11.8Schristos<!DOCTYPE html> 21.8Schristos<html lang="en"> 31.3Schristos<head> 41.3Schristos<title>How to Read the tz Database</title> 51.8Schristos<meta charset="UTF-8"> 61.8Schristos<style> 71.5Schristospre {margin-left: 2em; white-space: pre-wrap;} 81.8Schristospre.td {margin-left: 0;} 91.8Schristostd {text-align: center;} 101.8Schristostable {border: 1px outset;} 111.8Schristosth, td {border: 1px inset;} 121.8Schristostable.rule {border: none; margin: auto;} 131.8Schristostd.footnote {text-align: left;} 141.5Schristos</style> 151.3Schristos</head> 161.1Schristos<body> 171.1Schristos<h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 181.1SchristosDatabase</a> Source Files</h2> 191.1Schristos<h3>by Bill Seymour</h3> 201.9Schristos<p>This guide uses the <code>America/Chicago</code> and 211.1Schristos<code>Pacific/Honolulu</code> zones as examples of how to infer 221.4Schristostimes of day from the <a href="tz-link.html">tz database</a> 231.1Schristossource files. It might be helpful, but not absolutely necessary, 241.1Schristosfor the reader to have already downloaded the 251.2Schristoslatest release of the database and become familiar with the basic layout 261.1Schristosof the data files. The format is explained in the “man 271.1Schristospage” for the zic compiler, <code>zic.8.txt</code>, in 281.9Schristosthe <code>code</code> subdirectory. 291.9SchristosAlthough this guide covers many of the common cases, it is not a 301.9Schristoscomplete summary of what zic accepts; the man page is the 311.9Schristosauthoritative reference.</p> 321.1Schristos 331.1Schristos<p>We’ll begin by talking about the rules for changing between standard 341.1Schristosand daylight saving time since we’ll need that information when we talk 351.1Schristosabout the zones.</p> 361.1Schristos 371.1Schristos<p>First, let’s consider the special daylight saving time rules 381.1Schristosfor Chicago (from the <code>northamerica</code> file in 391.1Schristosthe <code>data</code> subdirectory):</p> 401.1Schristos 411.8Schristos<table> 421.1Schristos<tr> 431.1Schristos <th colspan="6">From the Source File</th> 441.1Schristos</tr> 451.1Schristos<tr> 461.8Schristos <td colspan="6"> 471.8Schristos <table class="rule"> 481.8Schristos <tr><td style="border:none;text-align:left"> 491.8Schristos<pre class="td"> 501.8Schristos#Rule NAME FROM TO - IN ON AT SAVE LETTER 511.1SchristosRule Chicago 1920 only - Jun 13 2:00 1:00 D 521.1SchristosRule Chicago 1920 1921 - Oct lastSun 2:00 0 S 531.1SchristosRule Chicago 1921 only - Mar lastSun 2:00 1:00 D 541.1SchristosRule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 551.1SchristosRule Chicago 1922 1954 - Sep lastSun 2:00 0 S 561.1SchristosRule Chicago 1955 1966 - Oct lastSun 2:00 0 S 571.1Schristos</pre> 581.1Schristos </td></tr></table></td> 591.1Schristos</tr> 601.1Schristos<tr> 611.1Schristos <th colspan="6">Reformatted a Bit</th> 621.1Schristos</tr> 631.1Schristos<tr> 641.1Schristos <th>From</th> 651.1Schristos <th>To</th> 661.1Schristos <th colspan="2">On</th> 671.1Schristos <th>At</th> 681.1Schristos <th>Action</th> 691.1Schristos</tr> 701.8Schristos<tr> 711.1Schristos <td colspan="2">1920 only</td> 721.1Schristos <td colspan="2">June 13<small><sup>th</sup></small></td> 731.1Schristos <td rowspan="6">02:00 local</td> 741.1Schristos <td>go to daylight saving time</td> 751.1Schristos</tr> 761.8Schristos<tr> 771.1Schristos <td>1920</td> 781.1Schristos <td>1921</td> 791.1Schristos <td rowspan="5">last Sunday</td> 801.1Schristos <td>in October</td> 811.1Schristos <td>return to standard time</td> 821.1Schristos</tr> 831.8Schristos<tr> 841.1Schristos <td colspan="2">1921 only</td> 851.1Schristos <td>in March</td> 861.1Schristos <td rowspan="2">go to daylight saving time</td> 871.1Schristos</tr> 881.8Schristos<tr> 891.1Schristos <td rowspan="2">1922</td> 901.1Schristos <td>1966</td> 911.1Schristos <td>in