1bc421551SDag-Erling Smørgrav<!DOCTYPE html> 2bc421551SDag-Erling Smørgrav<html lang="en"> 3bc421551SDag-Erling Smørgrav<head> 4bc421551SDag-Erling Smørgrav<title>How to Read the tz Database</title> 5bc421551SDag-Erling Smørgrav<meta charset="UTF-8"> 6bc421551SDag-Erling Smørgrav<style> 7bc421551SDag-Erling Smørgravpre {margin-left: 2em; white-space: pre-wrap;} 8bc421551SDag-Erling Smørgravpre.td {margin-left: 0;} 9bc421551SDag-Erling Smørgravtd {text-align: center;} 10bc421551SDag-Erling Smørgravtable {border: 1px outset;} 11bc421551SDag-Erling Smørgravth, td {border: 1px inset;} 12bc421551SDag-Erling Smørgravtable.rule {border: none; margin: auto;} 13bc421551SDag-Erling Smørgravtd.footnote {text-align: left;} 14bc421551SDag-Erling Smørgrav</style> 15bc421551SDag-Erling Smørgrav</head> 16bc421551SDag-Erling Smørgrav<body> 17bc421551SDag-Erling Smørgrav<h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 18bc421551SDag-Erling SmørgravDatabase</a> Source Files</h2> 19bc421551SDag-Erling Smørgrav<h3>by Bill Seymour</h3> 20bc421551SDag-Erling Smørgrav<p>This guide uses the <code>America/Chicago</code> and 21bc421551SDag-Erling Smørgrav<code>Pacific/Honolulu</code> zones as examples of how to infer 22bc421551SDag-Erling Smørgravtimes of day from the <a href="tz-link.html">tz database</a> 23bc421551SDag-Erling Smørgravsource files. It might be helpful, but not absolutely necessary, 24bc421551SDag-Erling Smørgravfor the reader to have already downloaded the 25bc421551SDag-Erling Smørgravlatest release of the database and become familiar with the basic layout 26bc421551SDag-Erling Smørgravof the data files. The format is explained in the “man 27bc421551SDag-Erling Smørgravpage” for the zic compiler, <code>zic.8.txt</code>, in 28bc421551SDag-Erling Smørgravthe <code>code</code> subdirectory. 29bc421551SDag-Erling SmørgravAlthough this guide covers many of the common cases, it is not a 30bc421551SDag-Erling Smørgravcomplete summary of what zic accepts; the man page is the 31bc421551SDag-Erling Smørgravauthoritative reference.</p> 32bc421551SDag-Erling Smørgrav 33bc421551SDag-Erling Smørgrav<p>We’ll begin by talking about the rules for changing between standard 34bc421551SDag-Erling Smørgravand daylight saving time since we’ll need that information when we talk 35bc421551SDag-Erling Smørgravabout the zones.</p> 36bc421551SDag-Erling Smørgrav 37bc421551SDag-Erling Smørgrav<p>First, let’s consider the special daylight saving time rules 38bc421551SDag-Erling Smørgravfor Chicago (from the <code>northamerica</code> file in 39bc421551SDag-Erling Smørgravthe <code>data</code> subdirectory):</p> 40bc421551SDag-Erling Smørgrav 41bc421551SDag-Erling Smørgrav<table> 42bc421551SDag-Erling Smørgrav<tr> 43bc421551SDag-Erling Smørgrav <th colspan="6">From the Source File</th> 44bc421551SDag-Erling Smørgrav</tr> 45bc421551SDag-Erling Smørgrav<tr> 46bc421551SDag-Erling Smørgrav <td colspan="6"> 47bc421551SDag-Erling Smørgrav <table class="rule"> 48bc421551SDag-Erling Smørgrav <tr><td style="border:none;text-align:left"> 49bc421551SDag-Erling Smørgrav<pre class="td"> 50bc421551SDag-Erling Smørgrav#Rule NAME FROM TO - IN ON AT SAVE LETTER 51bc421551SDag-Erling SmørgravRule Chicago 1920 only - Jun 13 2:00 1:00 D 52bc421551SDag-Erling SmørgravRule Chicago 1920 1921 - Oct lastSun 2:00 0 S 53bc421551SDag-Erling SmørgravRule Chicago 1921 only - Mar lastSun 2:00 1:00 D 54bc421551SDag-Erling SmørgravRule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 55bc421551SDag-Erling SmørgravRule Chicago 1922 1954 - Sep lastSun 2:00 0 S 56bc421551SDag-Erling SmørgravRule Chicago 1955 1966 - Oct lastSun 2:00 0 S 57bc421551SDag-Erling Smørgrav</pre> 58bc421551SDag-Erling Smørgrav </td></tr></table></td> 59bc421551SDag-Erling Smørgrav</tr> 60bc421551SDag-Erling Smørgrav<tr> 61bc421551SDag-Erling Smørgrav <th colspan="6">Reformatted a Bit</th> 62bc421551SDag-Erling Smørgrav</tr> 63bc421551SDag-Erling Smørgrav<tr> 64bc421551SDag-Erling Smørgrav <th>From</th> 65bc421551SDag-Erling Smørgrav <th>To</th> 66bc421551SDag-Erling Smørgrav <th colspan="2">On</th> 67bc421551SDag-Erling Smørgrav <th>At</th> 68bc421551SDag-Erling Smørgrav <th>Action</th> 69bc421551SDag-Erling Smørgrav</tr> 70bc421551SDag-Erling Smørgrav<tr> 71bc421551SDag-Erling Smørgrav <td colspan="2">1920 only</td> 72bc421551SDag-Erling Smørgrav <td colspan="2">June 13<small><sup>th</sup></small></td> 73bc421551SDag-Erling Smørgrav <td rowspan="6">02:00 local</td> 74bc421551SDag-Erling Smørgrav <td>go to daylight saving time</td> 75bc421551SDag-Erling Smørgrav</tr> 76bc421551SDag-Erling Smørgrav<tr> 77bc421551SDag-Erling Smørgrav <td>1920</td> 78bc421551SDag-Erling Smørgrav <td>1921</td> 79bc421551SDag-Erling Smørgrav <td rowspan="5">last Sunday</td> 80bc421551SDag-Erling Smørgrav <td>in October</td> 81bc421551SDag-Erling Smørgrav <td>return to standard time</td> 82bc421551SDag-Erling Smørgrav</tr> 83bc421551SDag-Erling Smørgrav<tr> 84bc421551SDag-Erling Smørgrav <td colspan="2">1921 only</td> 85bc421551SDag-Erling Smørgrav <td>in March</td> 86bc421551SDag-Erling Smørgrav <td rowspan="2">go to daylight saving time</td> 87bc421551SDag-Erling Smørgrav</tr> 88bc421551SDag-Erling Smørgrav<tr> 89bc421551SDag-Erling Smørgrav <td rowspan="2">1922</td> 90bc421551SDag-Erling Smørgrav <td>1966</td> 91bc421551SDag-Erling Smørgrav <td>in April</td> 92bc421551SDag-Erling Smørgrav</tr> 93bc421551SDag-Erling Smørgrav<tr> 94bc421551SDag-Erling Smørgrav <td>1954</td> 95bc421551SDag-Erling Smørgrav <td>in September</td> 