> 9636</a>. 1131bc421551SDag-Erling Smørgrav The daylight saving time rules to be used for a 1132bc421551SDag-Erling Smørgrav particular timezone are encoded in the 1133bc421551SDag-Erling Smørgrav <abbr>TZif</abbr> file; the format of the file allows <abbr>US</abbr>, 1134bc421551SDag-Erling Smørgrav Australian, and other rules to be encoded, and 1135bc421551SDag-Erling Smørgrav allows for situations where more than two time zone 1136bc421551SDag-Erling Smørgrav abbreviations are used. 1137bc421551SDag-Erling Smørgrav </p> 1138bc421551SDag-Erling Smørgrav <p> 1139a979394aSDag-Erling Smørgrav When the <code><abbr>tz</abbr></code> code was developed in the 1980s, 1140a979394aSDag-Erling Smørgrav it was recognized that allowing the <code>TZ</code> environment 1141bc421551SDag-Erling Smørgrav variable to take on values such as '<code>America/New_York</code>' 1142bc421551SDag-Erling Smørgrav might cause "old" programs (that expect <code>TZ</code> to have a 1143a979394aSDag-Erling Smørgrav certain format) to operate incorrectly; consideration was given to using 1144bc421551SDag-Erling Smørgrav some other environment variable (for example, <code>TIMEZONE</code>) 1145bc421551SDag-Erling Smørgrav to hold the string used to generate the <abbr>TZif</abbr> file's name. 1146bc421551SDag-Erling Smørgrav In the end, however, it was decided to continue using 1147bc421551SDag-Erling Smørgrav <code>TZ</code>: it is widely used for time zone purposes; 1148bc421551SDag-Erling Smørgrav separately maintaining both <code>TZ</code> 1149bc421551SDag-Erling Smørgrav and <code>TIMEZONE</code> seemed a nuisance; and systems where 1150bc421551SDag-Erling Smørgrav "new" forms of <code>TZ</code> might cause problems can simply 1151bc421551SDag-Erling Smørgrav use legacy <code>TZ</code> values such as "<code>EST5EDT</code>" which 1152bc421551SDag-Erling Smørgrav can be used by "new" programs as well as by "old" programs that 1153bc421551SDag-Erling Smørgrav assume pre-POSIX <code>TZ</code> values. 1154bc421551SDag-Erling Smørgrav </p> 1155bc421551SDag-Erling Smørgrav </li> 1156bc421551SDag-Erling Smørgrav <li> 1157bc421551SDag-Erling Smørgrav Functions <code>tzalloc</code>, <code>tzfree</code>, 1158bc421551SDag-Erling Smørgrav <code>localtime_rz</code>, and <code>mktime_z</code> for 1159bc421551SDag-Erling Smørgrav more-efficient thread-safe applications that need to use multiple 1160bc421551SDag-Erling Smørgrav timezones. 1161bc421551SDag-Erling Smørgrav The <code>tzalloc</code> and <code>tzfree</code> functions 1162bc421551SDag-Erling Smørgrav allocate and free objects of type <code>timezone_t</code>, 1163bc421551SDag-Erling Smørgrav and <code>localtime_rz</code> and <code>mktime_z</code> are 1164bc421551SDag-Erling Smørgrav like <code>localtime_r</code> and <code>mktime</code> with an 1165bc421551SDag-Erling Smørgrav extra <code>timezone_t</code> argument. 1166a979394aSDag-Erling Smørgrav The functions were inspired by <a href="https://netbsd.org">NetBSD</a>. 1167bc421551SDag-Erling Smørgrav </li> 1168bc421551SDag-Erling Smørgrav <li> 1169bc421551SDag-Erling Smørgrav Negative <code>time_t</code> values are supported, on systems 1170bc421551SDag-Erling Smørgrav where <code>time_t</code> is signed. 1171bc421551SDag-Erling Smørgrav </li> 1172bc421551SDag-Erling Smørgrav <li> 1173bc421551SDag-Erling Smørgrav These functions can account for leap seconds; 1174bc421551SDag-Erling Smørgrav see <a href="#leapsec">Leap seconds</a> below. 1175bc421551SDag-Erling Smørgrav </li> 1176bc421551SDag-Erling Smørgrav</ul> 1177bc421551SDag-Erling Smørgrav 1178bc421551SDag-Erling Smørgrav<h3 id="vestigial">POSIX features no longer needed</h3> 1179bc421551SDag-Erling Smørgrav<p> 1180bc421551SDag-Erling SmørgravPOSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a> 1181bc421551SDag-Erling Smørgravdefine some <a href="https://en.wikipedia.org/wiki/API"><abbr 1182bc421551SDag-Erling Smørgravtitle="application programming interface">API</abbr>s</a> that are vestigial: 1183bc421551SDag-Erling Smørgravthey are not needed, and are relics of a too-simple model that does 1184bc421551SDag-Erling Smørgravnot suffice to handle many real-world timestamps. 