April</td> 921.1Schristos</tr> 931.8Schristos<tr> 941.1Schristos <td>1954</td> 951.1Schristos <td>in September</td> 961.1Schristos <td rowspan="2">return to standard time</td> 971.1Schristos</tr> 981.8Schristos<tr> 991.1Schristos <td>1955</td> 1001.1Schristos <td>1966</td> 1011.1Schristos <td>in October</td> 1021.1Schristos</tr> 1031.1Schristos</table> 1041.1Schristos 1051.8Schristos<p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the 1061.8Schristosfirst and last calendar years defining a contiguous range over which a specific 1071.8SchristosRule line is to apply. The keyword <code>only</code> can be used in the 1081.8Schristos<code>TO</code> field to repeat the value of the <code>FROM</code> field in the 1091.8Schristosevent that a rule should only apply to a single year. Often, the keyword 1101.8Schristos<code>max</code> is used to extend a rule’s application into the 1111.8Schristosindefinite future; it is a platform-agnostic stand-in for the largest 1121.8Schristosrepresentable year. 1131.8Schristos 1141.8Schristos<p>The next column, <code>-</code>, is reserved; for compatibility with earlier 1151.8Schristosreleases, it always contains a hyphen, which acts as a kind of null value. 1161.8SchristosPrior to the 2020b release, it was called the <code>TYPE</code> field, though 1171.9Schristosit had not been used in the main data since the 2000e release. 1181.9SchristosAn obsolescent supplementary file used the 1191.8Schristosfield as a proof-of-concept to allow <code>zic</code> to apply a given Rule 1201.8Schristosline only to certain “types” of years within the specified range as 1211.8Schristosdictated by the output of a separate script, such as: only years which would 1221.8Schristoshave a US presidential election, or only years which wouldn’t. 1231.1Schristos 1241.7Schristos<p>The <code>SAVE</code> column contains the local (wall clock) offset from 1251.1Schristoslocal standard time. 1261.1SchristosThis is usually either zero for standard time or one hour for daylight 1271.1Schristossaving time; but there’s no reason, in principle, why it can’t 1281.1Schristostake on other values. 1291.1Schristos 1301.1Schristos<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 1311.1Schristoscolumn can contain a variable 1321.1Schristospart of the usual abbreviation of the time zone’s name, or it can just 1331.1Schristosbe a hyphen if there’s no variable part. For example, the abbreviation 1341.1Schristosused in the central time zone will be either “CST” or 1351.1Schristos“CDT”. The variable part is ‘S’ or ‘D’; 1361.1Schristosand, sure enough, that’s just what we find in 1371.1Schristosthe <code>LETTER</code> column 1381.1Schristosin the <code>Chicago</code> rules. More about this when we talk about 1391.1Schristos“Zone” lines. 1401.1Schristos 1411.1Schristos<p>One important thing to notice is that “Rule” lines 1421.1Schristoswant at once to be both <i>transitions</i> and <i>steady states</i>: 1431.1Schristos<ul> 1441.1Schristos<li>On the one hand, they represent transitions between standard and 1451.1Schristosdaylight saving time; and any number of Rule lines can be in effect 1461.1Schristosduring a given period (which will always be a non-empty set of 1471.1Schristoscontiguous calendar years).</li> 1481.1Schristos<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 1491.1Schristoscolumns contain state that exists between transitions. More about this 1501.1Schristoswhen we talk about the US rules.</li> 1511.1Schristos</ul> 1521.1Schristos 1531.1Schristos<p>In the example above, the transition to daylight saving time 1541.1Schristoshappened on the 13<small><sup>th</sup></small> of June in 1920, and on 1551.1Schristosthe last Sunday in March in 1921; but the return to standard time 1561.1Schristoshappened on the last Sunday in October in both of those 1571.1Schristosyears. Similarly, the rule for changing to daylight saving time was 1581.1Schristosthe same from 1922 to 1966; but the rule for returning to standard 1591.1Schristostime changed in 1955. Got it?