1bc421551SDag-Erling Smørgrav<!DOCTYPE html> 2bc421551SDag-Erling Smørgrav<html lang="en"> 3bc421551SDag-Erling Smørgrav<head> 4bc421551SDag-Erling Smørgrav<title>Time zone and daylight saving time data</title> 5bc421551SDag-Erling Smørgrav<meta charset="UTF-8"> 6bc421551SDag-Erling Smørgrav<style> 7bc421551SDag-Erling Smørgravpre {margin-left: 2em; white-space: pre-wrap;} 8bc421551SDag-Erling Smørgrav</style> 9bc421551SDag-Erling Smørgrav</head> 10bc421551SDag-Erling Smørgrav<body> 11bc421551SDag-Erling Smørgrav<h1>Time zone and daylight saving time data</h1> 12bc421551SDag-Erling Smørgrav<p> 13bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and 14bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a> 15bc421551SDag-Erling Smørgravrules are controlled by individual 16bc421551SDag-Erling Smørgravgovernments. They are sometimes changed with little notice, and their 17bc421551SDag-Erling Smørgravhistories and planned futures are often recorded only fitfully. Here 18bc421551SDag-Erling Smørgravis a summary of attempts to organize and record relevant data in this 19bc421551SDag-Erling Smørgravarea. 20bc421551SDag-Erling Smørgrav</p> 21bc421551SDag-Erling Smørgrav <h3>Outline</h3> 22bc421551SDag-Erling Smørgrav <nav> 23bc421551SDag-Erling Smørgrav <ul> 24bc421551SDag-Erling Smørgrav <li>The <code><abbr>tz</abbr></code> database product and process 25bc421551SDag-Erling Smørgrav <ul> 26bc421551SDag-Erling Smørgrav <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li> 27bc421551SDag-Erling Smørgrav <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li> 28bc421551SDag-Erling Smørgrav <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li> 2975411d15SDag-Erling Smørgrav <li><a href="#coordinating">Coordinating with governments and distributors</a></li> 30bc421551SDag-Erling Smørgrav <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li> 31bc421551SDag-Erling Smørgrav </ul> 32bc421551SDag-Erling Smørgrav </li> 33bc421551SDag-Erling Smørgrav <li>Uses of the <code><abbr>tz</abbr></code> database 34bc421551SDag-Erling Smørgrav <ul> 35bc421551SDag-Erling Smørgrav <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li> 36bc421551SDag-Erling Smørgrav <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li> 37bc421551SDag-Erling Smørgrav <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li> 38bc421551SDag-Erling Smørgrav <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li> 39bc421551SDag-Erling Smørgrav <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li> 40bc421551SDag-Erling Smørgrav </ul> 41bc421551SDag-Erling Smørgrav </li> 42bc421551SDag-Erling Smørgrav <li>Related data 43bc421551SDag-Erling Smørgrav <ul> 44bc421551SDag-Erling Smørgrav <li><a href="#other-dbs">Other time zone databases</a></li> 45bc421551SDag-Erling Smørgrav <li><a href="#maps">Maps</a></li> 46bc421551SDag-Erling Smørgrav <li><a href="#boundaries">Time zone boundaries</a></li> 47bc421551SDag-Erling Smørgrav </ul> 48bc421551SDag-Erling Smørgrav </li> 49bc421551SDag-Erling Smørgrav <li>Timekeeping concepts 50bc421551SDag-Erling Smørgrav <ul> 51bc421551SDag-Erling Smørgrav <li><a href="#civil">Civil time concepts and history</a></li> 52bc421551SDag-Erling Smørgrav <li><a href="#national">National histories of legal time</a></li> 53bc421551SDag-Erling Smørgrav <li><a href="#costs">Costs and benefits of time shifts</a></li> 54bc421551SDag-Erling Smørgrav <li><a href="#precision">Precision timekeeping</a></li> 55bc421551SDag-Erling Smørgrav <li><a href="#notation">Time notation</a></li> 56bc421551SDag-Erling Smørgrav </ul> 57bc421551SDag-Erling Smørgrav </li> 58bc421551SDag-Erling Smørgrav <li><a href="#see-also">See also</a></li> 59bc421551SDag-Erling Smørgrav </ul> 60bc421551SDag-Erling Smørgrav </nav> 61bc421551SDag-Erling Smørgrav 62bc421551SDag-Erling Smørgrav<section> 63bc421551SDag-Erling Smørgrav<h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2> 64bc421551SDag-Erling Smørgrav<p> 65bc421551SDag-Erling SmørgravThe <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a> 66bc421551SDag-Erling Smørgravtime zone database contains code and data 67bc421551SDag-Erling Smørgravthat represent the history of local time 68bc421551SDag-Erling Smørgravfor many representative locations around the globe. 69bc421551SDag-Erling SmørgravIt is updated periodically to reflect changes made by political bodies 70bc421551SDag-Erling Smørgravto time zone boundaries and daylight saving rules. 71bc421551SDag-Erling SmørgravThis database (known as <code><abbr>tz</abbr></code>, 72bc421551SDag-Erling Smørgrav<code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>) 73bc421551SDag-Erling Smørgravis used by several implementations, 74bc421551SDag-Erling Smørgravincluding 75bc421551SDag-Erling Smørgrav<a href="https://www.gnu.org/software/libc/">the 76bc421551SDag-Erling Smørgrav<abbr title="GNU's Not Unix">GNU</abbr> 77bc421551SDag-Erling SmørgravC Library</a> (used in 78bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>), 79bc421551SDag-Erling Smørgrav<a href="https://www.android.com">Android</a>, 80bc421551SDag-Erling Smørgrav<a href="https://www.freebsd.org">Free<abbr 81bc421551SDag-Erling Smørgravtitle="Berkeley Software Distribution">BSD</abbr></a>, 82bc421551SDag-Erling Smørgrav<a href="https://netbsd.org">Net<abbr>BSD</abbr></a>, 83bc421551SDag-Erling Smørgrav<a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>, 84bc421551SDag-Erling Smørgrav<a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>, 85bc421551SDag-Erling Smørgrav<a href="https://cygwin.com">Cygwin</a>, 86bc421551SDag-Erling Smørgrav<a href="https://mariadb.org">MariaDB</a>, 87bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>, 88*a979394aSDag-Erling Smørgrav<a href="https://musl.libc.org">musl libc</a>, 89bc421551SDag-Erling Smørgrav<a href="https://www.mysql.com">MySQL</a>, 90bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/WebOS"><abbr 91bc421551SDag-Erling Smørgravtitle="Web Operating System">webOS</abbr></a>, 92bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr 93bc421551SDag-Erling Smørgravtitle="Advanced Interactive eXecutive">AIX</abbr></a>, 94bc421551SDag-Erling Smørgrav<a href="https://www.apple.com/ios"><abbr 95bc421551SDag-Erling Smørgravtitle="iPhone OS">iOS</abbr></a>, 96bc421551SDag-Erling Smørgrav<a href="https://www.apple.com/macos">macOS</a>, 97bc421551SDag-Erling Smørgrav<a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>, 98bc421551SDag-Erling Smørgrav<a href="https://www.vmssoftware.com">Open<abbr 99bc421551SDag-Erling Smørgravtitle="Virtual Memory System">VMS</abbr></a>, 100bc421551SDag-Erling Smørgrav<a href="https://www.oracle.com/database/">Oracle Database</a>, and 101bc421551SDag-Erling Smørgrav<a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p> 102bc421551SDag-Erling Smørgrav<p> 103bc421551SDag-Erling SmørgravEach main entry in the database represents a <dfn>timezone</dfn> 104bc421551SDag-Erling Smørgravfor a set of civil-time clocks that have all agreed since 1970. 105bc421551SDag-Erling SmørgravTimezones are typically identified by continent or ocean and then by the 106bc421551SDag-Erling Smørgravname of the largest city within the region containing the clocks. 107bc421551SDag-Erling SmørgravFor example, <code>America/New_York</code> 108bc421551SDag-Erling Smørgravrepresents most of the <abbr title="United States">US</abbr> eastern time zone; 109bc421551SDag-Erling Smørgrav<code>America/Phoenix</code> represents most of Arizona, which 110bc421551SDag-Erling Smørgravuses mountain time without daylight saving time (<abbr>DST</abbr>); 111bc421551SDag-Erling Smørgrav<code>America/Detroit</code> represents most of Michigan, which uses 112bc421551SDag-Erling Smørgraveastern time but with different <abbr>DST</abbr> rules in 1975; 113bc421551SDag-Erling Smørgravand other entries represent smaller regions like Starke County, 114bc421551SDag-Erling SmørgravIndiana, which switched from central to eastern time in 1991 115bc421551SDag-Erling Smørgravand switched back in 2006. 116*a979394aSDag-Erling SmørgravTo use the database on a <a 117bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/POSIX"><abbr 118*a979394aSDag-Erling Smørgravtitle="Portable Operating System Interface">POSIX</abbr>.1-2024</a> 119bc421551SDag-Erling Smørgravimplementation set the <code><abbr>TZ</abbr></code> 120bc421551SDag-Erling Smørgravenvironment variable to the location's full name, 121bc421551SDag-Erling Smørgrave.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p> 122bc421551SDag-Erling Smørgrav<p> 123bc421551SDag-Erling SmørgravAssociated with each timezone is a history of offsets from 124bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Universal_Time">Universal 125bc421551SDag-Erling SmørgravTime</a> (<abbr>UT</abbr>), which is <a 126bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean 127bc421551SDag-Erling SmørgravTime</a> (<abbr>GMT</abbr>) with days beginning at midnight; 128bc421551SDag-Erling Smørgravfor timestamps after 1960 this is more precisely <a 129bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated 130bc421551SDag-Erling SmørgravUniversal Time</a> (<abbr>UTC</abbr>). 131bc421551SDag-Erling SmørgravThe database also records when daylight saving time was in use, 132bc421551SDag-Erling Smørgravalong with some time zone abbreviations such as <abbr>EST</abbr> 133bc421551SDag-Erling Smørgravfor Eastern Standard Time in the <abbr>US</abbr>.</p> 134bc421551SDag-Erling Smørgrav</section> 135bc421551SDag-Erling Smørgrav 136bc421551SDag-Erling Smørgrav<section> 137bc421551SDag-Erling Smørgrav<h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2> 138bc421551SDag-Erling Smørgrav<p> 139bc421551SDag-Erling SmørgravThe following <a 140bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download 141bc421551SDag-Erling Smørgravthe latest release's two 142bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a> 143bc421551SDag-Erling Smørgravto a <abbr>GNU</abbr>/Linux or similar host.</p> 144bc421551SDag-Erling Smørgrav<pre><code>mkdir tzdb 145bc421551SDag-Erling Smørgravcd tzdb 146bc421551SDag-Erling Smørgrav<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz 147bc421551SDag-Erling Smørgravwget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz 148bc421551SDag-Erling Smørgrav<a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf - 149bc421551SDag-Erling Smørgravgzip -dc tzdata-latest.tar.gz | tar -xf - 150bc421551SDag-Erling Smørgrav</code></pre> 151bc421551SDag-Erling Smørgrav<p>Alternatively, the following shell commands download the same 152bc421551SDag-Erling Smørgravrelease in a single-tarball format containing extra data 153bc421551SDag-Erling Smørgravuseful for regression testing:</p> 154bc421551SDag-Erling Smørgrav<pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a> 155bc421551SDag-Erling Smørgrav<a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf - 156bc421551SDag-Erling Smørgrav</code></pre> 157bc421551SDag-Erling Smørgrav<p>These commands use convenience links to the latest release 158bc421551SDag-Erling Smørgravof the <code><abbr>tz</abbr></code> database hosted by the 159bc421551SDag-Erling Smørgrav<a href="https://www.iana.org/time-zones">Time Zone Database website</a> 160bc421551SDag-Erling Smørgravof the <a href="https://www.iana.org">Internet Assigned Numbers 161bc421551SDag-Erling SmørgravAuthority (IANA)</a>. 162bc421551SDag-Erling SmørgravOlder releases are in files named 163bc421551SDag-Erling Smørgrav<code>tzcode<var>V</var>.tar.gz</code>, 164bc421551SDag-Erling Smørgrav<code>tzdata<var>V</var>.tar.gz</code>, and 165bc421551SDag-Erling Smørgrav<code>tzdb-<var>V</var>.tar.lz</code>, 166bc421551SDag-Erling Smørgravwhere <code><var>V</var></code> is the version. 167bc421551SDag-Erling SmørgravSince 1996, each version has been a four-digit year followed by 168bc421551SDag-Erling Smørgravlower-case letter (<samp>a</samp> through <samp>z</samp>, 169bc421551SDag-Erling Smørgravthen <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp> 170bc421551SDag-Erling Smørgravthrough <samp>zzz</samp>, and so on). 