1185bc421551SDag-Erling SmørgravAlthough the <code><abbr>tz</abbr></code> code supports these 1186bc421551SDag-Erling Smørgravvestigial <abbr>API</abbr>s for backwards compatibility, they should 1187bc421551SDag-Erling Smørgravbe avoided in portable applications. 1188bc421551SDag-Erling SmørgravThe vestigial <abbr>API</abbr>s are: 1189bc421551SDag-Erling Smørgrav</p> 1190bc421551SDag-Erling Smørgrav<ul> 1191bc421551SDag-Erling Smørgrav <li> 1192bc421551SDag-Erling Smørgrav The POSIX <code>tzname</code> variable does not suffice and is no 1193bc421551SDag-Erling Smørgrav longer needed. 1194a979394aSDag-Erling Smørgrav It is planned to be removed in a future edition of POSIX. 1195bc421551SDag-Erling Smørgrav To get a timestamp's time zone abbreviation, consult 1196bc421551SDag-Erling Smørgrav the <code>tm_zone</code> member if available; otherwise, 1197bc421551SDag-Erling Smørgrav use <code>strftime</code>'s <code>"%Z"</code> conversion 1198bc421551SDag-Erling Smørgrav specification. 1199bc421551SDag-Erling Smørgrav </li> 1200bc421551SDag-Erling Smørgrav <li> 1201bc421551SDag-Erling Smørgrav The POSIX <code>daylight</code> and <code>timezone</code> 1202bc421551SDag-Erling Smørgrav variables do not suffice and are no longer needed. 1203a979394aSDag-Erling Smørgrav They are planned to be removed in a future edition of POSIX. 1204bc421551SDag-Erling Smørgrav To get a timestamp's <abbr>UT</abbr> offset, consult 1205bc421551SDag-Erling Smørgrav the <code>tm_gmtoff</code> member if available; otherwise, 1206bc421551SDag-Erling Smørgrav subtract values returned by <code>localtime</code> 1207bc421551SDag-Erling Smørgrav and <code>gmtime</code> using the rules of the Gregorian calendar, 1208bc421551SDag-Erling Smørgrav or use <code>strftime</code>'s <code>"%z"</code> conversion 1209bc421551SDag-Erling Smørgrav specification if a string like <code>"+0900"</code> suffices. 1210bc421551SDag-Erling Smørgrav </li> 1211bc421551SDag-Erling Smørgrav <li> 1212bc421551SDag-Erling Smørgrav The <code>tm_isdst</code> member is almost never needed and most of 1213bc421551SDag-Erling Smørgrav its uses should be discouraged in favor of the abovementioned 1214bc421551SDag-Erling Smørgrav <abbr>API</abbr>s. 1215*967a49a2SDag-Erling Smørgrav It was intended as an index into the <code>tzname</code> variable, 1216*967a49a2SDag-Erling Smørgrav but as mentioned previously that usage is obsolete. 1217bc421551SDag-Erling Smørgrav Although it can still be used in arguments to 1218bc421551SDag-Erling Smørgrav <code>mktime</code> to disambiguate timestamps near 1219bc421551SDag-Erling Smørgrav a <abbr>DST</abbr> transition when the clock jumps back on 1220bc421551SDag-Erling Smørgrav platforms lacking <code>tm_gmtoff</code>, this 1221*967a49a2SDag-Erling Smørgrav disambiguation works only for proleptic <code>TZ</code> strings; 1222*967a49a2SDag-Erling Smørgrav it does not work in general for geographical timezones, 1223*967a49a2SDag-Erling Smørgrav such as when a location changes to a time zone with a 1224bc421551SDag-Erling Smørgrav lesser <abbr>UT</abbr> offset. 1225bc421551SDag-Erling Smørgrav </li> 1226bc421551SDag-Erling Smørgrav</ul> 1227bc421551SDag-Erling Smørgrav 1228bc421551SDag-Erling Smørgrav<h3 id="other-portability">Other portability notes</h3> 1229bc421551SDag-Erling Smørgrav<ul> 1230bc421551SDag-Erling Smørgrav <li> 1231bc421551SDag-Erling Smørgrav The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition 1232bc421551SDag-Erling Smørgrav UNIX</a> <code>timezone</code> function is not present in this 1233bc421551SDag-Erling Smørgrav package; it is impossible to reliably map <code>timezone</code>'s 1234bc421551SDag-Erling Smørgrav arguments (a "minutes west of <abbr>GMT</abbr>" value and a 1235bc421551SDag-Erling Smørgrav "daylight saving time in effect" flag) to a time zone 1236bc421551SDag-Erling Smørgrav abbreviation, and we refuse to guess. 