</p> 1601.1Schristos 1611.1Schristos<p>OK, now for the somewhat more interesting “US” rules:</p> 1621.1Schristos 1631.8Schristos<table> 1641.1Schristos<tr> 1651.1Schristos <th colspan="6">From the Source File</th> 1661.1Schristos</tr> 1671.1Schristos<tr> 1681.8Schristos <td colspan="6"> 1691.8Schristos <table class="rule"> 1701.8Schristos <tr><td style="border:none;text-align:left"> 1711.8Schristos<pre class="td"> 1721.8Schristos#Rule NAME FROM TO - IN ON AT SAVE LETTER/S 1731.1SchristosRule US 1918 1919 - Mar lastSun 2:00 1:00 D 1741.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 1751.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 1761.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 1771.1SchristosRule US 1945 only - Sep 30 2:00 0 S 1781.1SchristosRule US 1967 2006 - Oct lastSun 2:00 0 S 1791.1SchristosRule US 1967 1973 - Apr lastSun 2:00 1:00 D 1801.1SchristosRule US 1974 only - Jan 6 2:00 1:00 D 1811.1SchristosRule US 1975 only - Feb 23 2:00 1:00 D 1821.1SchristosRule US 1976 1986 - Apr lastSun 2:00 1:00 D 1831.1SchristosRule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 1841.1SchristosRule US 2007 max - Mar Sun>=8 2:00 1:00 D 1851.1SchristosRule US 2007 max - Nov Sun>=1 2:00 0 S 1861.1Schristos</pre> 1871.1Schristos </td></tr></table></td> 1881.1Schristos</tr> 1891.1Schristos<tr> 1901.1Schristos <th colspan="6">Reformatted a Bit</th> 1911.1Schristos</tr> 1921.1Schristos<tr> 1931.1Schristos <th>From</th> 1941.1Schristos <th>To</th> 1951.1Schristos <th colspan="2">On</th> 1961.1Schristos <th>At</th> 1971.1Schristos <th>Action</th> 1981.1Schristos</tr> 1991.8Schristos<tr> 2001.1Schristos <td rowspan="2">1918</td> 2011.1Schristos <td rowspan="2">1919</td> 2021.1Schristos <td rowspan="2">last Sunday</td> 2031.1Schristos <td>in March</td> 2041.1Schristos <td rowspan="3">02:00 local</td> 2051.1Schristos <td>go to daylight saving time</td> 2061.1Schristos</tr> 2071.8Schristos<tr> 2081.1Schristos <td>in October</td> 2091.1Schristos <td>return to standard time</td> 2101.1Schristos</tr> 2111.8Schristos<tr> 2121.1Schristos <td colspan="2">1942 only</td> 2131.1Schristos <td colspan="2">February 9<small><sup>th</sup></small></td> 2141.1Schristos <td>go to “war time”</td> 2151.1Schristos</tr> 2161.8Schristos<tr> 2171.1Schristos <td colspan="2" rowspan="2">1945 only</td> 2181.1Schristos <td colspan="2">August 14<small><sup>th</sup></small></td> 2191.1Schristos <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 2201.1Schristos <td> 2211.1Schristos rename “war time” to “peace<br>time;” 2221.1Schristos clocks don’t change 2231.1Schristos </td> 2241.1Schristos</tr> 2251.8Schristos<tr> 2261.1Schristos <td colspan="2">September 30<small><sup>th</sup></small></td> 2271.1Schristos <td rowspan="9">02:00 local</td> 2281.1Schristos <td rowspan="2">return to standard time</td> 2291.1Schristos</tr> 2301.8Schristos<tr> 2311.1Schristos <td rowspan="2">1967</td> 2321.1Schristos <td>2006</td> 2331.1Schristos <td rowspan="2">last Sunday</td> 2341.1Schristos <td>in October</td> 2351.1Schristos</tr> 2361.8Schristos<tr> 2371.1Schristos <td>1973</td> 2381.1Schristos <td>in April</td> 2391.1Schristos <td rowspan="6">go to daylight saving time</td> 2401.1Schristos</tr> 2411.8Schristos<tr> 2421.1Schristos <td colspan="2">1974 only</td> 2431.1Schristos <td colspan="2">January 6<small><sup>th</sup></small></td> 2441.1Schristos</tr> 2451.8Schristos<tr> 2461.1Schristos <td colspan="2">1975 only</td> 2471.1Schristos <td colspan="2">February 23<small><sup>rd</sup></small></td> 2481.1Schristos</tr> 2491.8Schristos<tr> 2501.1Schristos <td>1976</td> 2511.1Schristos <td>1986</td> 2521.1Schristos <td>last Sunday</td> 2531.1Schristos <td rowspan="2">in April</td> 2541.1Schristos</tr> 2551.8Schristos<tr> 2561.1Schristos <td>1987</td> 2571.1Schristos <td>2006</td> 2581.1Schristos <td>first Sunday</td> 2591.1Schristos</tr> 2601.8Schristos<tr> 2611.1Schristos <td rowspan="2">2007</td> 2621.1Schristos <td rowspan="2">present</td> 2631.1Schristos <td colspan="2">second Sunday in March</td> 2641.1Schristos</tr> 2651.8Schristos<tr> 2661.1Schristos <td colspan="2">first Sunday in November</td> 2671.1Schristos <td>return to standard time</td> 2681.1Schristos</tr> 2691.1Schristos</table> 2701.1Schristos 2711.1Schristos<p>There are two interesting things to note here.