171bc421551SDag-Erling SmørgravSince version 2022a, each release has been distributed in 172bc421551SDag-Erling Smørgrav<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX 173bc421551SDag-Erling Smørgravustar interchange format</a>, compressed as described above; 17475411d15SDag-Erling Smørgravolder releases use a nearly compatible format. 175bc421551SDag-Erling SmørgravSince version 2016h, each release has contained a text file named 17646c59934SDag-Erling Smørgrav"<code>version</code>" whose first (and currently only) line is the version. 177bc421551SDag-Erling SmørgravOlder releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>, 178bc421551SDag-Erling Smørgravand are also available in an 179bc421551SDag-Erling Smørgrav<a href="ftp://ftp.iana.org/tz/releases/"><abbr 180bc421551SDag-Erling Smørgravtitle="File Transfer Protocol">FTP</abbr> directory</a> via a 18175411d15SDag-Erling Smørgravless secure protocol.</p> 182bc421551SDag-Erling Smørgrav<p>Alternatively, a development repository of code and data can be 183bc421551SDag-Erling Smørgravretrieved from <a href="https://github.com">GitHub</a> via the shell 184bc421551SDag-Erling Smørgravcommand:</p> 185bc421551SDag-Erling Smørgrav<pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a> 186bc421551SDag-Erling Smørgrav</code></pre> 187bc421551SDag-Erling Smørgrav<p> 188bc421551SDag-Erling SmørgravSince version 2012e, each release has been tagged in development repositories. 189bc421551SDag-Erling SmørgravUntagged commits are less well tested and probably contain 190bc421551SDag-Erling Smørgravmore errors.</p> 191bc421551SDag-Erling Smørgrav<p> 192bc421551SDag-Erling SmørgravAfter obtaining the code and data files, see the 193bc421551SDag-Erling Smørgrav<code>README</code> file for what to do next. 194bc421551SDag-Erling SmørgravThe code lets you compile the <code><abbr>tz</abbr></code> source files into 195bc421551SDag-Erling Smørgravmachine-readable binary files, one for each location. The binary files 196*a979394aSDag-Erling Smørgravare in a special format specified by 197*a979394aSDag-Erling Smørgrav<a href="https://datatracker.ietf.org/doc/html/8536">The 198*a979394aSDag-Erling SmørgravTime Zone Information Format (<abbr>TZif</abbr>)</a> 199*a979394aSDag-Erling Smørgrav(Internet <abbr title="Request For Comments">RFC</abbr> 8536). 200bc421551SDag-Erling SmørgravThe code also lets 201bc421551SDag-Erling Smørgravyou read a <abbr>TZif</abbr> file and interpret timestamps for that 202bc421551SDag-Erling Smørgravlocation.</p> 203bc421551SDag-Erling Smørgrav</section> 204bc421551SDag-Erling Smørgrav 205bc421551SDag-Erling Smørgrav<section> 206bc421551SDag-Erling Smørgrav<h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2> 207bc421551SDag-Erling Smørgrav<p> 208bc421551SDag-Erling SmørgravThe <code><abbr>tz</abbr></code> code and data 209bc421551SDag-Erling Smørgravare by no means authoritative. If you find errors, please 210*a979394aSDag-Erling Smørgravemail changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>, 211*a979394aSDag-Erling Smørgravthe time zone mailing list. See 212*a979394aSDag-Erling Smørgrav<a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing 213*a979394aSDag-Erling Smørgravlist's main page</a> to subscribe or to browse its archive of old messages. 214*a979394aSDag-Erling Smørgrav<a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list 215bc421551SDag-Erling Smørgravdiscussions</a> and corresponding data changes can be 216bc421551SDag-Erling Smørgravgenerated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>. 217bc421551SDag-Erling Smørgrav</p> 218bc421551SDag-Erling Smørgrav<p> 219bc421551SDag-Erling SmørgravChanges to the <code><abbr>tz</abbr></code> code and data are often 220bc421551SDag-Erling Smørgravpropagated to clients via operating system updates, so 221bc421551SDag-Erling Smørgravclient <code><abbr>tz</abbr></code> data can often be corrected by 222bc421551SDag-Erling Smørgravapplying these updates. With GNU/Linux and similar systems, if your 223bc421551SDag-Erling Smørgravmaintenance provider has not yet adopted the 224bc421551SDag-Erling Smørgravlatest <code><abbr>tz</abbr></code> data, you can often short-circuit 225bc421551SDag-Erling Smørgravthe process by tailoring the generic instructions in 226bc421551SDag-Erling Smørgravthe <code><abbr>tz</abbr> README</code> file and installing the latest 227bc421551SDag-Erling Smørgravdata yourself. System-specific instructions for installing the 228bc421551SDag-Erling Smørgravlatest <code><abbr>tz</abbr></code> data have also been published 229*a979394aSDag-Erling Smørgravfor <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>, 230bc421551SDag-Erling Smørgrav<a 231bc421551SDag-Erling Smørgravhref="https://source.android.com/devices/tech/config/timezone-rules">Android</a>, 232bc421551SDag-Erling Smørgrav<a 233bc421551SDag-Erling Smørgravhref="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr 234bc421551SDag-Erling Smørgravtitle="International Components for Unicode">ICU</abbr></a>, 235bc421551SDag-Erling Smørgrav<a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr> 236bc421551SDag-Erling SmørgravJDK</a>, 237bc421551SDag-Erling Smørgrav<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a 238bc421551SDag-Erling Smørgravhref="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>, 239bc421551SDag-Erling Smørgrav<a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a 240bc421551SDag-Erling Smørgravhref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>. 241bc421551SDag-Erling Smørgrav</p> 242bc421551SDag-Erling Smørgrav<p>Since version 2013a, 243bc421551SDag-Erling Smørgravsources for the <code><abbr>tz</abbr></code> database have been 244bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr 245bc421551SDag-Erling Smørgravtitle="Unicode Transformation Format 8-bit">UTF-8</abbr></a> 246bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Text_file">text files</a> 247bc421551SDag-Erling Smørgravwith lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr 248bc421551SDag-Erling Smørgravtitle="linefeed">LF</abbr></a>, 249bc421551SDag-Erling Smørgravwhich can be modified by common text editors such 250bc421551SDag-Erling Smørgravas <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>, 251*a979394aSDag-Erling Smørgrav<a href="https://gedit-technology.github.io/apps/gedit/">gedit</a>, and 252bc421551SDag-Erling Smørgrav<a href="https://www.vim.org">vim</a>. 253bc421551SDag-Erling SmørgravSpecialized source-file editing can be done via the 254bc421551SDag-Erling Smørgrav<a href="https://packagecontrol.io/packages/zoneinfo">Sublime 255bc421551SDag-Erling Smørgravzoneinfo</a> package for <a 256bc421551SDag-Erling Smørgravhref="https://www.sublimetext.com">Sublime Text</a> and the <a 257bc421551SDag-Erling Smørgravhref="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode 258bc421551SDag-Erling Smørgravzoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual 259bc421551SDag-Erling SmørgravStudio Code</a>. 260bc421551SDag-Erling Smørgrav</p> 261bc421551SDag-Erling Smørgrav<p> 262bc421551SDag-Erling SmørgravFor further information about updates, please see 263bc421551SDag-Erling Smørgrav<a href="https://datatracker.ietf.org/doc/html/rfc6557">Procedures for 264*a979394aSDag-Erling SmørgravMaintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557). 265*a979394aSDag-Erling SmørgravMore detail can be 266bc421551SDag-Erling Smørgravfound in <a href="theory.html">Theory and pragmatics of the 267bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> code and data</a>. 268bc421551SDag-Erling Smørgrav<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a> 269bc421551SDag-Erling Smørgravdisplays changes between recent <code><abbr>tzdb</abbr></code> versions. 270bc421551SDag-Erling Smørgrav</p> 271bc421551SDag-Erling Smørgrav</section> 272bc421551SDag-Erling Smørgrav 273bc421551SDag-Erling Smørgrav<section> 27475411d15SDag-Erling Smørgrav<h2 id="coordinating">Coordinating with governments and distributors</h2> 27575411d15SDag-Erling Smørgrav<p> 27675411d15SDag-Erling SmørgravAs discussed in 27775411d15SDag-Erling Smørgrav"<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How 27875411d15SDag-Erling SmørgravTime Zones Are Coordinated</a>", the time zone database relies on 27975411d15SDag-Erling Smørgravcollaboration among governments, the time zone database volunteer 28075411d15SDag-Erling Smørgravcommunity, and data distributors downstream. 28175411d15SDag-Erling Smørgrav<p> 28275411d15SDag-Erling SmørgravIf your government plans to change its time zone boundaries or 28375411d15SDag-Erling Smørgravdaylight saving rules, please send email to <a 28475411d15SDag-Erling Smørgravhref="mailto:tz@iana.org"><code>tz@iana.org</code></a> well in advance, 28575411d15SDag-Erling Smørgravas this will lessen confusion and will coordinate updates to many cell phones, 28675411d15SDag-Erling Smørgravcomputers, and other devices around the world. 28775411d15SDag-Erling SmørgravIn your email, please cite the legislation or regulation that specifies 28875411d15SDag-Erling Smørgravthe change, so that it can be checked for details such as the exact times 28975411d15SDag-Erling Smørgravwhen clock transitions occur. 29075411d15SDag-Erling SmørgravIt is OK if a rule change is planned to affect clocks 29175411d15SDag-Erling Smørgravfar into the future, as a long-planned change can easily be reverted 29275411d15SDag-Erling Smørgravor otherwise altered with a year's notice before the change would have 29375411d15SDag-Erling Smørgravaffected clocks.</p> 29475411d15SDag-Erling Smørgrav<p> 29575411d15SDag-Erling SmørgravThere is no fixed schedule for <code><abbr>tzdb</abbr></code> releases. 29675411d15SDag-Erling SmørgravHowever, typically a release occurs every few months. 29775411d15SDag-Erling SmørgravMany downstream timezone data distributors wait for 29875411d15SDag-Erling Smørgrava <code><abbr>tzdb</abbr></code> release before they produce an update 29975411d15SDag-Erling Smørgravto time zone behavior in consumer devices and software products. 30075411d15SDag-Erling SmørgravAfter a release, various parties must integrate, test, 30175411d15SDag-Erling Smørgravand roll out an update before <a 30275411d15SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/End_user">end users</a> see changes. 30375411d15SDag-Erling SmørgravThese updates can be expensive, for both the <a 30475411d15SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Quality_assurance">quality 30575411d15SDag-Erling Smørgravassurance</a> process and the overall cost of shipping and installing 30675411d15SDag-Erling Smørgravupdates to each device's copy of <code><abbr>tzdb</abbr></code>. 30775411d15SDag-Erling SmørgravUpdates may be batched with other updates and may take substantial 30875411d15SDag-Erling Smørgravtime to reach end users after a release. 30975411d15SDag-Erling SmørgravOlder devices may no longer be supported and thus may never be updated, 31075411d15SDag-Erling Smørgravwhich means they will continue to use out-of-date rules.</p> 31175411d15SDag-Erling Smørgrav<p> 31275411d15SDag-Erling SmørgravFor these reasons any rule change should be promulgated at least a 31375411d15SDag-Erling Smørgravyear before it affects how clocks operate; otherwise, there is a good 31475411d15SDag-Erling Smørgravchance that many clocks will be wrong due to delays in propagating updates, 31575411d15SDag-Erling Smørgravand that residents will be confused or even actively resist the change. 31675411d15SDag-Erling SmørgravThe shorter the notice, the more likely clock problems will arise; see "<a 31775411d15SDag-Erling Smørgravhref="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On 31875411d15SDag-Erling Smørgravthe Timing of Time Zone Changes</a>" for examples. 