1237bc421551SDag-Erling Smørgrav Programs that in the past used the <code>timezone</code> function 1238bc421551SDag-Erling Smørgrav may now examine <code>localtime(&clock)->tm_zone</code> 1239bc421551SDag-Erling Smørgrav (if <code>TM_ZONE</code> is defined) or 1240*967a49a2SDag-Erling Smørgrav use <code>strftime</code> with a <code>%Z</code> conversion specification 1241*967a49a2SDag-Erling Smørgrav to learn the correct time 1242bc421551SDag-Erling Smørgrav zone abbreviation to use. 1243bc421551SDag-Erling Smørgrav </li> 1244bc421551SDag-Erling Smørgrav <li> 1245bc421551SDag-Erling Smørgrav The <a 1246bc421551SDag-Erling Smørgrav href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a> 1247bc421551SDag-Erling Smørgrav <code>gettimeofday</code> function is not 1248bc421551SDag-Erling Smørgrav used in this package. 1249bc421551SDag-Erling Smørgrav This formerly let users obtain the current <abbr>UTC</abbr> offset 1250bc421551SDag-Erling Smørgrav and <abbr>DST</abbr> flag, but this functionality was removed in 1251bc421551SDag-Erling Smørgrav later versions of <abbr>BSD</abbr>. 1252bc421551SDag-Erling Smørgrav </li> 1253bc421551SDag-Erling Smørgrav <li> 1254bc421551SDag-Erling Smørgrav In <abbr>SVR2</abbr>, time conversion fails for near-minimum or 1255bc421551SDag-Erling Smørgrav near-maximum <code>time_t</code> values when doing conversions 1256bc421551SDag-Erling Smørgrav for places that do not use <abbr>UT</abbr>. 1257bc421551SDag-Erling Smørgrav This package takes care to do these conversions correctly. 1258bc421551SDag-Erling Smørgrav A comment in the source code tells how to get compatibly wrong 1259bc421551SDag-Erling Smørgrav results. 1260bc421551SDag-Erling Smørgrav </li> 1261bc421551SDag-Erling Smørgrav <li> 1262bc421551SDag-Erling Smørgrav The functions that are conditionally compiled 126375411d15SDag-Erling Smørgrav if <code>STD_INSPIRED</code> is nonzero should, at this point, be 1264bc421551SDag-Erling Smørgrav looked on primarily as food for thought. 1265bc421551SDag-Erling Smørgrav They are not in any sense "standard compatible" – some are 1266bc421551SDag-Erling Smørgrav not, in fact, specified in <em>any</em> standard. 1267bc421551SDag-Erling Smørgrav They do, however, represent responses of various authors to 1268bc421551SDag-Erling Smørgrav standardization proposals. 1269bc421551SDag-Erling Smørgrav </li> 1270bc421551SDag-Erling Smørgrav <li> 1271bc421551SDag-Erling Smørgrav Other time conversion proposals, in particular those supported by the 1272bc421551SDag-Erling Smørgrav <a href="https://howardhinnant.github.io/date/tz.html">Time Zone 1273bc421551SDag-Erling Smørgrav Database Parser</a>, offer a wider selection of functions 1274bc421551SDag-Erling Smørgrav that provide capabilities beyond those provided here. 1275bc421551SDag-Erling Smørgrav The absence of such functions from this package is not meant to 1276bc421551SDag-Erling Smørgrav discourage the development, standardization, or use of such 1277bc421551SDag-Erling Smørgrav functions. 1278bc421551SDag-Erling Smørgrav Rather, their absence reflects the decision to make this package 1279bc421551SDag-Erling Smørgrav contain valid extensions to POSIX, to ensure its broad 1280bc421551SDag-Erling Smørgrav acceptability. 1281bc421551SDag-Erling Smørgrav If more powerful time conversion functions can be standardized, so 1282bc421551SDag-Erling Smørgrav much the better. 1283bc421551SDag-Erling Smørgrav </li> 1284bc421551SDag-Erling Smørgrav</ul> 1285bc421551SDag-Erling Smørgrav</section> 1286bc421551SDag-Erling Smørgrav 1287bc421551SDag-Erling Smørgrav<section> 1288bc421551SDag-Erling Smørgrav <h2 id="stability">Interface stability</h2> 1289bc421551SDag-Erling Smørgrav<p> 1290bc421551SDag-Erling SmørgravThe <code><abbr>tz</abbr></code> code and data supply the following interfaces: 1291bc421551SDag-Erling Smørgrav</p> 1292bc421551SDag-Erling Smørgrav 1293bc421551SDag-Erling Smørgrav<ul> 1294bc421551SDag-Erling Smørgrav <li> 1295bc421551SDag-Erling Smørgrav A set of timezone names as per 1296bc421551SDag-Erling Smørgrav "<a href="#naming">Timezone identifiers</a>" above. 1297bc421551SDag-Erling Smørgrav </li> 1298bc421551SDag-Erling Smørgrav <li> 1299bc421551SDag-Erling Smørgrav Library functions described in "<a href="#functions">Time and date 1300bc421551SDag-Erling Smørgrav functions</a>" above. 