</p> 2721.1Schristos 2731.1Schristos<p>First, the time that something happens (in the <code>AT</code> 2741.7Schristoscolumn) is not necessarily the local (wall clock) time. The time can be 2751.1Schristossuffixed with ‘s’ (for “standard”) to mean 2761.7Schristoslocal standard time, different from local (wall clock) time when observing 2771.7Schristosdaylight saving time; or it can be suffixed with ‘g’, 2781.1Schristos‘u’, or ‘z’, all three of which mean the 2791.1Schristosstandard time at the 2801.2Schristos<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 2811.1Schristos‘g’ stands for “<a 2821.1Schristoshref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 2831.1Schristos‘u’ stands for “<a 2841.1Schristoshref="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 2851.1Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 2861.1Schristos(whichever was official at the time); ‘z’ stands for the 2871.1Schristos<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 2881.1SchristosZ (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 2891.7SchristosThe time can also be suffixed with ‘w’ meaning local (wall 2901.7Schristosclock) time; but it usually isn’t because that’s the 2911.1Schristosdefault.</p> 2921.1Schristos 2931.1Schristos<p>Second, the day in the <code>ON</code> column, in addition to 2941.1Schristos“<code>lastSun</code>” or a particular day of the month, 2951.1Schristoscan have the form, “<code>Sun>=</code><i>x</i>” or 2961.1Schristos“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 2971.1Schristosof the month. For example, “<code>Sun>=8</code>” means 2981.1Schristos“the first Sunday on or after the eighth of the month,” in 2991.1Schristosother words, the second Sunday of the month. Furthermore, although 3001.1Schristosthere are no examples above, the weekday needn’t be 3011.1Schristos“<code>Sun</code>” in either form, but can be the usual 3021.1Schristosthree-character English abbreviation for any day of the week.</p> 3031.1Schristos 3041.1Schristos<p>And the US rules give us more examples of a couple of things 3051.1Schristosalready mentioned:</p> 3061.1Schristos 3071.1Schristos<ul> 3081.1Schristos<li>The rules for changing to and from daylight saving time are 3091.1Schristosactually <i>different sets</i> of rules; and the two sets can change 3101.1Schristosindependently. Consider, for example, that the rule for the return to 3111.1Schristosstandard time stayed the same from 1967 to 2006; but the rule for the 3121.1Schristostransition to daylight saving time changed several times in the same 3131.1Schristosperiod. There can also be periods, 1946 to 1966 for example, when no 3141.1Schristosrule from this group is in effect, and so either no transition 3151.1Schristoshappened in those years, or some other rule is in effect (perhaps a 3161.1Schristosstate or other more local rule).</li> 3171.1Schristos 3181.1Schristos<li>The <code>SAVE</code> and <code>LETTER</code> columns 3191.1Schristoscontain <i>steady state</i>, not transitions. Consider, for example, 3201.1Schristosthe transition from “war time” to “peace time” 3211.1Schristosthat happened on August 14, 1945. The “1:00” in 3221.1Schristosthe <code>SAVE</code> column is <i>not</i> an instruction to advance 3231.1Schristosthe clock an hour. It means that clocks should <i>be</i> one hour 3241.1Schristosahead of standard time, which they already are because of the previous 3251.1Schristosrule, so there should be no change.