31975411d15SDag-Erling Smørgrav</p> 32075411d15SDag-Erling Smørgrav</section> 32175411d15SDag-Erling Smørgrav 32275411d15SDag-Erling Smørgrav<section> 323bc421551SDag-Erling Smørgrav<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2> 324bc421551SDag-Erling Smørgrav<ul> 325bc421551SDag-Erling Smørgrav<li>The article 326bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is 327bc421551SDag-Erling Smørgravan encyclopedic summary.</li> 328bc421551SDag-Erling Smørgrav<li><a href="tz-how-to.html">How to Read the 329bc421551SDag-Erling Smørgravtz Database Source Files</a> explains the <code><abbr>tz</abbr></code> 330bc421551SDag-Erling Smørgravdatabase format.</li> 331bc421551SDag-Erling Smørgrav<li><a 332bc421551SDag-Erling Smørgravhref="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A 333bc421551SDag-Erling Smørgravliterary appreciation of the Olson/Zoneinfo/tz database</a> comments on the 334bc421551SDag-Erling Smørgravdatabase's style.</li> 335bc421551SDag-Erling Smørgrav<li><a href="https://doi.org/10.1145/3340301.3341125">What time is it: 336bc421551SDag-Erling Smørgravmanaging time in the internet</a> analyzes the database longitudinally.</li> 337bc421551SDag-Erling Smørgrav</ul> 338bc421551SDag-Erling Smørgrav</section> 339bc421551SDag-Erling Smørgrav 340bc421551SDag-Erling Smørgrav<section> 341bc421551SDag-Erling Smørgrav<h2 id="web">Web sites using recent versions of the 342bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> database</h2> 343bc421551SDag-Erling Smørgrav<p> 344bc421551SDag-Erling SmørgravThese are listed roughly in ascending order of complexity and fanciness. 345bc421551SDag-Erling Smørgrav</p> 346bc421551SDag-Erling Smørgrav<ul> 347bc421551SDag-Erling Smørgrav<li><a href="https://time.is">Time.is</a> shows locations' 348bc421551SDag-Erling Smørgravtime and zones.</li> 349bc421551SDag-Erling Smørgrav<li><a href="https://www.timejones.com">TimeJones.com</a>, 350bc421551SDag-Erling Smørgrav<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and 351bc421551SDag-Erling Smørgrav<a href="https://www.worldclock.com">The World Clock</a> 352bc421551SDag-Erling Smørgravare time zone converters.</li> 353bc421551SDag-Erling Smørgrav<li><a href="https://timezonedb.com/download">TimeZoneDB Database</a> 354bc421551SDag-Erling Smørgravpublishes <code><abbr>tzdb</abbr></code>-derived data in 355bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr 356bc421551SDag-Erling Smørgravtitle="comma-separated values">CSV</abbr></a> and 357bc421551SDag-Erling Smørgravin <a href="https://en.wikipedia.org/wiki/SQL"><abbr 358bc421551SDag-Erling Smørgravtitle="Structured Query Language">SQL</abbr></a> form.</li> 359bc421551SDag-Erling Smørgrav<li><a 360bc421551SDag-Erling Smørgravhref="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a> 361bc421551SDag-Erling Smørgravlets you see the <code><abbr>TZ</abbr></code> values directly.</li> 362bc421551SDag-Erling Smørgrav<li><a 363bc421551SDag-Erling Smørgravhref="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current 364bc421551SDag-Erling SmørgravTime in 1000 Places</a> uses descriptions of the values.</li> 365bc421551SDag-Erling Smørgrav<li><a href="https://www.timeanddate.com/worldclock/">The World Clock – 366bc421551SDag-Erling SmørgravWorldwide</a> lets you sort zone names and convert times.</li> 367bc421551SDag-Erling Smørgrav<li><a href="https://24timezones.com">24TimeZones</a> has a world 368bc421551SDag-Erling Smørgravtime map and a time converter.</li> 369bc421551SDag-Erling Smørgrav<li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a> 370bc421551SDag-Erling Smørgravcalculates the current time difference between locations.</li> 371bc421551SDag-Erling Smørgrav<li><a href="https://www.wx-now.com">Weather Now</a> and 372bc421551SDag-Erling Smørgrav<a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li> 373bc421551SDag-Erling Smørgrav</ul> 374bc421551SDag-Erling Smørgrav</section> 375bc421551SDag-Erling Smørgrav 376bc421551SDag-Erling Smørgrav<section> 377bc421551SDag-Erling Smørgrav<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2> 378bc421551SDag-Erling Smørgrav<ul> 379bc421551SDag-Erling Smørgrav<li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s 380bc421551SDag-Erling Smørgrav<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data 381bc421551SDag-Erling SmørgravDistribution Service (tzdist) working group</a> defined <a 382bc421551SDag-Erling Smørgravhref="https://datatracker.ietf.org/doc/html/rfc7808">TZDIST</a> 383bc421551SDag-Erling Smørgrav(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service, 384bc421551SDag-Erling Smørgravalong with <a href="https://datatracker.ietf.org/doc/html/rfc7809">CalDAV</a> 385bc421551SDag-Erling Smørgrav(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for 386bc421551SDag-Erling Smørgravtransferring time zone data by reference. 387bc421551SDag-Erling Smørgrav<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST 388bc421551SDag-Erling Smørgravimplementations</a> are available. 389bc421551SDag-Erling SmørgravThe <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis 390bc421551SDag-Erling Smørgravmailing list</a> discusses possible extensions.</li> 391bc421551SDag-Erling Smørgrav<li>The <a href="https://datatracker.ietf.org/doc/html/rfc5545"> 392bc421551SDag-Erling SmørgravInternet Calendaring and Scheduling Core Object Specification 393bc421551SDag-Erling Smørgrav(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445) 394bc421551SDag-Erling Smørgravcovers time zone 395bc421551SDag-Erling Smørgravdata; see its VTIMEZONE calendar component. 396bc421551SDag-Erling SmørgravThe iCalendar format requires specialized parsers and generators; a 397bc421551SDag-Erling Smørgravvariant <a href="https://datatracker.ietf.org/doc/html/rfc6321">xCal</a> 398bc421551SDag-Erling Smørgrav(Internet <abbr>RFC</abbr> 6321) uses 399bc421551SDag-Erling Smørgrav<a href="https://www.w3.org/XML/"><abbr 400bc421551SDag-Erling Smørgravtitle="Extensible Markup Language">XML</abbr></a> format, and a variant 401bc421551SDag-Erling Smørgrav<a href="https://datatracker.ietf.org/doc/html/rfc7265">jCal</a> 402bc421551SDag-Erling Smørgrav(Internet <abbr>RFC</abbr> 7265) 403*a979394aSDag-Erling Smørgravuses <a href="https://www.json.org/json-en.html"><abbr 404bc421551SDag-Erling Smørgravtitle="JavaScript Object Notation">JSON</abbr></a> format.</li> 405bc421551SDag-Erling Smørgrav</ul> 406bc421551SDag-Erling Smørgrav</section> 407bc421551SDag-Erling Smørgrav 408bc421551SDag-Erling Smørgrav<section> 409bc421551SDag-Erling Smørgrav<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2> 410bc421551SDag-Erling Smørgrav<p>Although some of these do not fully support 411bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code> 412bc421551SDag-Erling Smørgravdistributions you can generally work around compatibility problems by 413bc421551SDag-Erling Smørgravrunning the command <code>make rearguard_tarballs</code> and compiling 414bc421551SDag-Erling Smørgravfrom the resulting tarballs instead.</p> 415bc421551SDag-Erling Smørgrav<ul> 416*a979394aSDag-Erling Smørgrav<li><a href="https://github.com/libical/vzic">Vzic</a> is a <a 41775411d15SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/C_(programming_language)">C</a> 418bc421551SDag-Erling Smørgravprogram that compiles 419bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files. 420bc421551SDag-Erling SmørgravVzic is freely 421bc421551SDag-Erling Smørgravavailable under the <a 422bc421551SDag-Erling Smørgravhref="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> 423bc421551SDag-Erling SmørgravGeneral Public License (<abbr 424bc421551SDag-Erling Smørgravtitle="General Public License">GPL</abbr>)</a>.</li> 425bc421551SDag-Erling Smørgrav<li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz 426bc421551SDag-Erling Smørgravdatabase conversion utility</a> is like Vzic, except for the <a 427bc421551SDag-Erling Smørgravhref="https://dotnet.microsoft.com">.NET framework</a> 428bc421551SDag-Erling Smørgravand with a <abbr>BSD</abbr>-style license.</li> 429bc421551SDag-Erling Smørgrav<li><a 430bc421551SDag-Erling Smørgravhref="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a> 431bc421551SDag-Erling Smørgravcontains a script <code>parse_olson</code> that compiles 432bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a> 433bc421551SDag-Erling Smørgravmodules. It is part of the Perl <a 434bc421551SDag-Erling Smørgravhref="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>, 435bc421551SDag-Erling Smørgravwhich is freely 436bc421551SDag-Erling Smørgravavailable under both the <abbr>GPL</abbr> and the Perl Artistic 437bc421551SDag-Erling SmørgravLicense. DateTime::TimeZone also contains a script 438bc421551SDag-Erling Smørgrav<code>tests_from_zdump</code> that generates test cases for each clock 439bc421551SDag-Erling Smørgravtransition in the <code><abbr>tz</abbr></code> database.</li> 440bc421551SDag-Erling Smørgrav<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone 441bc421551SDag-Erling SmørgravDatabase Parser</a> is a 44275411d15SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and 443*a979394aSDag-Erling Smørgravruntime library with a <a 444*a979394aSDag-Erling Smørgravhref="https://en.cppreference.com/w/cpp/chrono"><code>std::chrono</code> API</a> 445*a979394aSDag-Erling Smørgravthat is a standard part of C++. 446bc421551SDag-Erling SmørgravIt is freely available under the 447bc421551SDag-Erling Smørgrav<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li> 448bc421551SDag-Erling Smørgrav<li><a id="ICU" href="https://icu.unicode.org">International Components for 449bc421551SDag-Erling SmørgravUnicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a 45075411d15SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a> 451bc421551SDag-Erling Smørgravlibraries for internationalization that 452bc421551SDag-Erling Smørgravhas a compiler from <code><abbr>tz</abbr></code> source 453bc421551SDag-Erling Smørgravand from <abbr title="Common Locale Data Repository">CLDR</abbr> data 454bc421551SDag-Erling Smørgrav(mentioned <a href="#CLDR">below</a>) 455bc421551SDag-Erling Smørgravinto an <abbr>ICU</abbr>-specific format. 456bc421551SDag-Erling Smørgrav<abbr>ICU</abbr> is freely available under a 457bc421551SDag-Erling Smørgrav<abbr>BSD</abbr>-style license.</li> 458bc421551SDag-Erling Smørgrav<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for 459bc421551SDag-Erling Smørgravthe <a href="https://elixir-lang.org">Elixir</a> language downloads 460bc421551SDag-Erling Smørgravand compiles <code><abbr>tz</abbr></code> source and exposes <abbr 461bc421551SDag-Erling Smørgravtitle="Application Program Interface">API</abbr>s for use. It is 462bc421551SDag-Erling Smørgravfreely available under the <abbr>MIT</abbr> license.</li> 463bc421551SDag-Erling Smørgrav<li>Java-based compilers and libraries include: 464bc421551SDag-Erling Smørgrav<ul> 465bc421551SDag-Erling Smørgrav<li>The <a 466bc421551SDag-Erling Smørgravhref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater 467bc421551SDag-Erling Smørgravtool</a> compiles <code><abbr>tz</abbr></code> source into the format used by 468*a979394aSDag-Erling Smørgrav<a href="https://openjdk.org">OpenJDK</a> and 469*a979394aSDag-Erling Smørgrav<a href="https://jdk.java.net">Oracle JDK</a>. 470bc421551SDag-Erling SmørgravAlthough its source code is proprietary, its executable is available under the 471bc421551SDag-Erling Smørgrav<a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE 472bc421551SDag-Erling SmørgravTimezone Updater License Agreement</a>.</li> 473bc421551SDag-Erling Smørgrav<li>The <a 474bc421551SDag-Erling Smørgravhref="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java 475bc421551SDag-Erling SmørgravSE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a 476bc421551SDag-Erling Smørgravhref="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>, 477bc421551SDag-Erling Smørgravwhich is freely available under a <abbr>BSD</abbr>-style license.