1301bc421551SDag-Erling Smørgrav </li> 1302bc421551SDag-Erling Smørgrav <li> 1303bc421551SDag-Erling Smørgrav The programs <code>tzselect</code>, <code>zdump</code>, 1304bc421551SDag-Erling Smørgrav and <code>zic</code>, documented in their man pages. 1305bc421551SDag-Erling Smørgrav </li> 1306bc421551SDag-Erling Smørgrav <li> 1307bc421551SDag-Erling Smørgrav The format of <code>zic</code> input files, documented in 1308bc421551SDag-Erling Smørgrav the <code>zic</code> man page. 1309bc421551SDag-Erling Smørgrav </li> 1310bc421551SDag-Erling Smørgrav <li> 1311bc421551SDag-Erling Smørgrav The format of <code>zic</code> output files, documented in 1312bc421551SDag-Erling Smørgrav the <code>tzfile</code> man page. 1313bc421551SDag-Erling Smørgrav </li> 1314bc421551SDag-Erling Smørgrav <li> 1315bc421551SDag-Erling Smørgrav The format of zone table files, documented in <code>zone1970.tab</code>. 1316bc421551SDag-Erling Smørgrav </li> 1317bc421551SDag-Erling Smørgrav <li> 1318bc421551SDag-Erling Smørgrav The format of the country code file, documented in <code>iso3166.tab</code>. 1319bc421551SDag-Erling Smørgrav </li> 1320bc421551SDag-Erling Smørgrav <li> 1321bc421551SDag-Erling Smørgrav The version number of the code and data, as the first line of 1322bc421551SDag-Erling Smørgrav the text file '<code>version</code>' in each release. 1323bc421551SDag-Erling Smørgrav </li> 1324bc421551SDag-Erling Smørgrav</ul> 1325bc421551SDag-Erling Smørgrav 1326bc421551SDag-Erling Smørgrav<p> 1327bc421551SDag-Erling SmørgravInterface changes in a release attempt to preserve compatibility with 1328bc421551SDag-Erling Smørgravrecent releases. 1329bc421551SDag-Erling SmørgravFor example, <code><abbr>tz</abbr></code> data files typically do not 133075411d15SDag-Erling Smørgravrely on recently added <code>zic</code> features, so that users can 1331bc421551SDag-Erling Smørgravrun older <code>zic</code> versions to process newer data files. 1332bc421551SDag-Erling Smørgrav<a href="tz-link.html#download">Downloading 1333bc421551SDag-Erling Smørgravthe <code><abbr>tz</abbr></code> database</a> describes how releases 1334bc421551SDag-Erling Smørgravare tagged and distributed. 1335bc421551SDag-Erling Smørgrav</p> 1336bc421551SDag-Erling Smørgrav 1337bc421551SDag-Erling Smørgrav<p> 1338bc421551SDag-Erling SmørgravInterfaces not listed above are less stable. 1339bc421551SDag-Erling SmørgravFor example, users should not rely on particular <abbr>UT</abbr> 1340bc421551SDag-Erling Smørgravoffsets or abbreviations for timestamps, as data entries are often 1341bc421551SDag-Erling Smørgravbased on guesswork and these guesses may be corrected or improved. 1342bc421551SDag-Erling Smørgrav</p> 1343bc421551SDag-Erling Smørgrav 1344bc421551SDag-Erling Smørgrav<p> 1345bc421551SDag-Erling SmørgravTimezone boundaries are not part of the stable interface. 1346bc421551SDag-Erling SmørgravFor example, even though the <samp>Asia/Bangkok</samp> timezone 1347bc421551SDag-Erling Smørgravcurrently includes Chang Mai, Hanoi, and Phnom Penh, this is not part 1348bc421551SDag-Erling Smørgravof the stable interface and the timezone can split at any time. 1349bc421551SDag-Erling SmørgravIf a calendar application records a future event in some location other 1350bc421551SDag-Erling Smørgravthan Bangkok by putting "<samp>Asia/Bangkok</samp>" in the event's record, 1351bc421551SDag-Erling Smørgravthe application should be robust in the presence of timezone splits 1352bc421551SDag-Erling Smørgravbetween now and the future time. 1353bc421551SDag-Erling Smørgrav</p> 1354bc421551SDag-Erling Smørgrav</section> 1355bc421551SDag-Erling Smørgrav 1356bc421551SDag-Erling Smørgrav<section> 1357bc421551SDag-Erling Smørgrav <h2 id="leapsec">Leap seconds</h2> 1358bc421551SDag-Erling Smørgrav<p> 135975411d15SDag-Erling SmørgravLeap seconds were introduced in 1972 to accommodate the 136075411d15SDag-Erling Smørgravdifference between atomic time and the less regular rotation of the earth. 