</li> 3261.1Schristos 3271.1Schristos</ul> 3281.1Schristos 3291.1Schristos<p>OK, now let’s look at a Zone record:</p> 3301.1Schristos 3311.8Schristos<table> 3321.1Schristos<tr> 3331.1Schristos <th colspan="5">From the Source File</th> 3341.1Schristos</tr> 3351.1Schristos<tr> 3361.8Schristos <td colspan="5"> 3371.8Schristos <table class="rule"> 3381.8Schristos <tr><td style="border:none;text-align:left"> 3391.8Schristos<pre class="td"> 3401.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 3411.1SchristosZone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 3421.1Schristos -6:00 US C%sT 1920 3431.1Schristos -6:00 Chicago C%sT 1936 Mar 1 2:00 3441.1Schristos -5:00 - EST 1936 Nov 15 2:00 3451.1Schristos -6:00 Chicago C%sT 1942 3461.1Schristos -6:00 US C%sT 1946 3471.1Schristos -6:00 Chicago C%sT 1967 3481.1Schristos -6:00 US C%sT 3491.1Schristos</pre> 3501.1Schristos </td></tr></table></td> 3511.1Schristos</tr> 3521.1Schristos<tr> 3531.1Schristos <th colspan="5">Columns Renamed</th> 3541.1Schristos</tr> 3551.1Schristos<tr> 3561.1Schristos <th rowspan="2">Standard Offset<br> 3571.1Schristos from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 3581.1Schristos Meridian</a></th> 3591.1Schristos <th rowspan="2">Daylight<br>Saving Time</th> 3601.1Schristos <th rowspan="2">Abbreviation(s)</th> 3611.1Schristos <th colspan="2">Ending at Local Time</th> 3621.1Schristos</tr> 3631.1Schristos<tr> 3641.1Schristos <th>Date</th> 3651.1Schristos <th>Time</th> 3661.1Schristos</tr> 3671.8Schristos<tr> 3681.1Schristos <td>−5:50:36</td> 3691.1Schristos <td>not observed</td> 3701.1Schristos <td>LMT</td> 3711.1Schristos <td>1883-11-18</td> 3721.1Schristos <td>12:09:24</td> 3731.1Schristos</tr> 3741.8Schristos<tr> 3751.1Schristos <td rowspan="2">−6:00:00</td> 3761.1Schristos <td>US rules</td> 3771.1Schristos <td rowspan="2">CST or CDT</td> 3781.1Schristos <td>1920-01-01</td> 3791.1Schristos <td>00:00:00</td> 3801.1Schristos</tr> 3811.8Schristos<tr> 3821.1Schristos <td>Chicago rules</td> 3831.1Schristos <td>1936-03-01</td> 3841.1Schristos <td rowspan="2">02:00:00</td> 3851.1Schristos</tr> 3861.8Schristos<tr> 3871.1Schristos <td>−5:00:00</td> 3881.1Schristos <td>not observed</td> 3891.1Schristos <td>EST</td> 3901.1Schristos <td>1936-11-15</td> 3911.1Schristos</tr> 3921.8Schristos<tr> 3931.1Schristos <td rowspan="4">−6:00:00</td> 3941.1Schristos <td>Chicago rules</td> 3951.1Schristos <td>CST or CDT</td> 3961.1Schristos <td>1942-01-01</td> 3971.1Schristos <td rowspan="3">00:00:00</td> 3981.1Schristos</tr> 3991.8Schristos<tr> 4001.1Schristos <td>US rules</td> 4011.1Schristos <td>CST, CWT or CPT</td> 4021.1Schristos <td>1946-01-01</td> 4031.1Schristos</tr> 4041.8Schristos<tr> 4051.1Schristos <td>Chicago rules</td> 4061.1Schristos <td rowspan="2">CST or CDT</td> 4071.1Schristos <td>1967-01-01</td> 4081.1Schristos</tr> 4091.8Schristos<tr> 4101.1Schristos <td>US rules</td> 4111.1Schristos <td colspan="2">—</td> 4121.1Schristos</tr> 4131.1Schristos</table> 4141.1Schristos 4151.1Schristos<p>There are a couple of interesting differences between Zones and Rules.</p> 4161.1Schristos 4171.1Schristos<p>First, and somewhat trivially, whereas Rules are considered to 4181.1Schristoscontain one or more records, a Zone is considered to be a single 4191.1Schristosrecord with zero or more <i>continuation lines</i>. Thus, the keyword, 4201.1Schristos“<code>Zone</code>,” and the zone name are not 4211.1Schristosrepeated. The last line is the one without anything in 4221.1Schristosthe <code>[UNTIL]</code> column.