96bc421551SDag-Erling Smørgrav <td rowspan="2">return to standard time</td> 97bc421551SDag-Erling Smørgrav</tr> 98bc421551SDag-Erling Smørgrav<tr> 99bc421551SDag-Erling Smørgrav <td>1955</td> 100bc421551SDag-Erling Smørgrav <td>1966</td> 101bc421551SDag-Erling Smørgrav <td>in October</td> 102bc421551SDag-Erling Smørgrav</tr> 103bc421551SDag-Erling Smørgrav</table> 104bc421551SDag-Erling Smørgrav 105bc421551SDag-Erling Smørgrav<p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the 106bc421551SDag-Erling Smørgravfirst and last calendar years defining a contiguous range over which a specific 107bc421551SDag-Erling SmørgravRule line is to apply. The keyword <code>only</code> can be used in the 108bc421551SDag-Erling Smørgrav<code>TO</code> field to repeat the value of the <code>FROM</code> field in the 109bc421551SDag-Erling Smørgravevent that a rule should only apply to a single year. Often, the keyword 110bc421551SDag-Erling Smørgrav<code>max</code> is used to extend a rule’s application into the 111bc421551SDag-Erling Smørgravindefinite future; it is a platform-agnostic stand-in for the largest 112bc421551SDag-Erling Smørgravrepresentable year. 113bc421551SDag-Erling Smørgrav 114bc421551SDag-Erling Smørgrav<p>The next column, <code>-</code>, is reserved; for compatibility with earlier 115bc421551SDag-Erling Smørgravreleases, it always contains a hyphen, which acts as a kind of null value. 116bc421551SDag-Erling SmørgravPrior to the 2020b release, it was called the <code>TYPE</code> field, though 117bc421551SDag-Erling Smørgravit had not been used in the main data since the 2000e release. 118bc421551SDag-Erling SmørgravAn obsolescent supplementary file used the 119bc421551SDag-Erling Smørgravfield as a proof-of-concept to allow <code>zic</code> to apply a given Rule 120bc421551SDag-Erling Smørgravline only to certain “types” of years within the specified range as 121bc421551SDag-Erling Smørgravdictated by the output of a separate script, such as: only years which would 122bc421551SDag-Erling Smørgravhave a US presidential election, or only years which wouldn’t. 123bc421551SDag-Erling Smørgrav 124bc421551SDag-Erling Smørgrav<p>The <code>SAVE</code> column contains the local (wall clock) offset from 125bc421551SDag-Erling Smørgravlocal standard time. 126bc421551SDag-Erling SmørgravThis is usually either zero for standard time or one hour for daylight 127bc421551SDag-Erling Smørgravsaving time; but there’s no reason, in principle, why it can’t 128bc421551SDag-Erling Smørgravtake on other values. 129bc421551SDag-Erling Smørgrav 130bc421551SDag-Erling Smørgrav<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 131bc421551SDag-Erling Smørgravcolumn can contain a variable 132bc421551SDag-Erling Smørgravpart of the usual abbreviation of the time zone’s name, or it can just 133bc421551SDag-Erling Smørgravbe a hyphen if there’s no variable part. For example, the abbreviation 134bc421551SDag-Erling Smørgravused in the central time zone will be either “CST” or 135bc421551SDag-Erling Smørgrav“CDT”. The variable part is ‘S’ or ‘D’; 136bc421551SDag-Erling Smørgravand, sure enough, that’s just what we find in 137bc421551SDag-Erling Smørgravthe <code>LETTER</code> column 138bc421551SDag-Erling Smørgravin the <code>Chicago</code> rules. More about this when we talk about 139bc421551SDag-Erling Smørgrav“Zone” lines. 140bc421551SDag-Erling Smørgrav 141bc421551SDag-Erling Smørgrav<p>One important thing to notice is that “Rule” lines 142bc421551SDag-Erling Smørgravwant at once to be both <i>transitions</i> and <i>steady states</i>: 143bc421551SDag-Erling Smørgrav<ul> 144bc421551SDag-Erling Smørgrav<li>On the one hand, they represent transitions between standard and 145bc421551SDag-Erling Smørgravdaylight saving time; and any number of Rule lines can be in effect 146bc421551SDag-Erling Smørgravduring a given period (which will always be a non-empty set of 147bc421551SDag-Erling Smørgravcontiguous calendar years).</li> 148bc421551SDag-Erling Smørgrav<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 149bc421551SDag-Erling Smørgravcolumns contain state that exists between transitions. More about this 150bc421551SDag-Erling Smørgravwhen we talk about the US rules.</li> 151bc421551SDag-Erling Smørgrav</ul> 152bc421551SDag-Erling Smørgrav 153bc421551SDag-Erling Smørgrav<p>In the example above, the transition to daylight saving time 154bc421551SDag-Erling Smørgravhappened on the 13<small><sup>th</sup></small> of June in 1920, and on 155bc421551SDag-Erling Smørgravthe last Sunday in March in 1921; but the return to standard time 156bc421551SDag-Erling Smørgravhappened on the last Sunday in October in both of those 157bc421551SDag-Erling Smørgravyears. Similarly, the rule for changing to daylight saving time was 158bc421551SDag-Erling Smørgravthe same from 1922 to 1966; but the rule for returning to standard 159bc421551SDag-Erling Smørgravtime changed in 1955. Got it?