</li> 478bc421551SDag-Erling Smørgrav<li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date 479bc421551SDag-Erling Smørgravand time <abbr>API</abbr></a> contains a class 480bc421551SDag-Erling Smørgrav<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles 481bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> source into a binary format. It inspired 482bc421551SDag-Erling SmørgravJava 8 <code>java.time</code>, which its users should migrate to once 483bc421551SDag-Erling Smørgravthey can assume Java 8 or later. It is available under the <a 484bc421551SDag-Erling Smørgravhref="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li> 485bc421551SDag-Erling Smørgrav<li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a 486bc421551SDag-Erling Smørgravhref="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a> 487bc421551SDag-Erling Smørgravare alternatives to TZUpdater. IANA Updater's license is unclear; 488bc421551SDag-Erling SmørgravZIUpdater is licensed under the <abbr>GPL</abbr>.</li> 489bc421551SDag-Erling Smørgrav<li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and 490bc421551SDag-Erling Smørgravtime library for Android</a> and 491*a979394aSDag-Erling Smørgrav<a href="https://github.com/MenoData/Time4J">Time4J: Advanced date, 492bc421551SDag-Erling Smørgravtime and interval library for Java</a> compile 493bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> source into a binary format. 494bc421551SDag-Erling SmørgravTime4A is available under the Apache License and Time4J is 495bc421551SDag-Erling Smørgravavailable under the <a 496bc421551SDag-Erling Smørgravhref="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser 497bc421551SDag-Erling SmørgravGeneral Public License (<abbr title="Lesser General Public 498bc421551SDag-Erling SmørgravLicense">LGPL</abbr>)</a>.</li> 499bc421551SDag-Erling Smørgrav<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and 500bc421551SDag-Erling SmørgravJava-based libraries.</li> 501bc421551SDag-Erling Smørgrav</ul> 502bc421551SDag-Erling Smørgrav<li><a href="https://nodatime.org">Noda Time – Date and 503bc421551SDag-Erling Smørgravtime <abbr>API</abbr> for .NET</a> 504bc421551SDag-Erling Smørgravis like Joda-Time and Time4J, but for the .NET framework instead of Java. 505bc421551SDag-Erling SmørgravIt is freely available under the Apache License.</li> 506bc421551SDag-Erling Smørgrav<li>Many modern 507bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a> 508bc421551SDag-Erling Smørgravruntimes support <code><abbr>tz</abbr></code> natively via the 50946c59934SDag-Erling Smørgrav<code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>. 510bc421551SDag-Erling SmørgravThis can be used as-is or with most of the following libraries, 51146c59934SDag-Erling Smørgravmany of which also support runtimes lacking the <code>timeZone</code> option. 512bc421551SDag-Erling Smørgrav<ul> 513bc421551SDag-Erling Smørgrav<li>The <a 51446c59934SDag-Erling Smørgravhref="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code> 515bc421551SDag-Erling Smørgravtimezone polyfill</a> 516bc421551SDag-Erling Smørgravis freely available under a <abbr>BSD</abbr>-style license.</li> 517*a979394aSDag-Erling Smørgrav<li>The <a href="https://date-fns.org">date-fns</a> 518bc421551SDag-Erling Smørgravlibrary manipulates timezone-aware timestamps in browsers and 519bc421551SDag-Erling Smørgravin <a href="https://nodejs.org/en/">Node.js</a>. 520bc421551SDag-Erling SmørgravIt is freely available under the <abbr>MIT</abbr> license.</li> 521bc421551SDag-Erling Smørgrav<li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a 522bc421551SDag-Erling Smørgravminimalist replacement for the date and time API of 523bc421551SDag-Erling Smørgravthe <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date 524bc421551SDag-Erling Smørgravmanipulation library. 525bc421551SDag-Erling SmørgravIt is freely available under the <abbr>MIT</abbr> license.</li> 526bc421551SDag-Erling Smørgrav<li><a href="https://moment.github.io/luxon/">Luxon</a> improves 52746c59934SDag-Erling Smørgravtimezone support for the <code>Intl</code> API. 528bc421551SDag-Erling SmørgravIt is freely available under the <abbr>MIT</abbr> license.</li> 529bc421551SDag-Erling Smørgrav<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a 530bc421551SDag-Erling SmørgravMoment.js plugin. 531bc421551SDag-Erling SmørgravIt is freely available under the <abbr>MIT</abbr> license.</li> 532bc421551SDag-Erling Smørgrav<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a 533bc421551SDag-Erling SmørgravJavaScript library that supports date arithmetic that is time zone 534bc421551SDag-Erling Smørgravaware. It is freely available under the <abbr>MIT</abbr> license.</li> 535bc421551SDag-Erling Smørgrav<li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a> 536bc421551SDag-Erling Smørgravsupports live <code><abbr>tzdb</abbr></code> updates, 537bc421551SDag-Erling Smørgravastronomical and atomic time, a command-line interface, 538bc421551SDag-Erling Smørgravand full <a 539bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>. 540bc421551SDag-Erling SmørgravIts companion <a 541bc421551SDag-Erling Smørgravhref="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a> 542bc421551SDag-Erling Smørgravcan generate <abbr>TZif</abbr> and other files, and a companion website 543bc421551SDag-Erling Smørgrav<a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you 544bc421551SDag-Erling Smørgravconvert timestamps, view transition histories, and download code and data. 545bc421551SDag-Erling SmørgravIt is freely available under the <abbr>MIT</abbr> license.</li> 546bc421551SDag-Erling Smørgrav</ul> 547bc421551SDag-Erling SmørgravThe proposed <a 54846c59934SDag-Erling Smørgravhref="https://github.com/tc39/proposal-temporal"><code>Temporal</code> 549bc421551SDag-Erling Smørgravobjects</a> let programs access an abstract view of 550bc421551SDag-Erling Smørgrav<code><abbr>tzdb</abbr></code> data, and are designed to replace <a 551bc421551SDag-Erling Smørgravhref="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's 55246c59934SDag-Erling Smørgravproblematic <code>Date</code> objects</a> when working with dates and times. 553*a979394aSDag-Erling Smørgrav<li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a 554bc421551SDag-Erling Smørgravcompiler from <code><abbr>tz</abbr></code> source into 555*a979394aSDag-Erling Smørgrav<a href="https://julialang.org">Julia</a>. It is freely available 556bc421551SDag-Erling Smørgravunder the <abbr>MIT</abbr> license.</li> 557bc421551SDag-Erling Smørgrav<li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> – 558bc421551SDag-Erling Smørgrav<abbr>IANA</abbr> Time Zone Database for Delphi/<abbr 559bc421551SDag-Erling Smørgravtitle="Free Pascal Compiler">FPC</abbr></a> 560bc421551SDag-Erling Smørgravcompiles from <code><abbr>tz</abbr></code> source into 561bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a> 562bc421551SDag-Erling Smørgravas compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a> 563bc421551SDag-Erling Smørgravand <a 564bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>. 565bc421551SDag-Erling SmørgravIt is freely available under a <abbr>BSD</abbr>-style license.</li> 566*a979394aSDag-Erling Smørgrav<li><a href="https://pythonhosted.org/pytz/">pytz – World Timezone 567bc421551SDag-Erling SmørgravDefinitions for Python</a> compiles <code><abbr>tz</abbr></code> source into 568bc421551SDag-Erling Smørgrav<a href="https://www.python.org">Python</a>. 569bc421551SDag-Erling SmørgravIt is freely available under a <abbr>BSD</abbr>-style license. 570bc421551SDag-Erling SmørgravIn code that can assume Python 3.6 or later it is largely superseded; see <a 571bc421551SDag-Erling Smørgravhref="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz: 572bc421551SDag-Erling SmørgravThe Fastest Footgun in the West</a>.</li> 573bc421551SDag-Erling Smørgrav<li><a href="https://tzinfo.github.io">TZInfo – 574bc421551SDag-Erling SmørgravRuby Timezone Library</a> 575bc421551SDag-Erling Smørgravcompiles <code><abbr>tz</abbr></code> source into 576bc421551SDag-Erling Smørgrav<a href="https://www.ruby-lang.org/en/">Ruby</a>. 577bc421551SDag-Erling SmørgravIt is freely available under the <abbr>MIT</abbr> license.</li> 578bc421551SDag-Erling Smørgrav<li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time 579bc421551SDag-Erling SmørgravLibrary</a> is 580bc421551SDag-Erling Smørgrava <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class 581bc421551SDag-Erling Smørgravlibrary that compiles <code><abbr>tz</abbr></code> source into a time 582bc421551SDag-Erling Smørgravzone repository whose format 583bc421551SDag-Erling Smørgravis either proprietary or an <abbr>XML</abbr>-encoded 584bc421551SDag-Erling Smørgravrepresentation.</li> 585bc421551SDag-Erling Smørgrav<li><a id="Tcl" href="https://tcl.tk">Tcl</a> 586bc421551SDag-Erling Smørgravcontains a developer-oriented parser that compiles <code><abbr>tz</abbr></code> 587bc421551SDag-Erling Smørgravsource into text files, along with a runtime that can read those 588bc421551SDag-Erling Smørgravfiles. Tcl is freely available under a <abbr>BSD</abbr>-style 589bc421551SDag-Erling Smørgravlicense.</li> 590bc421551SDag-Erling Smørgrav</ul> 591bc421551SDag-Erling Smørgrav</section> 592bc421551SDag-Erling Smørgrav 593bc421551SDag-Erling Smørgrav<section> 594bc421551SDag-Erling Smørgrav<h2 id="TZif">Other <abbr>TZif</abbr> readers</h2> 595bc421551SDag-Erling Smørgrav<ul> 596bc421551SDag-Erling Smørgrav<li>The <a 597bc421551SDag-Erling Smørgravhref="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C 598bc421551SDag-Erling SmørgravLibrary</a> 599bc421551SDag-Erling Smørgravhas an independent, thread-safe implementation of 600bc421551SDag-Erling Smørgrava <abbr>TZif</abbr> file reader. 601bc421551SDag-Erling SmørgravThis library is freely available under the LGPL 602bc421551SDag-Erling Smørgravand is widely used in <abbr>GNU</abbr>/Linux systems.</li> 603bc421551SDag-Erling Smørgrav<li><a href="https://www.gnome.org">GNOME</a>'s 604bc421551SDag-Erling Smørgrav<a href="https://developer.gnome.org/glib/">GLib</a> has 605bc421551SDag-Erling Smørgrava <abbr>TZif</abbr> file reader written in C that 606bc421551SDag-Erling Smørgravcreates a <code>GTimeZone</code> object representing sets 607bc421551SDag-Erling Smørgravof <abbr>UT</abbr> offsets. 608bc421551SDag-Erling SmørgravIt is freely available under the <abbr>LGPL</abbr>.</li> 609bc421551SDag-Erling Smørgrav<li>The 610bc421551SDag-Erling Smørgrav<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s 611bc421551SDag-Erling Smørgrav<code>baltzo::TimeZoneUtil</code> component contains a C++ 612bc421551SDag-Erling Smørgravimplementation of a <abbr>TZif</abbr> file reader. It is freely available under 613bc421551SDag-Erling Smørgravthe Apache License.</li> 614bc421551SDag-Erling Smørgrav<li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++ 615bc421551SDag-Erling Smørgravlibrary that translates between <abbr>UT</abbr> and civil time and 616bc421551SDag-Erling Smørgravcan read <abbr>TZif</abbr> files. It is freely available under the Apache 617bc421551SDag-Erling SmørgravLicense.</li> 61846c59934SDag-Erling Smørgrav<li>The 61946c59934SDag-Erling Smørgrav<a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code> 62046c59934SDag-Erling Smørgravpackage</a> contains Python code 62146c59934SDag-Erling Smørgravto generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map 622*a979394aSDag-Erling Smørgrav<code><abbr>tz</abbr></code> settings to proleptic TZ approximations. 