</p> 4231.1Schristos 4241.1Schristos<p>Second, and more fundamentally, each line of a Zone represents a 4251.1Schristossteady state, not a transition between states. The state exists from 4261.1Schristosthe date and time in the previous line’s <code>[UNTIL]</code> 4271.1Schristoscolumn up to the date and time in the current 4281.1Schristosline’s <code>[UNTIL]</code> column. In other words, the date and 4291.1Schristostime in the <code>[UNTIL]</code> column is the instant that separates 4301.1Schristosthis state from the next. Where that would be ambiguous because 4311.1Schristoswe’re setting our clocks back, the <code>[UNTIL]</code> column 4321.1Schristosspecifies the first occurrence of the instant. The state specified by 4331.1Schristosthe last line, the one without anything in the <code>[UNTIL]</code> 4341.1Schristoscolumn, continues to the present.</p> 4351.1Schristos 4361.1Schristos<p>The first line typically specifies the mean solar time observed 4371.1Schristosbefore the introduction of standard time. Since there’s no line before 4381.1Schristosthat, it has no beginning. <code>8-) </code> For some places near the <a 4391.1Schristoshref="https://en.wikipedia.org/wiki/International_Date_Line">International 4401.1SchristosDate Line</a>, the first <i>two</i> lines will show solar times 4411.1Schristosdiffering by 24 hours; this corresponds to a movement of the Date 4421.1SchristosLine. For example:</p> 4431.1Schristos 4441.1Schristos<pre> 4451.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 4461.1SchristosZone America/Juneau 15:02:19 - LMT 1867 Oct 18 4471.1Schristos -8:57:41 - LMT ... 4481.1Schristos</pre> 4491.1Schristos 4501.1Schristos<p>When Alaska was purchased from Russia in 1867, the Date Line moved 4511.1Schristosfrom the Alaska/Canada border to the Bering Strait; and the time in 4521.1SchristosAlaska was then 24 hours earlier than it had 4531.1Schristosbeen. <code><aside></code>(6 October in the Julian calendar, 4541.1Schristoswhich Russia was still using then for religious reasons, was followed 4551.1Schristosby <i>a second instance of the same day with a different name</i>, 18 4561.1SchristosOctober in the Gregorian calendar. Isn’t civil time 4571.1Schristoswonderful? <code>8-)</code>)<code></aside></code></p> 4581.1Schristos 4591.1Schristos<p>The abbreviation, “LMT” stands for “local mean 4601.1Schristostime”, which is an invention of 4611.1Schristosthe <a href="https://en.wikipedia.org/wiki/Tz_database">tz 4621.1Schristosdatabase</a> and was probably never actually used during the 4631.1Schristosperiod. Furthermore, the value is almost certainly wrong except in the 4641.1Schristosarchetypal place after which the zone is named. (The tz database 4651.1Schristosusually doesn’t provide a separate Zone record for places where 4661.1Schristosnothing significant happened after 1970.)</p> 4671.1Schristos 4681.1Schristos<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 4691.1Schristos<ul> 4701.1Schristos<li>A hyphen, a kind of null value, means that we have not set our 4711.1Schristosclocks ahead of standard time.</li> 4721.1Schristos 4731.1Schristos<li>An amount of time (usually but not necessarily “1:00” 4741.1Schristosmeaning one hour) means that we have set our clocks ahead by that 4751.1Schristosamount.</li> 4761.1Schristos 4771.1Schristos<li>Some alphabetic string means that we <i>might have</i> set our 4781.1Schristosclocks ahead; and we need to check the rule the name of which is the 4791.1Schristosgiven alphabetic string.</li> 4801.1Schristos</ul> 4811.1Schristos 4821.1Schristos<p>An example of a specific amount of time is:</p> 4831.1Schristos<pre> 4841.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 4851.6SchristosZone Pacific/Honolulu ... 1933 Apr 30 2:00 4861.6Schristos -10:30 1:00 HDT 1933 May 21 12:00 4871.1Schristos ... 4881.1Schristos</pre> 4891.1Schristos 4901.1Schristos<p>Hawaii tried daylight saving time for three weeks in 1933 and 4911.1Schristosdecided they didn’t like it. <code>8-) </code>Note that 4921.7Schristosthe <code>STDOFF</code> column always contains the standard time 4931.7Schristosoffset, so the local (wall clock) time during this period was GMT − 4941.1Schristos10:30 + 1:00 = GMT − 9:30.