</p> 160bc421551SDag-Erling Smørgrav 161bc421551SDag-Erling Smørgrav<p>OK, now for the somewhat more interesting “US” rules:</p> 162bc421551SDag-Erling Smørgrav 163bc421551SDag-Erling Smørgrav<table> 164bc421551SDag-Erling Smørgrav<tr> 165bc421551SDag-Erling Smørgrav <th colspan="6">From the Source File</th> 166bc421551SDag-Erling Smørgrav</tr> 167bc421551SDag-Erling Smørgrav<tr> 168bc421551SDag-Erling Smørgrav <td colspan="6"> 169bc421551SDag-Erling Smørgrav <table class="rule"> 170bc421551SDag-Erling Smørgrav <tr><td style="border:none;text-align:left"> 171bc421551SDag-Erling Smørgrav<pre class="td"> 172bc421551SDag-Erling Smørgrav#Rule NAME FROM TO - IN ON AT SAVE LETTER/S 173bc421551SDag-Erling SmørgravRule US 1918 1919 - Mar lastSun 2:00 1:00 D 174bc421551SDag-Erling SmørgravRule US 1918 1919 - Oct lastSun 2:00 0 S 175bc421551SDag-Erling SmørgravRule US 1942 only - Feb 9 2:00 1:00 W # War 176bc421551SDag-Erling SmørgravRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 177bc421551SDag-Erling SmørgravRule US 1945 only - Sep 30 2:00 0 S 178bc421551SDag-Erling SmørgravRule US 1967 2006 - Oct lastSun 2:00 0 S 179bc421551SDag-Erling SmørgravRule US 1967 1973 - Apr lastSun 2:00 1:00 D 180bc421551SDag-Erling SmørgravRule US 1974 only - Jan 6 2:00 1:00 D 181bc421551SDag-Erling SmørgravRule US 1975 only - Feb 23 2:00 1:00 D 182bc421551SDag-Erling SmørgravRule US 1976 1986 - Apr lastSun 2:00 1:00 D 183bc421551SDag-Erling SmørgravRule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 184bc421551SDag-Erling SmørgravRule US 2007 max - Mar Sun>=8 2:00 1:00 D 185bc421551SDag-Erling SmørgravRule US 2007 max - Nov Sun>=1 2:00 0 S 186bc421551SDag-Erling Smørgrav</pre> 187bc421551SDag-Erling Smørgrav </td></tr></table></td> 188bc421551SDag-Erling Smørgrav</tr> 189bc421551SDag-Erling Smørgrav<tr> 190bc421551SDag-Erling Smørgrav <th colspan="6">Reformatted a Bit</th> 191bc421551SDag-Erling Smørgrav</tr> 192bc421551SDag-Erling Smørgrav<tr> 193bc421551SDag-Erling Smørgrav <th>From</th> 194bc421551SDag-Erling Smørgrav <th>To</th> 195bc421551SDag-Erling Smørgrav <th colspan="2">On</th> 196bc421551SDag-Erling Smørgrav <th>At</th> 197bc421551SDag-Erling Smørgrav <th>Action</th> 198bc421551SDag-Erling Smørgrav</tr> 199bc421551SDag-Erling Smørgrav<tr> 200bc421551SDag-Erling Smørgrav <td rowspan="2">1918</td> 201bc421551SDag-Erling Smørgrav <td rowspan="2">1919</td> 202bc421551SDag-Erling Smørgrav <td rowspan="2">last Sunday</td> 203bc421551SDag-Erling Smørgrav <td>in March</td> 204bc421551SDag-Erling Smørgrav <td rowspan="3">02:00 local</td> 205bc421551SDag-Erling Smørgrav <td>go to daylight saving time</td> 206bc421551SDag-Erling Smørgrav</tr> 207bc421551SDag-Erling Smørgrav<tr> 208bc421551SDag-Erling Smørgrav <td>in October</td> 209bc421551SDag-Erling Smørgrav <td>return to standard time</td> 210bc421551SDag-Erling Smørgrav</tr> 211bc421551SDag-Erling Smørgrav<tr> 212bc421551SDag-Erling Smørgrav <td colspan="2">1942 only</td> 213bc421551SDag-Erling Smørgrav <td colspan="2">February 9<small><sup>th</sup></small></td> 214bc421551SDag-Erling Smørgrav <td>go to “war time”</td> 215bc421551SDag-Erling Smørgrav</tr> 216bc421551SDag-Erling Smørgrav<tr> 217bc421551SDag-Erling Smørgrav <td colspan="2" rowspan="2">1945 only</td> 218bc421551SDag-Erling Smørgrav <td colspan="2">August 14<small><sup>th</sup></small></td> 219bc421551SDag-Erling Smørgrav <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 220bc421551SDag-Erling Smørgrav <td> 221bc421551SDag-Erling Smørgrav rename “war time” to “peace<br>time;” 222bc421551SDag-Erling Smørgrav clocks don’t change 223bc421551SDag-Erling Smørgrav </td> 224bc421551SDag-Erling Smørgrav</tr> 225bc421551SDag-Erling Smørgrav<tr> 226bc421551SDag-Erling Smørgrav <td colspan="2">September 30<small><sup>th</sup></small></td> 227bc421551SDag-Erling Smørgrav <td rowspan="9">02:00 local</td> 228bc421551SDag-Erling Smørgrav <td rowspan="2">return to standard time</td> 229bc421551SDag-Erling Smørgrav</tr> 230bc421551SDag-Erling Smørgrav<tr> 231bc421551SDag-Erling Smørgrav <td rowspan="2">1967</td> 232bc421551SDag-Erling Smørgrav <td>2006</td> 233bc421551SDag-Erling Smørgrav <td rowspan="2">last Sunday</td> 234bc421551SDag-Erling Smørgrav <td>in October</td> 235bc421551SDag-Erling Smørgrav</tr> 236bc421551SDag-Erling Smørgrav<tr> 237bc421551SDag-Erling Smørgrav <td>1973</td> 238bc421551SDag-Erling Smørgrav <td>in April</td> 239bc421551SDag-Erling Smørgrav <td rowspan="6">go to daylight saving time</td> 240bc421551SDag-Erling Smørgrav</tr> 241bc421551SDag-Erling Smørgrav<tr> 242bc421551SDag-Erling Smørgrav <td colspan="2">1974 only</td> 243bc421551SDag-Erling Smørgrav <td colspan="2">January 6<small><sup>th</sup></small></td> 244bc421551SDag-Erling Smørgrav</tr> 245bc421551SDag-Erling Smørgrav<tr> 246bc421551SDag-Erling Smørgrav <td colspan="2">1975 only</td> 247bc421551SDag-Erling Smørgrav <td colspan="2">February 23<small><sup>rd</sup></small></td> 248bc421551SDag-Erling Smørgrav</tr> 249bc421551SDag-Erling Smørgrav<tr> 250bc421551SDag-Erling Smørgrav <td>1976</td> 251bc421551SDag-Erling Smørgrav <td>1986</td> 252bc421551SDag-Erling Smørgrav <td>last Sunday</td> 253bc421551SDag-Erling Smørgrav <td rowspan="2">in April</td> 254bc421551SDag-Erling Smørgrav</tr> 255bc421551SDag-Erling Smørgrav<tr> 256bc421551SDag-Erling Smørgrav <td>1987</td> 257bc421551SDag-Erling Smørgrav <td>2006</td> 258bc421551SDag-Erling Smørgrav <td>first Sunday</td> 259bc421551SDag-Erling Smørgrav</tr> 260bc421551SDag-Erling Smørgrav<tr> 261bc421551SDag-Erling Smørgrav <td rowspan="2">2007</td> 262bc421551SDag-Erling Smørgrav <td rowspan="2">present</td> 263bc421551SDag-Erling Smørgrav <td colspan="2">second Sunday in March</td> 264bc421551SDag-Erling Smørgrav</tr> 265bc421551SDag-Erling Smørgrav<tr> 266bc421551SDag-Erling Smørgrav <td colspan="2">first Sunday in November</td> 267bc421551SDag-Erling Smørgrav <td>return to standard time</td> 268bc421551SDag-Erling Smørgrav</tr> 269bc421551SDag-Erling Smørgrav</table> 270bc421551SDag-Erling Smørgrav 271bc421551SDag-Erling Smørgrav<p>There are two interesting things to note here.