1361a979394aSDag-Erling SmørgravUnfortunately they have caused so many problems with civil 1362a979394aSDag-Erling Smørgravtimekeeping that there are 1363a979394aSDag-Erling Smørgrav<a href="https://www.bipm.org/en/cgpm-2022/resolution-4">plans 1364a979394aSDag-Erling Smørgravto discontinue them by 2035</a>. 1365a979394aSDag-Erling SmørgravEven if these plans come to fruition, a record of leap seconds will still be 1366a979394aSDag-Erling Smørgravneeded to resolve timestamps from 1972 through 2035, 1367a979394aSDag-Erling Smørgravand there may also be a need to record whatever mechanism replaces them. 136875411d15SDag-Erling Smørgrav</p> 136975411d15SDag-Erling Smørgrav 137075411d15SDag-Erling Smørgrav<p> 1371bc421551SDag-Erling SmørgravThe <code><abbr>tz</abbr></code> code and data can account for leap seconds, 1372bc421551SDag-Erling Smørgravthanks to code contributed by Bradley White. 1373bc421551SDag-Erling SmørgravHowever, the leap second support of this package is rarely used directly 1374bc421551SDag-Erling Smørgravbecause POSIX requires leap seconds to be excluded and many 1375bc421551SDag-Erling Smørgravsoftware packages would mishandle leap seconds if they were present. 1376bc421551SDag-Erling SmørgravInstead, leap seconds are more commonly handled by occasionally adjusting 1377bc421551SDag-Erling Smørgravthe operating system kernel clock as described in 1378bc421551SDag-Erling Smørgrav<a href="tz-link.html#precision">Precision timekeeping</a>, 1379bc421551SDag-Erling Smørgravand this package by default installs a <samp>leapseconds</samp> file 1380bc421551SDag-Erling Smørgravcommonly used by 1381bc421551SDag-Erling Smørgrav<a href="https://www.ntp.org"><abbr title="Network Time Protocol">NTP</abbr></a> 1382bc421551SDag-Erling Smørgravsoftware that adjusts the kernel clock. 1383bc421551SDag-Erling SmørgravHowever, kernel-clock twiddling approximates UTC only roughly, 138475411d15SDag-Erling Smørgravand systems needing more precise UTC can use this package's leap 1385bc421551SDag-Erling Smørgravsecond support directly. 1386bc421551SDag-Erling Smørgrav</p> 1387bc421551SDag-Erling Smørgrav 1388bc421551SDag-Erling Smørgrav<p> 138975411d15SDag-Erling SmørgravThe directly supported mechanism assumes that <code>time_t</code> 1390bc421551SDag-Erling Smørgravcounts of seconds since the POSIX epoch normally include leap seconds, 1391bc421551SDag-Erling Smørgravas opposed to POSIX <code>time_t</code> counts which exclude leap seconds. 1392bc421551SDag-Erling SmørgravThis modified timescale is converted to <abbr>UTC</abbr> 1393bc421551SDag-Erling Smørgravat the same point that time zone and <abbr>DST</abbr> 1394bc421551SDag-Erling Smørgravadjustments are applied – 1395bc421551SDag-Erling Smørgravnamely, at calls to <code>localtime</code> and analogous functions – 1396bc421551SDag-Erling Smørgravand the process is driven by leap second information 1397bc421551SDag-Erling Smørgravstored in alternate versions of the <abbr>TZif</abbr> files. 1398bc421551SDag-Erling SmørgravBecause a leap second adjustment may be needed even 1399bc421551SDag-Erling Smørgravif no time zone correction is desired, 1400bc421551SDag-Erling Smørgravcalls to <code>gmtime</code>-like functions 1401bc421551SDag-Erling Smørgravalso need to consult a <abbr>TZif</abbr> file, 1402bc421551SDag-Erling Smørgravconventionally named <samp><abbr>Etc/UTC</abbr></samp> 1403bc421551SDag-Erling Smørgrav(<samp><abbr>GMT</abbr></samp> in previous versions), 1404bc421551SDag-Erling Smørgravto see whether leap second corrections are needed. 1405bc421551SDag-Erling SmørgravTo convert an application's <code>time_t</code> timestamps to or from 1406bc421551SDag-Erling SmørgravPOSIX <code>time_t</code> timestamps (for use when, say, 1407bc421551SDag-Erling Smørgravembedding or interpreting timestamps in portable 1408bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Tar_(computing)"><code>tar</code></a> 1409bc421551SDag-Erling Smørgravfiles), 1410bc421551SDag-Erling Smørgravthe application can call the utility functions 1411bc421551SDag-Erling Smørgrav<code>time2posix</code> and <code>posix2time</code> 1412bc421551SDag-Erling Smørgravincluded with this package. 