</p> 4951.1Schristos 4961.1Schristos<p>The <code>FORMAT</code> column specifies the usual abbreviation of 4971.9Schristosthe time zone name. It should have one of four forms:</p> 4981.1Schristos<ul> 4991.1Schristos 5001.9Schristos<li>a time zone abbreviation that is a string of three or more 5011.9Schristoscharacters that are either ASCII alphanumerics, 5021.9Schristos“<code>+</code>”, or “<code>-</code>”</li> 5031.9Schristos 5041.9Schristos<li>the string “%z”, in which case the 5051.9Schristos“<code>%z</code>” will be replaced by a numeric time zone 5061.9Schristosabbreviation</li> 5071.1Schristos 5081.9Schristos<li>a pair of time zone abbreviations separated by a slash 5091.1Schristos(‘<code>/</code>’), in which case the first string is the 5101.1Schristosabbreviation for the standard time name and the second string is the 5111.1Schristosabbreviation for the daylight saving time name</li> 5121.1Schristos 5131.9Schristos<li>a string containing “<code>%s</code>”, in which case 5141.1Schristosthe “<code>%s</code>” will be replaced by the text in the 5151.9Schristosappropriate Rule’s <code>LETTER</code> column, and the resulting 5161.9Schristosstring should be a time zone abbreviation</li> 5171.1Schristos</ul> 5181.1Schristos 5191.1Schristos<p>The last two make sense only if there’s a named rule in effect.</p> 5201.1Schristos 5211.1Schristos<p>An example of a slash is:</p> 5221.1Schristos<pre> 5231.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 5241.1SchristosZone Europe/London ... 1996 5251.1Schristos 0:00 EU GMT/BST 5261.1Schristos</pre> 5271.1Schristos 5281.1Schristos<p>The current time in the UK is called either Greenwich mean time or 5291.1SchristosBritish summer time.</p> 5301.1Schristos 5311.1Schristos<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 5321.1Schristoshappens when switching to a named rule. To what values should 5331.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 5341.1Schristos 5351.1Schristos<ul> 5361.1Schristos<li>If at least one transition has happened, use 5371.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data from the most 5381.1Schristosrecent.</li> 5391.1Schristos 5401.1Schristos<li>If switching to a named rule before any transition has happened, 5411.1Schristosassume standard time (<code>SAVE</code> zero), and use 5421.1Schristosthe <code>LETTER</code> data from the earliest transition with 5431.1Schristosa <code>SAVE</code> of zero. 5441.1Schristos 5451.1Schristos</ul> 5461.1Schristos 5471.1Schristos<p>And three last things about the <code>FORMAT</code> column:</p> 5481.1Schristos<ul> 5491.1Schristos 5501.1Schristos<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 5511.10Schristosdatabase</a> gives abbreviations for time zones 5521.10Schristosin popular English-language usage. For 5531.1Schristosexample, the last line in 5541.1Schristos<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 5551.1Schristos“HST” for “Hawaii standard time” even though the 5561.3Schristos<a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 5571.1Schristosname for that time zone is “Hawaii-Aleutian standard time.” 5581.1SchristosThis author has read that there are also some places in Australia where 5591.1Schristospopular time zone names differ from the legal ones. 5601.1Schristos 5611.1Schristos<li>No attempt is made to <a 5621.1Schristoshref="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 5631.1Schristosthe abbreviations. They are intended to be the values returned through the 5641.1Schristos<code>"%Z"</code> format specifier to 5651.1Schristos<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 5661.7Schristos<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 5671.1Schristosfunction in the 5681.7Schristos<a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 5691.1Schristos 5701.10Schristos<li>If there is no generally accepted abbreviation for a time zone, 5711.1Schristosa numeric offset is used instead, e.g., <code>+07</code> for 7 hours 5721.1Schristosahead of Greenwich. By convention, <code>-00</code> is used in a 5731.1Schristoszone while uninhabited, where the offset is zero but in some sense 5741.1Schristosthe true offset is undefined. 