</p> 272bc421551SDag-Erling Smørgrav 273bc421551SDag-Erling Smørgrav<p>First, the time that something happens (in the <code>AT</code> 274bc421551SDag-Erling Smørgravcolumn) is not necessarily the local (wall clock) time. The time can be 275bc421551SDag-Erling Smørgravsuffixed with ‘s’ (for “standard”) to mean 276bc421551SDag-Erling Smørgravlocal standard time, different from local (wall clock) time when observing 277bc421551SDag-Erling Smørgravdaylight saving time; or it can be suffixed with ‘g’, 278bc421551SDag-Erling Smørgrav‘u’, or ‘z’, all three of which mean the 279bc421551SDag-Erling Smørgravstandard time at the 280bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 281bc421551SDag-Erling Smørgrav‘g’ stands for “<a 282bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 283bc421551SDag-Erling Smørgrav‘u’ stands for “<a 284bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 285bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 286bc421551SDag-Erling Smørgrav(whichever was official at the time); ‘z’ stands for the 287bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 288bc421551SDag-Erling SmørgravZ (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 289bc421551SDag-Erling SmørgravThe time can also be suffixed with ‘w’ meaning local (wall 290bc421551SDag-Erling Smørgravclock) time; but it usually isn’t because that’s the 291bc421551SDag-Erling Smørgravdefault.</p> 292bc421551SDag-Erling Smørgrav 293bc421551SDag-Erling Smørgrav<p>Second, the day in the <code>ON</code> column, in addition to 294bc421551SDag-Erling Smørgrav“<code>lastSun</code>” or a particular day of the month, 295bc421551SDag-Erling Smørgravcan have the form, “<code>Sun>=</code><i>x</i>” or 296bc421551SDag-Erling Smørgrav“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 297bc421551SDag-Erling Smørgravof the month. For example, “<code>Sun>=8</code>” means 298bc421551SDag-Erling Smørgrav“the first Sunday on or after the eighth of the month,” in 299bc421551SDag-Erling Smørgravother words, the second Sunday of the month. Furthermore, although 300bc421551SDag-Erling Smørgravthere are no examples above, the weekday needn’t be 301bc421551SDag-Erling Smørgrav“<code>Sun</code>” in either form, but can be the usual 302bc421551SDag-Erling Smørgravthree-character English abbreviation for any day of the week.</p> 303bc421551SDag-Erling Smørgrav 304bc421551SDag-Erling Smørgrav<p>And the US rules give us more examples of a couple of things 305bc421551SDag-Erling Smørgravalready mentioned:</p> 306bc421551SDag-Erling Smørgrav 307bc421551SDag-Erling Smørgrav<ul> 308bc421551SDag-Erling Smørgrav<li>The rules for changing to and from daylight saving time are 309bc421551SDag-Erling Smørgravactually <i>different sets</i> of rules; and the two sets can change 310bc421551SDag-Erling Smørgravindependently. Consider, for example, that the rule for the return to 311bc421551SDag-Erling Smørgravstandard time stayed the same from 1967 to 2006; but the rule for the 312bc421551SDag-Erling Smørgravtransition to daylight saving time changed several times in the same 313bc421551SDag-Erling Smørgravperiod. There can also be periods, 1946 to 1966 for example, when no 314bc421551SDag-Erling Smørgravrule from this group is in effect, and so either no transition 315bc421551SDag-Erling Smørgravhappened in those years, or some other rule is in effect (perhaps a 316bc421551SDag-Erling Smørgravstate or other more local rule).</li> 317bc421551SDag-Erling Smørgrav 318bc421551SDag-Erling Smørgrav<li>The <code>SAVE</code> and <code>LETTER</code> columns 319bc421551SDag-Erling Smørgravcontain <i>steady state</i>, not transitions. Consider, for example, 320bc421551SDag-Erling Smørgravthe transition from “war time” to “peace time” 321bc421551SDag-Erling Smørgravthat happened on August 14, 1945. The “1:00” in 322bc421551SDag-Erling Smørgravthe <code>SAVE</code> column is <i>not</i> an instruction to advance 323bc421551SDag-Erling Smørgravthe clock an hour. It means that clocks should <i>be</i> one hour 324bc421551SDag-Erling Smørgravahead of standard time, which they already are because of the previous 325bc421551SDag-Erling Smørgravrule, so there should be no change.</li> 326bc421551SDag-Erling Smørgrav 327bc421551SDag-Erling Smørgrav</ul> 328bc421551SDag-Erling Smørgrav 329bc421551SDag-Erling Smørgrav<p>OK, now let’s look at a Zone record:</p> 330bc421551SDag-Erling Smørgrav 331bc421551SDag-Erling Smørgrav<table> 332bc421551SDag-Erling Smørgrav<tr> 333bc421551SDag-Erling Smørgrav <th colspan="5">From the Source File</th> 334bc421551SDag-Erling Smørgrav</tr> 335bc421551SDag-Erling Smørgrav<tr> 336bc421551SDag-Erling Smørgrav <td colspan="5"> 337bc421551SDag-Erling Smørgrav <table class="rule"> 338bc421551SDag-Erling Smørgrav <tr><td style="border:none;text-align:left"> 339bc421551SDag-Erling Smørgrav<pre class="td"> 340bc421551SDag-Erling Smørgrav#Zone NAME STDOFF RULES FORMAT [UNTIL] 341bc421551SDag-Erling SmørgravZone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 342bc421551SDag-Erling Smørgrav -6:00 US C%sT 1920 343bc421551SDag-Erling Smørgrav -6:00 Chicago C%sT 1936 Mar 1 2:00 344bc421551SDag-Erling Smørgrav -5:00 - EST 1936 Nov 15 2:00 345bc421551SDag-Erling Smørgrav -6:00 Chicago C%sT 1942 346bc421551SDag-Erling Smørgrav -6:00 US C%sT 1946 347bc421551SDag-Erling Smørgrav -6:00 Chicago C%sT 1967 348bc421551SDag-Erling Smørgrav -6:00 US C%sT 349bc421551SDag-Erling Smørgrav</pre> 350bc421551SDag-Erling Smørgrav </td></tr></table></td> 