62346c59934SDag-Erling SmørgravFor example, it maps <code>"Africa/Cairo"</code> 62446c59934SDag-Erling Smørgravto <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>, 62546c59934SDag-Erling Smørgravan approximation valid for Cairo timestamps from 2023 on. 626*a979394aSDag-Erling SmørgravThis can help porting to platforms that support only proleptic TZ. 62746c59934SDag-Erling SmørgravThe package is freely available under the MIT license.</li> 628bc421551SDag-Erling Smørgrav<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C 629bc421551SDag-Erling Smørgravlibrary that reads <abbr>TZif</abbr> files and converts 630bc421551SDag-Erling Smørgravtimestamps from one time zone or format to another. 631bc421551SDag-Erling SmørgravIt is used by <a href="https://secure.php.net"><abbr 632bc421551SDag-Erling Smørgravtitle="PHP: Hypertext Preprocessor">PHP</abbr></a>, 633bc421551SDag-Erling Smørgrav<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>, 634bc421551SDag-Erling Smørgravand <a href="https://www.mongodb.com">MongoDB</a>. 635bc421551SDag-Erling SmørgravIt is freely available under the <abbr>MIT</abbr> license.</li> 636bc421551SDag-Erling Smørgrav<li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a 637bc421551SDag-Erling Smørgrav<abbr>TZif</abbr> file reader.</li> 638bc421551SDag-Erling Smørgrav<li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile"> 639bc421551SDag-Erling SmørgravDateTime::TimeZone::Tzfile</a> 640bc421551SDag-Erling Smørgravis a <abbr>TZif</abbr> file reader written in Perl. 641bc421551SDag-Erling SmørgravIt is freely available under the same terms as Perl 642bc421551SDag-Erling Smørgrav(dual <abbr>GPL</abbr> and Artistic license).</li> 643bc421551SDag-Erling Smørgrav<li>Python has a <a id="python-zoneinfo" 644bc421551SDag-Erling Smørgravhref="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code> 645bc421551SDag-Erling Smørgravclass</a> that reads <abbr>TZif</abbr> data and creates objects 646bc421551SDag-Erling Smørgravthat represent <code><abbr>tzdb</abbr></code> timezones. 647bc421551SDag-Erling SmørgravPython is freely available under the 648bc421551SDag-Erling Smørgrav<a href="https://docs.python.org/3/license.html">Python Software Foundation 649bc421551SDag-Erling SmørgravLicense</a>. 650*a979394aSDag-Erling SmørgravA companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module 651bc421551SDag-Erling Smørgrav<a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a> 652bc421551SDag-Erling Smørgravsupplies TZif data if the underlying system data cannot be found; 653bc421551SDag-Erling Smørgravit is freely available under the Apache License.</li> 654bc421551SDag-Erling Smørgrav<li>The 655bc421551SDag-Erling Smørgravpublic-domain <a href="https://github.com/dbaron/tz.js">tz.js</a> 656bc421551SDag-Erling Smørgravlibrary contains a Python tool that 657bc421551SDag-Erling Smørgravconverts <abbr>TZif</abbr> data into 658bc421551SDag-Erling Smørgrav<abbr>JSON</abbr>-format data suitable for use 659bc421551SDag-Erling Smørgravin its JavaScript library for time zone conversion. Dates before 1970 660bc421551SDag-Erling Smørgravare not supported.</li> 661bc421551SDag-Erling Smørgrav<li>The <a 662bc421551SDag-Erling Smørgravhref="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a> 663bc421551SDag-Erling Smørgravpackage contains <a href="https://www.haskell.org">Haskell</a> code that 664bc421551SDag-Erling Smørgravparses and uses <abbr>TZif</abbr> data. It is freely 665bc421551SDag-Erling Smørgravavailable under a <abbr>BSD</abbr>-style license.</li> 666bc421551SDag-Erling Smørgrav</ul> 667bc421551SDag-Erling Smørgrav</section> 668bc421551SDag-Erling Smørgrav 669bc421551SDag-Erling Smørgrav<section> 670bc421551SDag-Erling Smørgrav<h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2> 671bc421551SDag-Erling Smørgrav<ul> 672bc421551SDag-Erling Smørgrav<li><a href="https://foxclocks.org">FoxClocks</a> 673bc421551SDag-Erling Smørgravis an extension for <a href="https://www.google.com/chrome/">Google 67446c59934SDag-Erling SmørgravChrome</a>, <a 675bc421551SDag-Erling Smørgravhref="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a 676bc421551SDag-Erling Smørgravhref="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>. 677bc421551SDag-Erling SmørgravIt displays multiple clocks in the application window, and has a mapping 678bc421551SDag-Erling Smørgravinterface to <a href="https://www.google.com/earth/">Google Earth</a>. 679bc421551SDag-Erling SmørgravIt is freely available under the <abbr>GPL</abbr>.</li> 680bc421551SDag-Erling Smørgrav<li><a href="https://golang.org">Go programming language</a> 681bc421551SDag-Erling Smørgravimplementations contain a copy of a 32-bit subset of a recent 682bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> database in a 683bc421551SDag-Erling SmørgravGo-specific format.</li> 684bc421551SDag-Erling Smørgrav<li>Microsoft Windows 8.1 685bc421551SDag-Erling Smørgravand later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr> 686bc421551SDag-Erling Smørgravdata (mentioned <a href="#CLDR">below</a>) used by the 687bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> / 688bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes 689bc421551SDag-Erling Smørgrav<a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and 690bc421551SDag-Erling Smørgrav<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>. 691bc421551SDag-Erling Smørgrav<a id="System.TimeZoneInfo" 692bc421551SDag-Erling Smørgravhref="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring 693bc421551SDag-Erling SmørgravWindows Time Zones with <code>System.TimeZoneInfo</code></a> describes 694bc421551SDag-Erling Smørgravthe older, proprietary method of Microsoft Windows 2000 and later, 695bc421551SDag-Erling Smørgravwhich stores time zone data in the 696bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The 697bc421551SDag-Erling Smørgrav<a 698bc421551SDag-Erling Smørgravhref="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone → 699bc421551SDag-Erling SmørgravTzid table</a> or <a 700bc421551SDag-Erling Smørgravhref="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr> 701bc421551SDag-Erling Smørgravfile</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs 702bc421551SDag-Erling Smørgravto <code><abbr>tz</abbr></code> names. 703bc421551SDag-Erling SmørgravThese mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library, 704bc421551SDag-Erling Smørgravor the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>. 705bc421551SDag-Erling Smørgrav<li><a 706bc421551SDag-Erling Smørgravhref="https://www.oracle.com/java/index.html">Oracle 707bc421551SDag-Erling SmørgravJava</a> contains a copy of a subset of a recent 708bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> database in a 709bc421551SDag-Erling SmørgravJava-specific format.</li> 710bc421551SDag-Erling Smørgrav</ul> 711bc421551SDag-Erling Smørgrav</section> 712bc421551SDag-Erling Smørgrav 713bc421551SDag-Erling Smørgrav<section> 714bc421551SDag-Erling Smørgrav<h2 id="other-dbs">Other time zone databases</h2> 715bc421551SDag-Erling Smørgrav<ul> 716bc421551SDag-Erling Smørgrav<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a> 717bc421551SDag-Erling Smørgravis Astrodienst's Web version of Shanks and Pottenger's out-of-print 718bc421551SDag-Erling Smørgravtime zone history atlases 719bc421551SDag-Erling Smørgrav<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and 720bc421551SDag-Erling Smørgrav<a href="https://www.worldcat.org/oclc/76950459">for the world</a>. 721bc421551SDag-Erling SmørgravAlthough these extensive atlases 722bc421551SDag-Erling Smørgrav<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were 723bc421551SDag-Erling Smørgravsources for much of the older <code><abbr>tz</abbr></code> data</a>, 724bc421551SDag-Erling Smørgravthey are unreliable as Shanks appears to have 725bc421551SDag-Erling Smørgravguessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no 726bc421551SDag-Erling Smørgravsources and do not indicate which entries are guesswork.</li> 727bc421551SDag-Erling Smørgrav<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in 728bc421551SDag-Erling Smørgravits own <code>tztab</code>(4) format.</li> 729bc421551SDag-Erling Smørgrav<li>Microsoft Windows has proprietary data mentioned 730bc421551SDag-Erling Smørgrav<a href="#System.TimeZoneInfo">above</a>.</li> 731bc421551SDag-Erling Smørgrav<li><a href="https://www.worldtimeserver.com">World Time Server</a> 732bc421551SDag-Erling Smørgravis another time zone database.</li> 733bc421551SDag-Erling Smørgrav<li>The <a 734bc421551SDag-Erling Smørgravhref="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard 735bc421551SDag-Erling SmørgravSchedules Information Manual</a> of the 736bc421551SDag-Erling SmørgravInternational Air Transport Association 737bc421551SDag-Erling Smørgravgives current time zone rules for airports served by commercial aviation.</li> 738bc421551SDag-Erling Smørgrav</ul> 739bc421551SDag-Erling Smørgrav</section> 740bc421551SDag-Erling Smørgrav 741bc421551SDag-Erling Smørgrav<section> 742bc421551SDag-Erling Smørgrav<h2 id="maps">Maps</h2> 743bc421551SDag-Erling Smørgrav<ul> 744bc421551SDag-Erling Smørgrav<li>The <a 745bc421551SDag-Erling Smørgravhref="https://www.cia.gov/the-world-factbook/maps/world-regional/">World 746bc421551SDag-Erling Smørgravand Regional Maps section</a> of <em>The World Factbook</em>, published by the 747bc421551SDag-Erling Smørgrav<a href="https://www.cia.gov">US Central Intelligence 748bc421551SDag-Erling SmørgravAgency (<abbr 749bc421551SDag-Erling Smørgravtitle="Central Intelligence Agency">CIA</abbr>)</a>, contains a time 750bc421551SDag-Erling Smørgravzone map; the 751bc421551SDag-Erling Smørgrav<a 752bc421551SDag-Erling Smørgravhref="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda 753bc421551SDag-Erling SmørgravLibrary Map Collection</a> 754bc421551SDag-Erling Smørgravof the University of Texas at Austin has copies of 755bc421551SDag-Erling Smørgravrecent editions. 756bc421551SDag-Erling SmørgravThe pictorial quality is good, 757bc421551SDag-Erling Smørgravbut the maps do not indicate daylight saving time, 758bc421551SDag-Erling Smørgravand parts of the data are a few years out of date.</li> 759bc421551SDag-Erling Smørgrav<li><a href="https://www.worldtimezone.com">World Time Zone Map 760bc421551SDag-Erling Smørgravwith current time</a> 761bc421551SDag-Erling Smørgravhas several fancy time zone maps; it covers Russia particularly well. 762bc421551SDag-Erling SmørgravThe maps' pictorial quality is not quite as good as the 763bc421551SDag-Erling Smørgrav<abbr>CIA</abbr>'s 764bc421551SDag-Erling Smørgravbut the maps are more up to date.</li> 765bc421551SDag-Erling Smørgrav<li><a 766bc421551SDag-Erling Smørgravhref="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How 767bc421551SDag-Erling Smørgravmuch is time wrong around the world?</a> maps the difference between 768bc421551SDag-Erling Smørgravmean solar and standard time, highlighting areas such as western China 769bc421551SDag-Erling Smørgravwhere the two differ greatly. It's a bit out of date, unfortunately.</li> 770bc421551SDag-Erling Smørgrav</ul> 771bc421551SDag-Erling Smørgrav</section> 772bc421551SDag-Erling Smørgrav 773bc421551SDag-Erling Smørgrav<section> 774bc421551SDag-Erling Smørgrav<h2 id="boundaries">Time zone boundaries</h2> 775bc421551SDag-Erling Smørgrav<p>Geographical boundaries between timezones are available 776bc421551SDag-Erling Smørgravfrom several <a 777bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Internet_geolocation">Internet 778bc421551SDag-Erling Smørgravgeolocation</a> 779bc421551SDag-Erling Smørgravservices and other sources.</p> 780bc421551SDag-Erling Smørgrav<ul> 781bc421551SDag-Erling Smørgrav<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone 782bc421551SDag-Erling SmørgravBoundary Builder</a> extracts 783bc421551SDag-Erling Smørgrav<a href="https://www.openstreetmap.org">Open Street Map</a> data to build 784bc421551SDag-Erling Smørgravboundaries of <code><abbr>tzdb</abbr></code> timezones. 