1413bc421551SDag-Erling Smørgrav</p> 1414bc421551SDag-Erling Smørgrav 1415bc421551SDag-Erling Smørgrav<p> 1416bc421551SDag-Erling SmørgravIf the POSIX-compatible <abbr>TZif</abbr> file set is installed 1417bc421551SDag-Erling Smørgravin a directory whose basename is <samp>zoneinfo</samp>, the 1418bc421551SDag-Erling Smørgravleap-second-aware file set is by default installed in a separate 1419bc421551SDag-Erling Smørgravdirectory <samp>zoneinfo-leaps</samp>. 1420bc421551SDag-Erling SmørgravAlthough each process can have its own time zone by setting 1421bc421551SDag-Erling Smørgravits <code>TZ</code> environment variable, there is no support for some 1422bc421551SDag-Erling Smørgravprocesses being leap-second aware while other processes are 1423bc421551SDag-Erling SmørgravPOSIX-compatible; the leap-second choice is system-wide. 1424bc421551SDag-Erling SmørgravSo if you configure your kernel to count leap seconds, you should also 1425bc421551SDag-Erling Smørgravdiscard <samp>zoneinfo</samp> and rename <samp>zoneinfo-leaps</samp> 1426bc421551SDag-Erling Smørgravto <samp>zoneinfo</samp>. 1427bc421551SDag-Erling SmørgravAlternatively, you can install just one set of <abbr>TZif</abbr> files 1428bc421551SDag-Erling Smørgravin the first place; see the <code>REDO</code> variable in this package's 1429bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Makefile">makefile</a>. 1430bc421551SDag-Erling Smørgrav</p> 1431bc421551SDag-Erling Smørgrav</section> 1432bc421551SDag-Erling Smørgrav 1433bc421551SDag-Erling Smørgrav<section> 1434bc421551SDag-Erling Smørgrav <h2 id="calendar">Calendrical issues</h2> 1435bc421551SDag-Erling Smørgrav<p> 1436bc421551SDag-Erling SmørgravCalendrical issues are a bit out of scope for a time zone database, 1437bc421551SDag-Erling Smørgravbut they indicate the sort of problems that we would run into if we 1438bc421551SDag-Erling Smørgravextended the time zone database further into the past. 1439bc421551SDag-Erling SmørgravAn excellent resource in this area is Edward M. Reingold 1440bc421551SDag-Erling Smørgravand Nachum Dershowitz, <cite><a 1441bc421551SDag-Erling Smørgravhref="https://www.cambridge.org/fr/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical 1442bc421551SDag-Erling SmørgravCalculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018). 1443bc421551SDag-Erling SmørgravOther information and sources are given in the file '<code>calendars</code>' 1444bc421551SDag-Erling Smørgravin the <code><abbr>tz</abbr></code> distribution. 1445bc421551SDag-Erling SmørgravThey sometimes disagree. 1446bc421551SDag-Erling Smørgrav</p> 1447bc421551SDag-Erling Smørgrav</section> 1448bc421551SDag-Erling Smørgrav 1449bc421551SDag-Erling Smørgrav<section> 145075411d15SDag-Erling Smørgrav <h2 id="planets">Time and time zones off Earth</h2> 145175411d15SDag-Erling Smørgrav<p> 145275411d15SDag-Erling SmørgravThe European Space Agency is <a 145375411d15SDag-Erling Smørgravhref='https://www.esa.int/Applications/Navigation/Telling_time_on_the_Moon'>considering</a> 145475411d15SDag-Erling Smørgravthe establishment of a reference timescale for the Moon, which has 145575411d15SDag-Erling Smørgravdays roughly equivalent to 29.5 Earth days, and where relativistic 145675411d15SDag-Erling Smørgraveffects cause clocks to tick slightly faster than on Earth. 1457a979394aSDag-Erling SmørgravAlso, <abbr title="National Aeronautics and Space Administration">NASA</abbr> 1458a979394aSDag-Erling Smørgravhas been <a 1459a979394aSDag-Erling Smørgravhref='https://www.whitehouse.gov/wp-content/uploads/2024/04/Celestial-Time-Standardization-Policy.pdf'>ordered</a> 1460a979394aSDag-Erling Smørgravto consider the establishment of Coordinated Lunar Time (<abbr>LTC</abbr>). 1461a979394aSDag-Erling SmørgravIt is not yet known whether the US and European efforts will result in 1462a979394aSDag-Erling Smørgravmultiple timescales on the Moon. 146375411d15SDag-Erling Smørgrav</p> 146475411d15SDag-Erling Smørgrav 1465bc421551SDag-Erling Smørgrav<p> 1466bc421551SDag-Erling SmørgravSome people's work schedules have used 1467bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars">Mars time</a>. 