351bc421551SDag-Erling Smørgrav</tr> 352bc421551SDag-Erling Smørgrav<tr> 353bc421551SDag-Erling Smørgrav <th colspan="5">Columns Renamed</th> 354bc421551SDag-Erling Smørgrav</tr> 355bc421551SDag-Erling Smørgrav<tr> 356bc421551SDag-Erling Smørgrav <th rowspan="2">Standard Offset<br> 357bc421551SDag-Erling Smørgrav from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 358bc421551SDag-Erling Smørgrav Meridian</a></th> 359bc421551SDag-Erling Smørgrav <th rowspan="2">Daylight<br>Saving Time</th> 360bc421551SDag-Erling Smørgrav <th rowspan="2">Abbreviation(s)</th> 361bc421551SDag-Erling Smørgrav <th colspan="2">Ending at Local Time</th> 362bc421551SDag-Erling Smørgrav</tr> 363bc421551SDag-Erling Smørgrav<tr> 364bc421551SDag-Erling Smørgrav <th>Date</th> 365bc421551SDag-Erling Smørgrav <th>Time</th> 366bc421551SDag-Erling Smørgrav</tr> 367bc421551SDag-Erling Smørgrav<tr> 368bc421551SDag-Erling Smørgrav <td>−5:50:36</td> 369bc421551SDag-Erling Smørgrav <td>not observed</td> 370bc421551SDag-Erling Smørgrav <td>LMT</td> 371bc421551SDag-Erling Smørgrav <td>1883-11-18</td> 372bc421551SDag-Erling Smørgrav <td>12:09:24</td> 373bc421551SDag-Erling Smørgrav</tr> 374bc421551SDag-Erling Smørgrav<tr> 375bc421551SDag-Erling Smørgrav <td rowspan="2">−6:00:00</td> 376bc421551SDag-Erling Smørgrav <td>US rules</td> 377bc421551SDag-Erling Smørgrav <td rowspan="2">CST or CDT</td> 378bc421551SDag-Erling Smørgrav <td>1920-01-01</td> 379bc421551SDag-Erling Smørgrav <td>00:00:00</td> 380bc421551SDag-Erling Smørgrav</tr> 381bc421551SDag-Erling Smørgrav<tr> 382bc421551SDag-Erling Smørgrav <td>Chicago rules</td> 383bc421551SDag-Erling Smørgrav <td>1936-03-01</td> 384bc421551SDag-Erling Smørgrav <td rowspan="2">02:00:00</td> 385bc421551SDag-Erling Smørgrav</tr> 386bc421551SDag-Erling Smørgrav<tr> 387bc421551SDag-Erling Smørgrav <td>−5:00:00</td> 388bc421551SDag-Erling Smørgrav <td>not observed</td> 389bc421551SDag-Erling Smørgrav <td>EST</td> 390bc421551SDag-Erling Smørgrav <td>1936-11-15</td> 391bc421551SDag-Erling Smørgrav</tr> 392bc421551SDag-Erling Smørgrav<tr> 393bc421551SDag-Erling Smørgrav <td rowspan="4">−6:00:00</td> 394bc421551SDag-Erling Smørgrav <td>Chicago rules</td> 395bc421551SDag-Erling Smørgrav <td>CST or CDT</td> 396bc421551SDag-Erling Smørgrav <td>1942-01-01</td> 397bc421551SDag-Erling Smørgrav <td rowspan="3">00:00:00</td> 398bc421551SDag-Erling Smørgrav</tr> 399bc421551SDag-Erling Smørgrav<tr> 400bc421551SDag-Erling Smørgrav <td>US rules</td> 401bc421551SDag-Erling Smørgrav <td>CST, CWT or CPT</td> 402bc421551SDag-Erling Smørgrav <td>1946-01-01</td> 403bc421551SDag-Erling Smørgrav</tr> 404bc421551SDag-Erling Smørgrav<tr> 405bc421551SDag-Erling Smørgrav <td>Chicago rules</td> 406bc421551SDag-Erling Smørgrav <td rowspan="2">CST or CDT</td> 407bc421551SDag-Erling Smørgrav <td>1967-01-01</td> 408bc421551SDag-Erling Smørgrav</tr> 409bc421551SDag-Erling Smørgrav<tr> 410bc421551SDag-Erling Smørgrav <td>US rules</td> 411bc421551SDag-Erling Smørgrav <td colspan="2">—</td> 412bc421551SDag-Erling Smørgrav</tr> 413bc421551SDag-Erling Smørgrav</table> 414bc421551SDag-Erling Smørgrav 415bc421551SDag-Erling Smørgrav<p>There are a couple of interesting differences between Zones and Rules.</p> 416bc421551SDag-Erling Smørgrav 417bc421551SDag-Erling Smørgrav<p>First, and somewhat trivially, whereas Rules are considered to 418bc421551SDag-Erling Smørgravcontain one or more records, a Zone is considered to be a single 419bc421551SDag-Erling Smørgravrecord with zero or more <i>continuation lines</i>. Thus, the keyword, 420bc421551SDag-Erling Smørgrav“<code>Zone</code>,” and the zone name are not 421bc421551SDag-Erling Smørgravrepeated. The last line is the one without anything in 422bc421551SDag-Erling Smørgravthe <code>[UNTIL]</code> column.</p> 423bc421551SDag-Erling Smørgrav 424bc421551SDag-Erling Smørgrav<p>Second, and more fundamentally, each line of a Zone represents a 425bc421551SDag-Erling Smørgravsteady state, not a transition between states. The state exists from 426bc421551SDag-Erling Smørgravthe date and time in the previous line’s <code>[UNTIL]</code> 427bc421551SDag-Erling Smørgravcolumn up to the date and time in the current 428bc421551SDag-Erling Smørgravline’s <code>[UNTIL]</code> column. In other words, the date and 429bc421551SDag-Erling Smørgravtime in the <code>[UNTIL]</code> column is the instant that separates 430bc421551SDag-Erling Smørgravthis state from the next. Where that would be ambiguous because 431bc421551SDag-Erling Smørgravwe’re setting our clocks back, the <code>[UNTIL]</code> column 432bc421551SDag-Erling Smørgravspecifies the first occurrence of the instant. The state specified by 433bc421551SDag-Erling Smørgravthe last line, the one without anything in the <code>[UNTIL]</code> 434bc421551SDag-Erling Smørgravcolumn, continues to the present.</p> 435bc421551SDag-Erling Smørgrav 436bc421551SDag-Erling Smørgrav<p>The first line typically specifies the mean solar time observed 437bc421551SDag-Erling Smørgravbefore the introduction of standard time. Since there’s no line before 438bc421551SDag-Erling Smørgravthat, it has no beginning. <code>8-) </code> For some places near the <a 439bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/International_Date_Line">International 440bc421551SDag-Erling SmørgravDate Line</a>, the first <i>two</i> lines will show solar times 441bc421551SDag-Erling Smørgravdiffering by 24 hours; this corresponds to a movement of the Date 442bc421551SDag-Erling SmørgravLine. For example:</p> 443bc421551SDag-Erling Smørgrav 444bc421551SDag-Erling Smørgrav<pre> 445bc421551SDag-Erling Smørgrav#Zone NAME STDOFF RULES FORMAT [UNTIL] 446bc421551SDag-Erling SmørgravZone America/Juneau 15:02:19 - LMT 1867 Oct 18 447bc421551SDag-Erling Smørgrav -8:57:41 - LMT ... 