785bc421551SDag-Erling SmørgravIts code is freely available under the <abbr>MIT</abbr> license, and 786bc421551SDag-Erling Smørgravits data entries are freely available under the 787bc421551SDag-Erling Smørgrav<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons 78846c59934SDag-Erling SmørgravOpen Database License</a>. The borders appear to be quite accurate. 78946c59934SDag-Erling SmørgravIts main web page lists more than twenty libraries 79046c59934SDag-Erling Smørgravfor looking up a timezone name from a GPS coordinate.</li> 791bc421551SDag-Erling Smørgrav<li>Free access via a network API, if you register a key, is provided by 792bc421551SDag-Erling Smørgravthe <a 793bc421551SDag-Erling Smørgravhref="https://www.geonames.org/export/web-services.html#timezone">GeoNames 794bc421551SDag-Erling SmørgravTimezone web service</a>, the <a 795bc421551SDag-Erling Smørgravhref="https://developers.google.com/maps/documentation/timezone/intro">Google 796bc421551SDag-Erling SmørgravMaps Time Zone API</a>, and 797bc421551SDag-Erling Smørgravthe <a href="https://timezonedb.com/api">TimeZoneDB API</a>. 798bc421551SDag-Erling SmørgravCommercial network API access is provided 799bc421551SDag-Erling Smørgravby <a href="https://askgeo.com">AskGeo</a> 800bc421551SDag-Erling Smørgravand <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>. 801bc421551SDag-Erling Smørgrav</li> 802bc421551SDag-Erling Smørgrav<li>"<a 803bc421551SDag-Erling Smørgravhref="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How 804bc421551SDag-Erling Smørgravto get a time zone from a location using latitude and longitude 805bc421551SDag-Erling Smørgravcoordinates?</a>" discusses other geolocation possibilities.</li> 806bc421551SDag-Erling Smørgrav<li><a href="http://statoids.com/statoids.html">Administrative 807bc421551SDag-Erling SmørgravDivisions of Countries ("Statoids")</a> lists 808bc421551SDag-Erling Smørgravpolitical subdivision data related to time zones.</li> 809bc421551SDag-Erling Smørgrav<li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software 810bc421551SDag-Erling Smørgrav– GIS and Database Tools</a> includes a Manifold-format map of 81146c59934SDag-Erling Smørgravworld time zone boundaries circa 2007, distributed under the 812bc421551SDag-Erling Smørgrav<abbr>GPL</abbr>.</li> 813bc421551SDag-Erling Smørgrav<li>A ship within the <a 814bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Territorial_waters">territorial 815bc421551SDag-Erling Smørgravwaters</a> of any nation uses that nation's time. In international 816bc421551SDag-Erling Smørgravwaters, time zone boundaries are meridians 15° apart, except that 817bc421551SDag-Erling Smørgrav<abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5° 818bc421551SDag-Erling Smørgravwide and are separated by 819bc421551SDag-Erling Smørgravthe 180° meridian (not by the International Date Line, which is 820bc421551SDag-Erling Smørgravfor land and territorial waters only). A captain can change ship's 821bc421551SDag-Erling Smørgravclocks any time after entering a new time zone; midnight changes are 822bc421551SDag-Erling Smørgravcommon.</li> 823bc421551SDag-Erling Smørgrav</ul> 824bc421551SDag-Erling Smørgrav</section> 825bc421551SDag-Erling Smørgrav 826bc421551SDag-Erling Smørgrav<section> 827bc421551SDag-Erling Smørgrav<h2 id="civil">Civil time concepts and history</h2> 828bc421551SDag-Erling Smørgrav<ul> 829bc421551SDag-Erling Smørgrav<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A 830bc421551SDag-Erling SmørgravWalk through Time</a> 831bc421551SDag-Erling Smørgravsurveys the evolution of timekeeping.</li> 832bc421551SDag-Erling Smørgrav<li>The history of daylight saving time is surveyed in <a 833bc421551SDag-Erling Smørgravhref="http://www.webexhibits.org/daylightsaving/">About Daylight 834bc421551SDag-Erling SmørgravSaving Time – History, rationale, laws & dates</a> and summarized in 835bc421551SDag-Erling Smørgrav<a href="http://seizethedaylight.com/dst/">A Brief 836bc421551SDag-Erling SmørgravHistory of Daylight Saving Time</a>.</li> 837bc421551SDag-Erling Smørgrav<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time 838bc421551SDag-Erling SmørgravLords</a> discusses how authoritarians manipulate civil time.</li> 839bc421551SDag-Erling Smørgrav<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a> 840bc421551SDag-Erling Smørgravcontains guidelines and best practices for software applications that 841bc421551SDag-Erling Smørgravdeal with civil time.</li> 84246c59934SDag-Erling Smørgrav<li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of 843bc421551SDag-Erling Smørgravthe International Date Line</a> tells the story of the most important 844bc421551SDag-Erling Smørgravtime zone boundary.</li> 845bc421551SDag-Erling Smørgrav<li><a href="http://statoids.com/tconcept.html">Basic Time 846bc421551SDag-Erling SmørgravZone Concepts</a> discusses terminological issues behind time zones.</li> 847bc421551SDag-Erling Smørgrav</ul> 848bc421551SDag-Erling Smørgrav</section> 849bc421551SDag-Erling Smørgrav 850bc421551SDag-Erling Smørgrav<section> 851bc421551SDag-Erling Smørgrav<h2 id="national">National histories of legal time</h2> 852bc421551SDag-Erling Smørgrav<dl> 853bc421551SDag-Erling Smørgrav<dt>Australia</dt> 854bc421551SDag-Erling Smørgrav<dd>The Parliamentary Library commissioned a <a 85546c59934SDag-Erling Smørgravhref="https://parlinfo.aph.gov.au/parlInfo/download/library/prspub/359V6/upload_binary/359v60.pdf">research 856bc421551SDag-Erling Smørgravpaper on daylight saving time in Australia</a>. 857bc421551SDag-Erling SmørgravThe Bureau of Meteorology publishes a list of <a 858bc421551SDag-Erling Smørgravhref="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation 859bc421551SDag-Erling SmørgravDates of Daylight Savings Time within Australia</a>.</dd> 860bc421551SDag-Erling Smørgrav<dt>Belgium</dt> 861bc421551SDag-Erling Smørgrav<dd>The Royal Observatory of Belgium maintains a table of time in 862bc421551SDag-Erling SmørgravBelgium (in 86346c59934SDag-Erling Smørgrav<a href="https://robinfo.oma.be/nl/astro-info/tijd/" 864bc421551SDag-Erling Smørgravhreflang="nl">Dutch</a> and <a 86546c59934SDag-Erling Smørgravhref="https://robinfo.oma.be/fr/astro-info/heure/" 866bc421551SDag-Erling Smørgravhreflang="fr">French</a>).</dd> 867bc421551SDag-Erling Smørgrav<dt>Brazil</dt> 868bc421551SDag-Erling Smørgrav<dd>The Time Service Department of the National Observatory 869bc421551SDag-Erling Smørgravrecords <a href="http://pcdsh01.on.br/DecHV.html" 870bc421551SDag-Erling Smørgravhreflang="pt-BR">Brazil's daylight saving time decrees (in 871bc421551SDag-Erling SmørgravPortuguese)</a>.</dd> 872bc421551SDag-Erling Smørgrav<dt>Canada</dt> 873bc421551SDag-Erling Smørgrav<dd>National Research Council Canada publishes current 874bc421551SDag-Erling Smørgravand some older information about <a 875bc421551SDag-Erling Smørgravhref="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time 876bc421551SDag-Erling Smørgravzones and daylight saving time</a>.</dd> 877bc421551SDag-Erling Smørgrav<dt>Chile</dt> 878bc421551SDag-Erling Smørgrav<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a 879bc421551SDag-Erling Smørgrav<a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of 880bc421551SDag-Erling SmørgravChile's official time (in Spanish)</a>.</dd> 881bc421551SDag-Erling Smørgrav<dt>China</dt> 882bc421551SDag-Erling Smørgrav<dd>The Hong Kong Observatory maintains a 883bc421551SDag-Erling Smørgrav<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of 884bc421551SDag-Erling Smørgrav summer time in Hong Kong</a>, 885bc421551SDag-Erling Smørgravand Macau's Meteorological and Geophysical Bureau maintains a <a 886bc421551SDag-Erling Smørgravhref="https://www.smg.gov.mo/en/subpage/224/page/174">similar 887bc421551SDag-Erling Smørgravhistory for Macau</a>. 888bc421551SDag-Erling SmørgravUnfortunately the latter is incomplete and has errors.</dd> 889bc421551SDag-Erling Smørgrav<dt>Czech Republic</dt> 890bc421551SDag-Erling Smørgrav<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas" 891bc421551SDag-Erling Smørgravhreflang="cs">When daylight saving time starts and ends (in Czech)</a> 892bc421551SDag-Erling Smørgravsummarizes and cites historical <abbr>DST</abbr> regulations.</dd> 893bc421551SDag-Erling Smørgrav<dt>Germany</dt> 894bc421551SDag-Erling Smørgrav<dd>The National Institute for Science and Technology maintains the <a 895bc421551SDag-Erling Smørgravhref="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation 896bc421551SDag-Erling Smørgravof Legal Time in Germany</a>.</dd> 897bc421551SDag-Erling Smørgrav<dt>Israel</dt> 898*a979394aSDag-Erling Smørgrav<dd><a 899*a979394aSDag-Erling Smørgravhref="https://tz.cs.huji.ac.il">Israel Timezone Files</a> 900*a979394aSDag-Erling Smørgravlists official time-change announcements and laws since 1940, 901*a979394aSDag-Erling Smørgravalmost all in Hebrew.</dd> 902bc421551SDag-Erling Smørgrav<dt>Malaysia</dt> 903bc421551SDag-Erling Smørgrav<dd>See Singapore <a href="#Singapore">below</a>.</dd> 904bc421551SDag-Erling Smørgrav<dt>Mexico</dt> 905bc421551SDag-Erling Smørgrav<dd>The Investigation and Analysis Service of the Mexican Library of 906bc421551SDag-Erling SmørgravCongress has published a <a 907bc421551SDag-Erling Smørgravhref="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm" 908bc421551SDag-Erling Smørgravhreflang="es">history of Mexican local time (in Spanish)</a>.</dd> 909bc421551SDag-Erling Smørgrav<dt>Netherlands</dt> 91046c59934SDag-Erling Smørgrav<dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm" 911bc421551SDag-Erling Smørgravhreflang="nl">Legal time in the Netherlands (in Dutch)</a> 912bc421551SDag-Erling Smørgravcovers the history of local time in the Netherlands from ancient times.</dd> 913bc421551SDag-Erling Smørgrav<dt>New Zealand</dt> 914bc421551SDag-Erling Smørgrav<dd>The Department of Internal Affairs maintains a brief <a 915bc421551SDag-Erling Smørgravhref="https://www.dia.govt.nz/Daylight-Saving-History">History of 916bc421551SDag-Erling SmørgravDaylight Saving</a>.</dd> 91775411d15SDag-Erling Smørgrav<dt>Palestine</dt> 91875411d15SDag-Erling Smørgrav<dd>The Ministry of Telecom and IT publishes a <a 91946c59934SDag-Erling Smørgravhref="https://mtit.pna.ps/home/TimeZone" 92075411d15SDag-Erling Smørgravhreflang="ar">history of clock changes (in Arabic)</a>.</dd> 921bc421551SDag-Erling Smørgrav<dt>Portugal</dt> 922bc421551SDag-Erling Smørgrav<dd>The Lisbon Astronomical Observatory publishes a 923bc421551SDag-Erling Smørgrav<a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of 924bc421551SDag-Erling Smørgravlegal time (in Portuguese)</a>.</dd> 925bc421551SDag-Erling Smørgrav<dt>Singapore</dt> 926bc421551SDag-Erling Smørgrav<dd><a id="Singapore" 927bc421551SDag-Erling Smørgravhref="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why 928bc421551SDag-Erling Smørgravis Singapore in the "Wrong" Time Zone?</a> details the 929bc421551SDag-Erling Smørgravhistory of legal time in Singapore and Malaysia.</dd> 930bc421551SDag-Erling Smørgrav<dt>United Kingdom</dt> 931bc421551SDag-Erling Smørgrav<dd><a 932bc421551SDag-Erling Smørgravhref="https://www.polyomino.org.uk/british-time/">History of 933bc421551SDag-Erling Smørgravlegal time in Britain</a> discusses in detail the country 934bc421551SDag-Erling Smørgravwith perhaps the best-documented history of clock adjustments.</dd> 935bc421551SDag-Erling Smørgrav<dt>United States</dt> 936bc421551SDag-Erling Smørgrav<dd>The Department of Transportation's <a 937bc421551SDag-Erling Smørgravhref="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent 938bc421551SDag-Erling SmørgravTime Zone Proceedings</a> lists changes to time zone boundaries.