1468bc421551SDag-Erling SmørgravJet Propulsion Laboratory (JPL) coordinators kept Mars time on 1469bc421551SDag-Erling Smørgravand off during the 1470bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Mars_Pathfinder">Mars 1471bc421551SDag-Erling SmørgravPathfinder</a> mission (1997). 1472bc421551SDag-Erling SmørgravSome of their family members also adapted to Mars time. 1473bc421551SDag-Erling SmørgravDozens of special Mars watches were built for JPL workers who kept 1474bc421551SDag-Erling SmørgravMars time during the 1475bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars 1476bc421551SDag-Erling SmørgravExploration Rovers (MER)</a> mission (2004–2018). 1477bc421551SDag-Erling SmørgravThese timepieces looked like normal Seikos and Citizens but were adjusted 1478bc421551SDag-Erling Smørgravto use Mars seconds rather than terrestrial seconds, although 1479bc421551SDag-Erling Smørgravunfortunately the adjusted watches were unreliable and appear to have 1480bc421551SDag-Erling Smørgravhad only limited use. 1481bc421551SDag-Erling Smørgrav</p> 1482bc421551SDag-Erling Smørgrav 1483bc421551SDag-Erling Smørgrav<p> 1484bc421551SDag-Erling SmørgravA Mars solar day is called a "sol" and has a mean period equal to 1485bc421551SDag-Erling Smørgravabout 24 hours 39 minutes 35.244 seconds in terrestrial time. 1486bc421551SDag-Erling SmørgravIt is divided into a conventional 24-hour clock, so each Mars second 1487bc421551SDag-Erling Smørgravequals about 1.02749125 terrestrial seconds. 1488bc421551SDag-Erling Smørgrav(One MER worker noted, "If I am working Mars hours, and Mars hours are 1489bc421551SDag-Erling Smørgrav2.5% more than Earth hours, shouldn't I get an extra 2.5% pay raise?") 1490bc421551SDag-Erling Smørgrav</p> 1491bc421551SDag-Erling Smørgrav 1492bc421551SDag-Erling Smørgrav<p> 1493bc421551SDag-Erling SmørgravThe <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime 1494bc421551SDag-Erling Smørgravmeridian</a> of Mars goes through the center of the crater 1495bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in 1496bc421551SDag-Erling Smørgravhonor of the British astronomer who built the Greenwich telescope that 1497bc421551SDag-Erling Smørgravdefines Earth's prime meridian. 1498bc421551SDag-Erling SmørgravMean solar time on the Mars prime meridian is 1499bc421551SDag-Erling Smørgravcalled Mars Coordinated Time (<abbr>MTC</abbr>). 1500bc421551SDag-Erling Smørgrav</p> 1501bc421551SDag-Erling Smørgrav 1502bc421551SDag-Erling Smørgrav<p> 1503bc421551SDag-Erling SmørgravEach landed mission on Mars has adopted a different reference for 1504bc421551SDag-Erling Smørgravsolar timekeeping, so there is no real standard for Mars time zones. 1505bc421551SDag-Erling SmørgravFor example, the MER mission defined two time zones "Local 1506bc421551SDag-Erling SmørgravSolar Time A" and "Local Solar Time B" for its two missions, each zone 1507bc421551SDag-Erling Smørgravdesigned so that its time equals local true solar time at 1508bc421551SDag-Erling Smørgravapproximately the middle of the nominal mission. 1509bc421551SDag-Erling SmørgravThe A and B zones differ enough so that an MER worker assigned to 1510bc421551SDag-Erling Smørgravthe A zone might suffer "Mars lag" when switching to work in the B zone. 1511bc421551SDag-Erling SmørgravSuch a "time zone" is not particularly suited for any application 1512bc421551SDag-Erling Smørgravother than the mission itself. 1513bc421551SDag-Erling Smørgrav</p> 1514bc421551SDag-Erling Smørgrav 1515bc421551SDag-Erling Smørgrav<p> 1516bc421551SDag-Erling SmørgravMany calendars have been proposed for Mars, but none have achieved 1517bc421551SDag-Erling Smørgravwide acceptance. 1518bc421551SDag-Erling SmørgravAstronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a 1519bc421551SDag-Erling Smørgravsequential count of Mars solar days elapsed since about 1873-12-29 1520bc421551SDag-Erling Smørgrav12:00 <abbr>GMT</abbr>. 1521bc421551SDag-Erling Smørgrav</p> 1522bc421551SDag-Erling Smørgrav 1523bc421551SDag-Erling Smørgrav<p> 1524bc421551SDag-Erling SmørgravIn our solar system, Mars is the planet with time and calendar most 1525bc421551SDag-Erling Smørgravlike Earth's. 1526bc421551SDag-Erling SmørgravOn other planets, Sun-based time and calendars would work quite 1527bc421551SDag-Erling Smørgravdifferently. 