448bc421551SDag-Erling Smørgrav</pre> 449bc421551SDag-Erling Smørgrav 450bc421551SDag-Erling Smørgrav<p>When Alaska was purchased from Russia in 1867, the Date Line moved 451bc421551SDag-Erling Smørgravfrom the Alaska/Canada border to the Bering Strait; and the time in 452bc421551SDag-Erling SmørgravAlaska was then 24 hours earlier than it had 453bc421551SDag-Erling Smørgravbeen. <code><aside></code>(6 October in the Julian calendar, 454bc421551SDag-Erling Smørgravwhich Russia was still using then for religious reasons, was followed 455bc421551SDag-Erling Smørgravby <i>a second instance of the same day with a different name</i>, 18 456bc421551SDag-Erling SmørgravOctober in the Gregorian calendar. Isn’t civil time 457bc421551SDag-Erling Smørgravwonderful? <code>8-)</code>)<code></aside></code></p> 458bc421551SDag-Erling Smørgrav 459bc421551SDag-Erling Smørgrav<p>The abbreviation, “LMT” stands for “local mean 460bc421551SDag-Erling Smørgravtime”, which is an invention of 461bc421551SDag-Erling Smørgravthe <a href="https://en.wikipedia.org/wiki/Tz_database">tz 462bc421551SDag-Erling Smørgravdatabase</a> and was probably never actually used during the 463bc421551SDag-Erling Smørgravperiod. Furthermore, the value is almost certainly wrong except in the 464bc421551SDag-Erling Smørgravarchetypal place after which the zone is named. (The tz database 465bc421551SDag-Erling Smørgravusually doesn’t provide a separate Zone record for places where 466bc421551SDag-Erling Smørgravnothing significant happened after 1970.)</p> 467bc421551SDag-Erling Smørgrav 468bc421551SDag-Erling Smørgrav<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 469bc421551SDag-Erling Smørgrav<ul> 470bc421551SDag-Erling Smørgrav<li>A hyphen, a kind of null value, means that we have not set our 471bc421551SDag-Erling Smørgravclocks ahead of standard time.</li> 472bc421551SDag-Erling Smørgrav 473bc421551SDag-Erling Smørgrav<li>An amount of time (usually but not necessarily “1:00” 474bc421551SDag-Erling Smørgravmeaning one hour) means that we have set our clocks ahead by that 475bc421551SDag-Erling Smørgravamount.</li> 476bc421551SDag-Erling Smørgrav 477bc421551SDag-Erling Smørgrav<li>Some alphabetic string means that we <i>might have</i> set our 478bc421551SDag-Erling Smørgravclocks ahead; and we need to check the rule the name of which is the 479bc421551SDag-Erling Smørgravgiven alphabetic string.</li> 480bc421551SDag-Erling Smørgrav</ul> 481bc421551SDag-Erling Smørgrav 482bc421551SDag-Erling Smørgrav<p>An example of a specific amount of time is:</p> 483bc421551SDag-Erling Smørgrav<pre> 484bc421551SDag-Erling Smørgrav#Zone NAME STDOFF RULES FORMAT [UNTIL] 485bc421551SDag-Erling SmørgravZone Pacific/Honolulu ... 1933 Apr 30 2:00 486bc421551SDag-Erling Smørgrav -10:30 1:00 HDT 1933 May 21 12:00 487bc421551SDag-Erling Smørgrav ... 488bc421551SDag-Erling Smørgrav</pre> 489bc421551SDag-Erling Smørgrav 490bc421551SDag-Erling Smørgrav<p>Hawaii tried daylight saving time for three weeks in 1933 and 491bc421551SDag-Erling Smørgravdecided they didn’t like it. <code>8-) </code>Note that 492bc421551SDag-Erling Smørgravthe <code>STDOFF</code> column always contains the standard time 493bc421551SDag-Erling Smørgravoffset, so the local (wall clock) time during this period was GMT − 494bc421551SDag-Erling Smørgrav10:30 + 1:00 = GMT − 9:30.</p> 495bc421551SDag-Erling Smørgrav 496bc421551SDag-Erling Smørgrav<p>The <code>FORMAT</code> column specifies the usual abbreviation of 497bc421551SDag-Erling Smørgravthe time zone name. It should have one of four forms:</p> 498bc421551SDag-Erling Smørgrav<ul> 499bc421551SDag-Erling Smørgrav 500bc421551SDag-Erling Smørgrav<li>a time zone abbreviation that is a string of three or more 501bc421551SDag-Erling Smørgravcharacters that are either ASCII alphanumerics, 502bc421551SDag-Erling Smørgrav“<code>+</code>”, or “<code>-</code>”</li> 503bc421551SDag-Erling Smørgrav 504bc421551SDag-Erling Smørgrav<li>the string “%z”, in which case the 505bc421551SDag-Erling Smørgrav“<code>%z</code>” will be replaced by a numeric time zone 506bc421551SDag-Erling Smørgravabbreviation</li> 507bc421551SDag-Erling Smørgrav 508bc421551SDag-Erling Smørgrav<li>a pair of time zone abbreviations separated by a slash 509bc421551SDag-Erling Smørgrav(‘<code>/</code>’), in which case the first string is the 510bc421551SDag-Erling Smørgravabbreviation for the standard time name and the second string is the 511bc421551SDag-Erling Smørgravabbreviation for the daylight saving time name</li> 512bc421551SDag-Erling Smørgrav 513bc421551SDag-Erling Smørgrav<li>a string containing “<code>%s</code>”, in which case 514bc421551SDag-Erling Smørgravthe “<code>%s</code>” will be replaced by the text in the 515bc421551SDag-Erling Smørgravappropriate Rule’s <code>LETTER</code> column, and the resulting 516bc421551SDag-Erling Smørgravstring should be a time zone abbreviation</li> 517bc421551SDag-Erling Smørgrav</ul> 518bc421551SDag-Erling Smørgrav 519bc421551SDag-Erling Smørgrav<p>The last two make sense only if there’s a named rule in effect.</p> 520bc421551SDag-Erling Smørgrav 521bc421551SDag-Erling Smørgrav<p>An example of a slash is:</p> 522bc421551SDag-Erling Smørgrav<pre> 523bc421551SDag-Erling Smørgrav#Zone NAME STDOFF RULES FORMAT [UNTIL] 524bc421551SDag-Erling SmørgravZone Europe/London ... 