</dd> 939bc421551SDag-Erling Smørgrav<dt>Uruguay</dt> 940bc421551SDag-Erling Smørgrav<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan 941bc421551SDag-Erling SmørgravNavy (SOHMA) publishes an annual <a 942bc421551SDag-Erling Smørgravhref="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac 943bc421551SDag-Erling Smørgrav(in Spanish)</a>.</dd> 944bc421551SDag-Erling Smørgrav</dl> 945bc421551SDag-Erling Smørgrav</section> 946bc421551SDag-Erling Smørgrav 947bc421551SDag-Erling Smørgrav<section> 948bc421551SDag-Erling Smørgrav<h2 id="costs">Costs and benefits of time shifts</h2> 949bc421551SDag-Erling Smørgrav<p>Various sources argue for and against daylight saving time and time 950bc421551SDag-Erling Smørgravzone shifts, and many scientific studies have been conducted. This 951bc421551SDag-Erling Smørgravsection summarizes reviews and position statements based on 952bc421551SDag-Erling Smørgravscientific literature in the area.</p> 953bc421551SDag-Erling Smørgrav<ul> 95446c59934SDag-Erling Smørgrav<li>In 2022 the American Medical Association issued a 95546c59934SDag-Erling Smørgrav<a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement 95646c59934SDag-Erling Smørgravsupporting permanent standard time</a> on health grounds.</li> 957bc421551SDag-Erling Smørgrav<li>Carey RN, Sarma KM. 958bc421551SDag-Erling Smørgrav<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of 959bc421551SDag-Erling Smørgravdaylight saving time on road traffic collision risk: a systematic 960bc421551SDag-Erling Smørgravreview</a>. 961bc421551SDag-Erling Smørgrav<em>BMJ Open.</em> 2017;7(6):e014319. doi:<a href="https://doi.org/10.1136/bmjopen-2016-014319">10.1136/bmjopen-2016-014319</a>. 962bc421551SDag-Erling SmørgravThis reviews research literature and concludes that the evidence 963bc421551SDag-Erling Smørgravneither supports nor refutes road safety benefits from 964bc421551SDag-Erling Smørgravshifts in time zones.</li> 965bc421551SDag-Erling Smørgrav<li>Havranek T, Herman D, Irsova D. 966bc421551SDag-Erling Smørgrav<a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does 96746c59934SDag-Erling Smørgravdaylight saving save electricity? A meta-analysis</a>. 968bc421551SDag-Erling Smørgrav<em>Energy J.</em> 2018;39(2):35–61. 969bc421551SDag-Erling Smørgravdoi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>. 970bc421551SDag-Erling SmørgravThis analyzes research literature and concludes, "Electricity savings 971bc421551SDag-Erling Smørgravare larger for countries farther away from the equator, while 972bc421551SDag-Erling Smørgravsubtropical regions consume more electricity because of <abbr>DST</abbr>."</li> 97346c59934SDag-Erling Smørgrav<li>Malow BA. <a 97446c59934SDag-Erling Smørgravhref="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time 97546c59934SDag-Erling Smørgravto abolish the clock change and adopt permanent 97646c59934SDag-Erling Smørgravstandard time in the United States: 97746c59934SDag-Erling Smørgrava Sleep Research Society position statement</a>. 97846c59934SDag-Erling Smørgrav<em>Sleep.</em> 2022;45(12):zsac236. 97946c59934SDag-Erling Smørgravdoi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>. 98046c59934SDag-Erling SmørgravAfter reviewing the scientific literature, the Sleep Research Society 98146c59934SDag-Erling Smørgravadvocates permanent standard time due to its health benefits. 98246c59934SDag-Erling Smørgrav<li>Rishi MA, Cheng JY, Strang AR <em>et al</em>. 98346c59934SDag-Erling Smørgrav<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time 98446c59934SDag-Erling Smørgravis the optimal choice for health and safety: 985bc421551SDag-Erling Smørgravan American Academy of Sleep Medicine position statement</a>. 98646c59934SDag-Erling Smørgrav<em>J Clin Sleep Med.</em> 2024;20(1):121–125. 98746c59934SDag-Erling Smørgravdoi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>. 98846c59934SDag-Erling SmørgravThe AASM argues for permanent standard time due to health and safety risks 98946c59934SDag-Erling Smørgravand economic costs of both <abbr>DST</abbr> transitions and 99046c59934SDag-Erling Smørgravpermanent <abbr>DST</abbr>.</li> 991bc421551SDag-Erling Smørgrav<li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>. 99246c59934SDag-Erling Smørgrav<a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why 993bc421551SDag-Erling Smørgravshould we abolish Daylight Saving Time?</a> 99446c59934SDag-Erling Smørgrav<em>J Biol Rhythms.</em> 2019;34(3):227–230. 995bc421551SDag-Erling Smørgravdoi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>. 99646c59934SDag-Erling SmørgravThe Society for Research on Biological Rhythms 997bc421551SDag-Erling Smørgravopposes DST changes and permanent DST, and advocates that governments adopt 998bc421551SDag-Erling Smørgrav"permanent Standard Time for the health and safety of their citizens".</li> 999bc421551SDag-Erling Smørgrav</ul> 1000bc421551SDag-Erling Smørgrav</section> 1001bc421551SDag-Erling Smørgrav 1002bc421551SDag-Erling Smørgrav<section> 1003bc421551SDag-Erling Smørgrav<h2 id="precision">Precision timekeeping</h2> 1004bc421551SDag-Erling Smørgrav<ul> 1005bc421551SDag-Erling Smørgrav<li><a 1006bc421551SDag-Erling Smørgravhref="http://leapsecond.com/hpan/an1289.pdf">The 1007bc421551SDag-Erling SmørgravScience of Timekeeping</a> is a thorough introduction 1008bc421551SDag-Erling Smørgravto the theory and practice of precision timekeeping.</li> 1009bc421551SDag-Erling Smørgrav<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of 101075411d15SDag-Erling SmørgravTime 2016</a> contains several freely readable papers.</li> 1011bc421551SDag-Erling Smørgrav<li><a href="https://www.ntp.org"><abbr 1012bc421551SDag-Erling Smørgravtitle="Network Time Protocol">NTP</abbr>: The Network 1013bc421551SDag-Erling SmørgravTime Protocol</a> (Internet <abbr>RFC</abbr> 5905) 1014bc421551SDag-Erling Smørgravdiscusses how to synchronize clocks of 1015bc421551SDag-Erling SmørgravInternet hosts.</li> 1016bc421551SDag-Erling Smørgrav<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a> 1017bc421551SDag-Erling Smørgravfamily of software algorithms can achieve accuracy to a few tens of 1018bc421551SDag-Erling Smørgravnanoseconds in scalable server farms without special hardware.</li> 1019bc421551SDag-Erling Smørgrav<li>The <a 1020bc421551SDag-Erling Smørgravhref="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision 1021bc421551SDag-Erling SmørgravTime Protocol</a> (<abbr 1022bc421551SDag-Erling Smørgravtitle="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588) 1023bc421551SDag-Erling Smørgravcan achieve submicrosecond clock accuracy on a local area network 1024bc421551SDag-Erling Smørgravwith special-purpose hardware.</li> 1025bc421551SDag-Erling Smørgrav<li><a 1026bc421551SDag-Erling Smørgravhref="https://datatracker.ietf.org/doc/html/rfc4833">Timezone 1027bc421551SDag-Erling SmørgravOptions for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> 1028bc421551SDag-Erling Smørgrav(Internet <abbr>RFC</abbr> 4833) 1029bc421551SDag-Erling Smørgravspecifies a <a 1030bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a> 1031bc421551SDag-Erling Smørgravoption for a server to configure 1032bc421551SDag-Erling Smørgrava client's time zone and daylight saving settings automatically.</li> 1033bc421551SDag-Erling Smørgrav<li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time 1034bc421551SDag-Erling SmørgravScales</a> describes astronomical time scales like 1035bc421551SDag-Erling Smørgrav<abbr title="Terrestrial Dynamic Time">TDT</abbr>, 1036bc421551SDag-Erling Smørgrav<abbr title="Geocentric Coordinate Time">TCG</abbr>, and 1037bc421551SDag-Erling Smørgrav<abbr title="Barycentric Dynamic Time">TDB</abbr>. 1038bc421551SDag-Erling Smørgrav<li>The <a href="https://www.iau.org"><abbr 1039bc421551SDag-Erling Smørgravtitle="International Astronomical Union">IAU</abbr></a>'s <a 1040bc421551SDag-Erling Smørgravhref="https://www.iausofa.org"><abbr 1041bc421551SDag-Erling Smørgravtitle="Standards Of Fundamental Astronomy">SOFA</abbr></a> 1042bc421551SDag-Erling Smørgravcollection contains C and <a 1043bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Fortran">Fortran</a> 1044bc421551SDag-Erling Smørgravcode for converting among time scales like 1045bc421551SDag-Erling Smørgrav<abbr title="International Atomic Time">TAI</abbr>, 1046bc421551SDag-Erling Smørgrav<abbr>TDB</abbr>, <abbr>TDT</abbr> and 1047bc421551SDag-Erling Smørgrav<abbr>UTC</abbr>. It is freely available under the 1048bc421551SDag-Erling Smørgrav<a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li> 1049bc421551SDag-Erling Smørgrav<li><a 1050bc421551SDag-Erling Smørgravhref="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock 1051bc421551SDag-Erling Smørgrav– Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the 1052bc421551SDag-Erling Smørgravdiverse local time 1053bc421551SDag-Erling Smørgravscales used by each landed mission on Mars.</li> 1054bc421551SDag-Erling Smørgrav<li><a href="http://leapsecond.com">LeapSecond.com</a> is 1055bc421551SDag-Erling Smørgravdedicated not only to leap seconds but to precise time and frequency 1056bc421551SDag-Erling Smørgravin general. It covers the state of the art in amateur timekeeping, and 1057bc421551SDag-Erling Smørgravhow the art has progressed over the past few decades.</li> 1058bc421551SDag-Erling Smørgrav<li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a 1059bc421551SDag-Erling Smørgravhref="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency 1060bc421551SDag-Erling Smørgravand time-signal emissions</a>, International Telecommunication Union – 1061bc421551SDag-Erling SmørgravRadiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li> 1062bc421551SDag-Erling Smørgrav<li><a 1063bc421551SDag-Erling Smørgravhref="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr 1064bc421551SDag-Erling Smørgravtitle="International Earth Rotation and Reference Systems Service">IERS</abbr> 1065bc421551SDag-Erling SmørgravBulletins</a> contains official publications of the International 1066bc421551SDag-Erling SmørgravEarth Rotation and Reference Systems Service, which decides when leap 1067bc421551SDag-Erling Smørgravseconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds 1068bc421551SDag-Erling Smørgravvia an optional "<code>right</code>" configuration where a computer's internal 1069bc421551SDag-Erling Smørgrav<code>time_t</code> integer clock counts every <abbr>TAI</abbr> second, 1070bc421551SDag-Erling Smørgravas opposed to the default "<code>posix</code>" configuration 1071bc421551SDag-Erling Smørgravwhere the internal clock ignores leap seconds. 1072bc421551SDag-Erling SmørgravThe two configurations agree for timestamps starting with 1972-01-01 00:00:00 1073bc421551SDag-Erling Smørgrav<abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for 1074bc421551SDag-Erling Smørgravtimestamps starting with <code>time_t</code> 78 796 800, 1075bc421551SDag-Erling Smørgravwhich corresponds to the first leap second 1076bc421551SDag-Erling Smørgrav1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration, 1077bc421551SDag-Erling Smørgravand to 1078bc421551SDag-Erling Smørgrav1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration. 1079bc421551SDag-Erling SmørgravIn practice the two configurations also agree for timestamps before 1080bc421551SDag-Erling Smørgrav1972 even though the historical situation is messy, partly because 1081bc421551SDag-Erling Smørgravneither <abbr>UTC</abbr> nor <abbr>TAI</abbr> 108275411d15SDag-Erling Smørgravis well-defined for sufficiently old timestamps.</li> 1083*a979394aSDag-Erling Smørgrav<li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The 1084*a979394aSDag-Erling Smørgrav<abbr>NTP</abbr> Leap Second File</a> covers the text file 1085*a979394aSDag-Erling Smørgrav<code>leap-seconds.list</code>, which lists the currently known leap seconds. 