1528bc421551SDag-Erling SmørgravFor example, although Mercury's 1529bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal 1530bc421551SDag-Erling Smørgravrotation period</a> is 58.646 Earth days, Mercury revolves around the 1531bc421551SDag-Erling SmørgravSun so rapidly that an observer on Mercury's equator would see a 1532bc421551SDag-Erling Smørgravsunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a 1533bc421551SDag-Erling SmørgravMercury day. 1534bc421551SDag-Erling SmørgravVenus is more complicated, partly because its rotation is slightly 1535bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>: 1536bc421551SDag-Erling Smørgravits year is 1.92 of its days. 1537bc421551SDag-Erling SmørgravGas giants like Jupiter are trickier still, as their polar and 1538bc421551SDag-Erling Smørgravequatorial regions rotate at different rates, so that the length of a 1539bc421551SDag-Erling Smørgravday depends on latitude. 1540bc421551SDag-Erling SmørgravThis effect is most pronounced on Neptune, where the day is about 12 1541bc421551SDag-Erling Smørgravhours at the poles and 18 hours at the equator. 1542bc421551SDag-Erling Smørgrav</p> 1543bc421551SDag-Erling Smørgrav 1544bc421551SDag-Erling Smørgrav<p> 1545bc421551SDag-Erling SmørgravAlthough the <code><abbr>tz</abbr></code> database does not support 1546bc421551SDag-Erling Smørgravtime on other planets, it is documented here in the hopes that support 1547bc421551SDag-Erling Smørgravwill be added eventually. 1548bc421551SDag-Erling Smørgrav</p> 1549bc421551SDag-Erling Smørgrav 1550bc421551SDag-Erling Smørgrav<p> 1551bc421551SDag-Erling SmørgravSources for time on other planets: 1552bc421551SDag-Erling Smørgrav</p> 1553bc421551SDag-Erling Smørgrav 1554bc421551SDag-Erling Smørgrav<ul> 1555bc421551SDag-Erling Smørgrav <li> 1556bc421551SDag-Erling Smørgrav Michael Allison and Robert Schmunk, 1557bc421551SDag-Erling Smørgrav "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical 1558bc421551SDag-Erling Smørgrav Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>" 1559bc421551SDag-Erling Smørgrav (2020-03-08). 1560bc421551SDag-Erling Smørgrav </li> 1561bc421551SDag-Erling Smørgrav <li> 1562bc421551SDag-Erling Smørgrav Zara Mirmalek, 1563bc421551SDag-Erling Smørgrav <em><a href="https://mitpress.mit.edu/books/making-time-mars">Making 1564bc421551SDag-Erling Smørgrav Time on Mars</a></em>, MIT Press (March 2020), ISBN 978-0262043854. 1565bc421551SDag-Erling Smørgrav </li> 1566bc421551SDag-Erling Smørgrav <li> 1567bc421551SDag-Erling Smørgrav Jia-Rui Chong, 1568bc421551SDag-Erling Smørgrav "<a href="https://www.latimes.com/archives/la-xpm-2004-jan-14-sci-marstime14-story.html">Workdays 1569bc421551SDag-Erling Smørgrav Fit for a Martian</a>", <cite>Los Angeles Times</cite> 1570bc421551SDag-Erling Smørgrav (2004-01-14), pp A1, A20–A21. 1571bc421551SDag-Erling Smørgrav </li> 1572bc421551SDag-Erling Smørgrav <li> 1573bc421551SDag-Erling Smørgrav Tom Chmielewski, 1574bc421551SDag-Erling Smørgrav "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet 1575bc421551SDag-Erling Smørgrav Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26) 1576bc421551SDag-Erling Smørgrav </li> 1577bc421551SDag-Erling Smørgrav <li> 1578bc421551SDag-Erling Smørgrav Matt Williams, 1579bc421551SDag-Erling Smørgrav "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How 1580bc421551SDag-Erling Smørgrav long is a day on the other planets of the solar system?</a>" 1581bc421551SDag-Erling Smørgrav (2016-01-20). 1582bc421551SDag-Erling Smørgrav </li> 1583bc421551SDag-Erling Smørgrav</ul> 1584bc421551SDag-Erling Smørgrav</section> 1585bc421551SDag-Erling Smørgrav 1586bc421551SDag-Erling Smørgrav<footer> 1587bc421551SDag-Erling Smørgrav <hr> 1588bc421551SDag-Erling Smørgrav This file is in the public domain, so clarified as of 2009-05-17 by 1589bc421551SDag-Erling Smørgrav Arthur David Olson. 1590bc421551SDag-Erling Smørgrav</footer> 1591bc421551SDag-Erling Smørgrav</body> 1592bc421551SDag-Erling Smørgrav</html> 1593