1996 525bc421551SDag-Erling Smørgrav 0:00 EU GMT/BST 526bc421551SDag-Erling Smørgrav</pre> 527bc421551SDag-Erling Smørgrav 528bc421551SDag-Erling Smørgrav<p>The current time in the UK is called either Greenwich mean time or 529bc421551SDag-Erling SmørgravBritish summer time.</p> 530bc421551SDag-Erling Smørgrav 531bc421551SDag-Erling Smørgrav<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 532bc421551SDag-Erling Smørgravhappens when switching to a named rule. To what values should 533bc421551SDag-Erling Smørgravthe <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 534bc421551SDag-Erling Smørgrav 535bc421551SDag-Erling Smørgrav<ul> 536bc421551SDag-Erling Smørgrav<li>If at least one transition has happened, use 537bc421551SDag-Erling Smørgravthe <code>SAVE</code> and <code>LETTER</code> data from the most 538bc421551SDag-Erling Smørgravrecent.</li> 539bc421551SDag-Erling Smørgrav 540bc421551SDag-Erling Smørgrav<li>If switching to a named rule before any transition has happened, 541bc421551SDag-Erling Smørgravassume standard time (<code>SAVE</code> zero), and use 542bc421551SDag-Erling Smørgravthe <code>LETTER</code> data from the earliest transition with 543bc421551SDag-Erling Smørgrava <code>SAVE</code> of zero. 544bc421551SDag-Erling Smørgrav 545bc421551SDag-Erling Smørgrav</ul> 546bc421551SDag-Erling Smørgrav 547bc421551SDag-Erling Smørgrav<p>And three last things about the <code>FORMAT</code> column:</p> 548bc421551SDag-Erling Smørgrav<ul> 549bc421551SDag-Erling Smørgrav 550bc421551SDag-Erling Smørgrav<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 551*75411d15SDag-Erling Smørgravdatabase</a> gives abbreviations for time zones 552*75411d15SDag-Erling Smørgravin popular English-language usage. For 553bc421551SDag-Erling Smørgravexample, the last line in 554bc421551SDag-Erling Smørgrav<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 555bc421551SDag-Erling Smørgrav“HST” for “Hawaii standard time” even though the 556bc421551SDag-Erling Smørgrav<a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 557bc421551SDag-Erling Smørgravname for that time zone is “Hawaii-Aleutian standard time.” 558bc421551SDag-Erling SmørgravThis author has read that there are also some places in Australia where 559bc421551SDag-Erling Smørgravpopular time zone names differ from the legal ones. 560bc421551SDag-Erling Smørgrav 561bc421551SDag-Erling Smørgrav<li>No attempt is made to <a 562bc421551SDag-Erling Smørgravhref="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 563bc421551SDag-Erling Smørgravthe abbreviations. They are intended to be the values returned through the 564bc421551SDag-Erling Smørgrav<code>"%Z"</code> format specifier to 565bc421551SDag-Erling Smørgrav<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 566bc421551SDag-Erling Smørgrav<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 567bc421551SDag-Erling Smørgravfunction in the 568bc421551SDag-Erling Smørgrav<a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 569bc421551SDag-Erling Smørgrav 570*75411d15SDag-Erling Smørgrav<li>If there is no generally accepted abbreviation for a time zone, 571bc421551SDag-Erling Smørgrava numeric offset is used instead, e.g., <code>+07</code> for 7 hours 572bc421551SDag-Erling Smørgravahead of Greenwich. By convention, <code>-00</code> is used in a 573bc421551SDag-Erling Smørgravzone while uninhabited, where the offset is zero but in some sense 574bc421551SDag-Erling Smørgravthe true offset is undefined. 575bc421551SDag-Erling Smørgrav</ul> 576bc421551SDag-Erling Smørgrav 577bc421551SDag-Erling Smørgrav<p>As a final example, here’s the complete history for Hawaii:</p> 578bc421551SDag-Erling Smørgrav 579bc421551SDag-Erling Smørgrav<table> 580bc421551SDag-Erling Smørgrav<tr> 581bc421551SDag-Erling Smørgrav <th colspan="6">Relevant Excerpts from the US Rules</th> 582bc421551SDag-Erling Smørgrav</tr> 583bc421551SDag-Erling Smørgrav<tr> 584bc421551SDag-Erling Smørgrav <td colspan="6"> 585bc421551SDag-Erling Smørgrav <table class="rule"> 586bc421551SDag-Erling Smørgrav <tr><td style="border:none;text-align:left"> 587bc421551SDag-Erling Smørgrav<pre class="td"> 588bc421551SDag-Erling Smørgrav#Rule NAME FROM TO - IN ON AT SAVE LETTER/S 589bc421551SDag-Erling SmørgravRule US 1918 1919 - Oct lastSun 2:00 0 S 590bc421551SDag-Erling SmørgravRule US 1942 only - Feb 9 2:00 1:00 W # War 591bc421551SDag-Erling SmørgravRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 592bc421551SDag-Erling SmørgravRule US 1945 only - Sep lastSun 2:00 0 S 593bc421551SDag-Erling Smørgrav</pre> 594bc421551SDag-Erling Smørgrav </td></tr></table></td> 595bc421551SDag-Erling Smørgrav</tr> 596bc421551SDag-Erling Smørgrav<tr> 597bc421551SDag-Erling Smørgrav <th colspan="6">The Zone Record</th> 598bc421551SDag-Erling Smørgrav</tr> 599bc421551SDag-Erling Smørgrav<tr> 600bc421551SDag-Erling Smørgrav <td colspan="6"> 601bc421551SDag-Erling Smørgrav <table class="rule"> 602bc421551SDag-Erling Smørgrav <tr><td style="border:none;text-align:left"> 603bc421551SDag-Erling Smørgrav<pre class="td"> 604bc421551SDag-Erling Smørgrav#Zone NAME STDOFF RULES FORMAT [UNTIL] 605bc421551SDag-Erling SmørgravZone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 606bc421551SDag-Erling Smørgrav -10:30 - HST 1933 Apr 30 2:00 607bc421551SDag-Erling Smørgrav -10:30 1:00 HDT 1933 May 21 2:00 608bc421551SDag-Erling Smørgrav -10:30 US H%sT 1947 Jun 8 2:00 609bc421551SDag-Erling Smørgrav -10:00 - HST 610bc421551SDag-Erling Smørgrav</pre> 611bc421551SDag-Erling Smørgrav </td></tr></table></td> 612bc421551SDag-Erling Smørgrav</tr> 613bc421551SDag-Erling Smørgrav<tr> 614bc421551SDag-Erling Smørgrav <th colspan="6">What We Infer</th> 615bc421551SDag-Erling Smørgrav</tr> 616bc421551SDag-Erling Smørgrav<tr> 617bc421551SDag-Erling Smørgrav <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 618bc421551SDag-Erling Smørgrav <th rowspan="2">Adjust<br>Clocks</th> 619bc421551SDag-Erling Smørgrav <th colspan="2">Time Zone</th> 620bc421551SDag-Erling Smørgrav <th colspan="2">Ending at Local Time</th> 621bc421551SDag-Erling Smørgrav</tr> 622bc421551SDag-Erling Smørgrav<tr> 623bc421551SDag-Erling Smørgrav <th>Abbrv.