1086*a979394aSDag-Erling SmørgravThe <abbr>IERS</abbr> maintains this file, and a copy is distributed by 1087*a979394aSDag-Erling Smørgrav<code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like 1088*a979394aSDag-Erling Smørgrav<a href="https://www.ntp.org">classic 1089*a979394aSDag-Erling Smørgrav<code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a> 1090*a979394aSDag-Erling Smørgravand <a href="https://ntpsec.org">NTPsec</a>. 1091*a979394aSDag-Erling SmørgravThe <code><abbr>tz</abbr></code> database also distributes leap second 1092*a979394aSDag-Erling Smørgravinformation in a differently-formatted <code>leapseconds</code> text file, 1093*a979394aSDag-Erling Smørgravas well as in the "<code>right</code>" configuration in binary form; for 1094*a979394aSDag-Erling Smørgravexample, <code>right/UTC</code> can be used 1095*a979394aSDag-Erling Smørgravby <a href="https://chrony-project.org"><code>chrony</code></a>, 1096*a979394aSDag-Erling Smørgravanother <abbr>NTP</abbr> implementation.</li> 1097bc421551SDag-Erling Smørgrav<li><a href="https://developers.google.com/time/smear">Leap Smear</a> 1098bc421551SDag-Erling Smørgravdiscusses how to gradually adjust <abbr>POSIX</abbr> clocks near a 1099bc421551SDag-Erling Smørgravleap second so that they disagree with <abbr>UTC</abbr> by at most a 1100bc421551SDag-Erling Smørgravhalf second, even though every <abbr>POSIX</abbr> minute has exactly 1101bc421551SDag-Erling Smørgravsixty seconds. This approach works with the default <code><abbr>tz</abbr></code> 1102bc421551SDag-Erling Smørgrav"<code>posix</code>" configuration, is <a 1103bc421551SDag-Erling Smørgravhref="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by 1104*a979394aSDag-Erling Smørgravthe abovementioned <abbr>NTP</abbr> implementations, <a 1105bc421551SDag-Erling Smørgravhref="https://github.com/google/unsmear">supports</a> conversion between 1106bc421551SDag-Erling Smørgrav<abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major 1107bc421551SDag-Erling Smørgravcloud service providers. However, according to 1108bc421551SDag-Erling Smørgrav<a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">§3.7.1 of 1109bc421551SDag-Erling SmørgravNetwork Time Protocol Best Current Practices</a> 1110bc421551SDag-Erling Smørgrav(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for 1111bc421551SDag-Erling Smørgravapplications requiring accurate <abbr>UTC</abbr> or civil time, 1112bc421551SDag-Erling Smørgravand is intended for use only in single, well-controlled environments.</li> 1113bc421551SDag-Erling Smørgrav<li>The <a 1114bc421551SDag-Erling Smørgravhref="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap 1115bc421551SDag-Erling SmørgravSecond Discussion List</a> covers <a 1116bc421551SDag-Erling Smørgravhref="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy 1117bc421551SDag-Erling Smørgravand Klepczynski's 1999 proposal to discontinue leap seconds</a>, 1118bc421551SDag-Erling Smørgravdiscussed further in 1119bc421551SDag-Erling Smørgrav<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The 1120bc421551SDag-Erling Smørgravleap second: its history and possible future</a>. 1121bc421551SDag-Erling Smørgrav<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr> 1122bc421551SDag-Erling Smørgravmight be redefined 1123bc421551SDag-Erling Smørgravwithout Leap Seconds</a> gives pointers on this 112475411d15SDag-Erling Smørgravcontentious issue. 112575411d15SDag-Erling SmørgravThe General Conference on Weights and Measures 112646c59934SDag-Erling Smørgrav<a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a> 1127*a979394aSDag-Erling Smørgravto discontinue the use of leap seconds by 2035, and requested that no 1128*a979394aSDag-Erling Smørgravdiscontinuous adjustments be made to UTC for at least a century. 112946c59934SDag-Erling SmørgravThe World Radiocommunication Conference <a 113046c59934SDag-Erling Smørgravhref="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved 113146c59934SDag-Erling Smørgravin 2023</a> to cooperate with this process. 1132*a979394aSDag-Erling Smørgrav<a href="https://www.preprints.org/manuscript/202406.0043/v1">A proposal 1133*a979394aSDag-Erling Smørgravto change the leap-second adjustments to Coordinated Universal Time</a> 1134*a979394aSDag-Erling Smørgrav(doi:<a href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>) 1135*a979394aSDag-Erling Smørgravwould replace leap seconds with 13-second leap smears occurring once per 1136*a979394aSDag-Erling Smørgravdecade until 2100, with leap smears after that gradually increasing in size. 1137*a979394aSDag-Erling SmørgravHowever, there is still no consensus on whether this is the best way 1138*a979394aSDag-Erling Smørgravto replace leap seconds. 113975411d15SDag-Erling Smørgrav</li> 1140bc421551SDag-Erling Smørgrav</ul> 1141bc421551SDag-Erling Smørgrav</section> 1142bc421551SDag-Erling Smørgrav 1143bc421551SDag-Erling Smørgrav<section> 1144bc421551SDag-Erling Smørgrav<h2 id="notation">Time notation</h2> 1145bc421551SDag-Erling Smørgrav<ul> 1146bc421551SDag-Erling Smørgrav<li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data 1147bc421551SDag-Erling SmørgravRepository (<abbr>CLDR</abbr>) Project</a> has localizations for time 1148bc421551SDag-Erling Smørgravzone names, abbreviations, identifiers, and formats. For example, it 1149bc421551SDag-Erling Smørgravcontains French translations for "Eastern European Summer Time", 1150bc421551SDag-Erling Smørgrav"<abbr title="Eastern European Summer Time">EEST</abbr>", and 1151bc421551SDag-Erling Smørgrav"Bucharest". Its 1152bc421551SDag-Erling Smørgrav<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type 1153bc421551SDag-Erling Smørgravcharts</a> show these values for many locales. Data values are available in 1154bc421551SDag-Erling Smørgravboth <abbr title="Locale Data Markup Language">LDML</abbr> 1155bc421551SDag-Erling Smørgrav(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>. 1156bc421551SDag-Erling Smørgrav<li> 1157bc421551SDag-Erling Smørgrav<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of 1158bc421551SDag-Erling Smørgravthe international standard date and time notation</a> covers 1159bc421551SDag-Erling Smørgrav<a 1160bc421551SDag-Erling Smørgravhref="https://www.iso.org/standard/70907.html"><em><abbr 1161bc421551SDag-Erling Smørgravtitle="International Organization for Standardization">ISO</abbr> 1162bc421551SDag-Erling Smørgrav8601-1:2019 – Date and time – Representations for information 1163bc421551SDag-Erling Smørgravinterchange – Part 1: Basic rules</em></a>.</li> 1164bc421551SDag-Erling Smørgrav<li> 1165bc421551SDag-Erling Smørgrav<a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr> 1166bc421551SDag-Erling SmørgravSchema: Datatypes – dateTime</a> specifies a format inspired by 1167bc421551SDag-Erling Smørgrav<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li> 1168bc421551SDag-Erling Smørgrav<li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">§3.3 of 1169bc421551SDag-Erling SmørgravInternet Message Format</a> (Internet <abbr>RFC</abbr> 5322) 1170bc421551SDag-Erling Smørgravspecifies the time notation used in email and <a 1171bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a> 1172bc421551SDag-Erling Smørgravheaders.</li> 1173bc421551SDag-Erling Smørgrav<li> 1174bc421551SDag-Erling Smørgrav<a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time 1175bc421551SDag-Erling Smørgravon the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) 1176*a979394aSDag-Erling Smørgravspecifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols. 1177*a979394aSDag-Erling SmørgravAn extension, <a href="https://datatracker.ietf.org/doc/html/rfc9557">Date 1178*a979394aSDag-Erling Smørgravand Time on the Internet: Timestamps with Additional Information</a> 1179*a979394aSDag-Erling Smørgrav(Internet <abbr>RFC</abbr> 9557) extends this profile 1180*a979394aSDag-Erling Smørgravto let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp 1181*a979394aSDag-Erling Smørgravvia suffixes like "<code>[Asia/Tokyo]</code>". 1182bc421551SDag-Erling Smørgrav<li> 1183bc421551SDag-Erling Smørgrav<a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time 1184bc421551SDag-Erling SmørgravFormats on the Web</a> surveys web- and Internet-oriented date and time 1185bc421551SDag-Erling Smørgravformats.</li> 1186bc421551SDag-Erling Smørgrav<li>Alphabetic time zone abbreviations should not be used as unique 1187bc421551SDag-Erling Smørgravidentifiers for <abbr>UT</abbr> offsets as they are ambiguous in 1188bc421551SDag-Erling Smørgravpractice. For example, in English-speaking North America 1189bc421551SDag-Erling Smørgrav"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>, 1190bc421551SDag-Erling Smørgravbut in China it denotes 8 hours ahead of <abbr>UT</abbr>, 1191bc421551SDag-Erling Smørgravand French-speaking North Americans prefer 1192bc421551SDag-Erling Smørgrav"<abbr title="Heure Normale du Centre">HNC</abbr>" to 1193bc421551SDag-Erling Smørgrav"<abbr>CST</abbr>". The <code><abbr>tz</abbr></code> 1194bc421551SDag-Erling Smørgravdatabase contains English abbreviations for many timestamps; 1195bc421551SDag-Erling Smørgravunfortunately some of these abbreviations were merely the database maintainers' 1196bc421551SDag-Erling Smørgravinventions, and these have been removed when possible.</li> 1197bc421551SDag-Erling Smørgrav<li>Numeric time zone abbreviations typically count hours east of 1198bc421551SDag-Erling Smørgrav<abbr>UT</abbr>, e.g., +09 for Japan and 1199*a979394aSDag-Erling Smørgrav−10 for Hawaii. However, <abbr>POSIX</abbr> proleptic 1200*a979394aSDag-Erling Smørgrav<code><abbr>TZ</abbr></code> settings use the opposite convention. 1201bc421551SDag-Erling SmørgravFor example, one might use <code><abbr>TZ</abbr>="<abbr 1202bc421551SDag-Erling Smørgravtitle="Japan Standard Time">JST</abbr>-9"</code> and 1203bc421551SDag-Erling Smørgrav<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code> 1204bc421551SDag-Erling Smørgravfor Japan and Hawaii, respectively. If the 1205bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> database is available, it is usually better to use 1206bc421551SDag-Erling Smørgravsettings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and 1207bc421551SDag-Erling Smørgrav<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid 1208bc421551SDag-Erling Smørgravconfusion, handle old timestamps better, and insulate you better from 1209bc421551SDag-Erling Smørgravany future changes to the rules. One should never set 1210bc421551SDag-Erling Smørgrav<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like 1211bc421551SDag-Erling Smørgrav<code>"GMT-9"</code>, though, since this would incorrectly imply that 1212bc421551SDag-Erling Smørgravlocal time is nine hours ahead of <abbr>UT</abbr> and the time zone 1213bc421551SDag-Erling Smørgravis called "<abbr>GMT</abbr>".</li> 1214bc421551SDag-Erling Smørgrav</ul> 1215bc421551SDag-Erling Smørgrav</section> 1216bc421551SDag-Erling Smørgrav 1217bc421551SDag-Erling Smørgrav<section> 1218bc421551SDag-Erling Smørgrav<h2 id="see-also">See also</h2> 1219bc421551SDag-Erling Smørgrav<ul> 1220bc421551SDag-Erling Smørgrav<li><a href="theory.html">Theory and pragmatics of the 1221bc421551SDag-Erling Smørgrav<code><abbr>tz</abbr></code> code and data</a></li> 1222bc421551SDag-Erling Smørgrav<li><a href="tz-art.html">Time and the Arts</a></li> 1223bc421551SDag-Erling Smørgrav</ul> 1224bc421551SDag-Erling Smørgrav</section> 1225bc421551SDag-Erling Smørgrav 1226bc421551SDag-Erling Smørgrav<footer> 1227bc421551SDag-Erling Smørgrav<hr> 1228bc421551SDag-Erling SmørgravThis web page is in the public domain, so clarified as of 1229bc421551SDag-Erling Smørgrav2009-05-17 by Arthur David Olson. 1230bc421551SDag-Erling Smørgrav<br> 1231bc421551SDag-Erling SmørgravPlease send corrections to this web page to the 1232bc421551SDag-Erling Smørgrav<a href="mailto:tz@iana.org">time zone mailing list</a>. 1233bc421551SDag-Erling Smørgrav</footer> 1234bc421551SDag-Erling Smørgrav</body> 1235bc421551SDag-Erling Smørgrav</html> 1236