</th> 624bc421551SDag-Erling Smørgrav <th>Name</th> 625bc421551SDag-Erling Smørgrav <th>Date</th> 626bc421551SDag-Erling Smørgrav <th>Time</th> 627bc421551SDag-Erling Smørgrav</tr> 628bc421551SDag-Erling Smørgrav<tr> 629bc421551SDag-Erling Smørgrav <td>−10:31:26</td> 630bc421551SDag-Erling Smørgrav <td>—</td> 631bc421551SDag-Erling Smørgrav <td>LMT</td> 632bc421551SDag-Erling Smørgrav <td>local mean time</td> 633bc421551SDag-Erling Smørgrav <td>1896-01-13</td> 634bc421551SDag-Erling Smørgrav <td>12:00</td> 635bc421551SDag-Erling Smørgrav</tr> 636bc421551SDag-Erling Smørgrav<tr> 637bc421551SDag-Erling Smørgrav <td>−10:30</td> 638bc421551SDag-Erling Smørgrav <td>+0:01:26</td> 639bc421551SDag-Erling Smørgrav <td>HST</td> 640bc421551SDag-Erling Smørgrav <td>Hawaii standard time</td> 641bc421551SDag-Erling Smørgrav <td>1933-04-30</td> 642bc421551SDag-Erling Smørgrav <td>02:00</td> 643bc421551SDag-Erling Smørgrav</tr> 644bc421551SDag-Erling Smørgrav<tr> 645bc421551SDag-Erling Smørgrav <td>−9:30</td> 646bc421551SDag-Erling Smørgrav <td>+1:00</td> 647bc421551SDag-Erling Smørgrav <td>HDT</td> 648bc421551SDag-Erling Smørgrav <td>Hawaii daylight time</td> 649bc421551SDag-Erling Smørgrav <td>1933-05-21</td> 650bc421551SDag-Erling Smørgrav <td>12:00</td> 651bc421551SDag-Erling Smørgrav</tr> 652bc421551SDag-Erling Smørgrav<tr> 653bc421551SDag-Erling Smørgrav <td>−10:30¹</td> 654bc421551SDag-Erling Smørgrav <td>−1:00¹</td> 655bc421551SDag-Erling Smørgrav <td>HST¹</td> 656bc421551SDag-Erling Smørgrav <td>Hawaii standard time</td> 657bc421551SDag-Erling Smørgrav <td>1942-02-09</td> 658bc421551SDag-Erling Smørgrav <td>02:00</td> 659bc421551SDag-Erling Smørgrav</tr> 660bc421551SDag-Erling Smørgrav<tr> 661bc421551SDag-Erling Smørgrav <td rowspan="2">−9:30</td> 662bc421551SDag-Erling Smørgrav <td>+1:00</td> 663bc421551SDag-Erling Smørgrav <td>HWT</td> 664bc421551SDag-Erling Smørgrav <td>Hawaii war time</td> 665bc421551SDag-Erling Smørgrav <td>1945-08-14</td> 666bc421551SDag-Erling Smørgrav <td>13:30²</td> 667bc421551SDag-Erling Smørgrav</tr> 668bc421551SDag-Erling Smørgrav<tr> 669bc421551SDag-Erling Smørgrav <td>0</td> 670bc421551SDag-Erling Smørgrav <td>HPT</td> 671bc421551SDag-Erling Smørgrav <td>Hawaii peace time</td> 672bc421551SDag-Erling Smørgrav <td>1945-09-30</td> 673bc421551SDag-Erling Smørgrav <td rowspan="2">02:00</td> 674bc421551SDag-Erling Smørgrav</tr> 675bc421551SDag-Erling Smørgrav<tr> 676bc421551SDag-Erling Smørgrav <td>−10:30</td> 677bc421551SDag-Erling Smørgrav <td>−1:00</td> 678bc421551SDag-Erling Smørgrav <td rowspan="2">HST</td> 679bc421551SDag-Erling Smørgrav <td rowspan="2">Hawaii standard time</td> 680bc421551SDag-Erling Smørgrav <td>1947-06-08</td> 681bc421551SDag-Erling Smørgrav</tr> 682bc421551SDag-Erling Smørgrav<tr> 683bc421551SDag-Erling Smørgrav <td>−10:00³</td> 684bc421551SDag-Erling Smørgrav <td>+0:30³</td> 685bc421551SDag-Erling Smørgrav <td colspan="2">—</td> 686bc421551SDag-Erling Smørgrav</tr> 687bc421551SDag-Erling Smørgrav<tr> 688bc421551SDag-Erling Smørgrav <td colspan="6" class="footnote"> 689bc421551SDag-Erling Smørgrav ¹Switching to US rules…most recent transition (in 1919) was to standard time 690bc421551SDag-Erling Smørgrav </td> 691bc421551SDag-Erling Smørgrav</tr> 692bc421551SDag-Erling Smørgrav<tr> 693bc421551SDag-Erling Smørgrav <td colspan="6" class="footnote"> 694bc421551SDag-Erling Smørgrav ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 695bc421551SDag-Erling Smørgrav + (−9:30) = 13:30 local 696bc421551SDag-Erling Smørgrav </td> 697bc421551SDag-Erling Smørgrav</tr> 698bc421551SDag-Erling Smørgrav<tr> 699bc421551SDag-Erling Smørgrav <td colspan="6" class="footnote"> 700bc421551SDag-Erling Smørgrav ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 701bc421551SDag-Erling Smørgrav the civil time in Hawaii has been 702bc421551SDag-Erling Smørgrav <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 703bc421551SDag-Erling Smørgrav − 10:00 year-round. 704bc421551SDag-Erling Smørgrav </td> 705bc421551SDag-Erling Smørgrav</tr> 706bc421551SDag-Erling Smørgrav</table> 707bc421551SDag-Erling Smørgrav 708bc421551SDag-Erling Smørgrav<p>There will be a short quiz later. <code>8-)</code></p> 709bc421551SDag-Erling Smørgrav 710bc421551SDag-Erling Smørgrav<hr> 711bc421551SDag-Erling Smørgrav<address> 712bc421551SDag-Erling SmørgravThis web page is in the public domain, so clarified as of 713bc421551SDag-Erling Smørgrav2015-10-20 by Bill Seymour. 714bc421551SDag-Erling Smørgrav<br> 715bc421551SDag-Erling SmørgravAll suggestions and corrections will be welcome; all flames will be amusing. 716bc421551SDag-Erling SmørgravMail to was at pobox dot com. 717bc421551SDag-Erling Smørgrav</address> 718bc421551SDag-Erling Smørgrav</body> 719bc421551SDag-Erling Smørgrav</html> 720