5751.1Schristos</ul> 5761.1Schristos 5771.1Schristos<p>As a final example, here’s the complete history for Hawaii:</p> 5781.1Schristos 5791.8Schristos<table> 5801.1Schristos<tr> 5811.1Schristos <th colspan="6">Relevant Excerpts from the US Rules</th> 5821.1Schristos</tr> 5831.1Schristos<tr> 5841.8Schristos <td colspan="6"> 5851.8Schristos <table class="rule"> 5861.8Schristos <tr><td style="border:none;text-align:left"> 5871.8Schristos<pre class="td"> 5881.8Schristos#Rule NAME FROM TO - IN ON AT SAVE LETTER/S 5891.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 5901.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 5911.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 5921.6SchristosRule US 1945 only - Sep lastSun 2:00 0 S 5931.1Schristos</pre> 5941.1Schristos </td></tr></table></td> 5951.1Schristos</tr> 5961.1Schristos<tr> 5971.1Schristos <th colspan="6">The Zone Record</th> 5981.1Schristos</tr> 5991.1Schristos<tr> 6001.8Schristos <td colspan="6"> 6011.8Schristos <table class="rule"> 6021.8Schristos <tr><td style="border:none;text-align:left"> 6031.8Schristos<pre class="td"> 6041.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 6051.6SchristosZone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 6061.1Schristos -10:30 - HST 1933 Apr 30 2:00 6071.1Schristos -10:30 1:00 HDT 1933 May 21 2:00 6081.1Schristos -10:30 US H%sT 1947 Jun 8 2:00 6091.1Schristos -10:00 - HST 6101.1Schristos</pre> 6111.1Schristos </td></tr></table></td> 6121.1Schristos</tr> 6131.1Schristos<tr> 6141.1Schristos <th colspan="6">What We Infer</th> 6151.1Schristos</tr> 6161.1Schristos<tr> 6171.1Schristos <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 6181.1Schristos <th rowspan="2">Adjust<br>Clocks</th> 6191.1Schristos <th colspan="2">Time Zone</th> 6201.1Schristos <th colspan="2">Ending at Local Time</th> 6211.1Schristos</tr> 6221.1Schristos<tr> 6231.1Schristos <th>Abbrv.</th> 6241.1Schristos <th>Name</th> 6251.1Schristos <th>Date</th> 6261.1Schristos <th>Time</th> 6271.1Schristos</tr> 6281.8Schristos<tr> 6291.1Schristos <td>−10:31:26</td> 6301.1Schristos <td>—</td> 6311.1Schristos <td>LMT</td> 6321.1Schristos <td>local mean time</td> 6331.6Schristos <td>1896-01-13</td> 6341.1Schristos <td>12:00</td> 6351.1Schristos</tr> 6361.8Schristos<tr> 6371.1Schristos <td>−10:30</td> 6381.1Schristos <td>+0:01:26</td> 6391.1Schristos <td>HST</td> 6401.1Schristos <td>Hawaii standard time</td> 6411.1Schristos <td>1933-04-30</td> 6421.6Schristos <td>02:00</td> 6431.1Schristos</tr> 6441.8Schristos<tr> 6451.1Schristos <td>−9:30</td> 6461.1Schristos <td>+1:00</td> 6471.1Schristos <td>HDT</td> 6481.1Schristos <td>Hawaii daylight time</td> 6491.1Schristos <td>1933-05-21</td> 6501.6Schristos <td>12:00</td> 6511.1Schristos</tr> 6521.8Schristos<tr> 6531.1Schristos <td>−10:30¹</td> 6541.1Schristos <td>−1:00¹</td> 6551.1Schristos <td>HST¹</td> 6561.1Schristos <td>Hawaii standard time</td> 6571.1Schristos <td>1942-02-09</td> 6581.6Schristos <td>02:00</td> 6591.1Schristos</tr> 6601.8Schristos<tr> 6611.1Schristos <td rowspan="2">−9:30</td> 6621.1Schristos <td>+1:00</td> 6631.1Schristos <td>HWT</td> 6641.1Schristos <td>Hawaii war time</td> 6651.1Schristos <td>1945-08-14</td> 6661.1Schristos <td>13:30²</td> 6671.1Schristos</tr> 6681.8Schristos<tr> 6691.1Schristos <td>0</td> 6701.1Schristos <td>HPT</td> 6711.1Schristos <td>Hawaii peace time</td> 6721.1Schristos <td>1945-09-30</td> 6731.1Schristos <td rowspan="2">02:00</td> 6741.1Schristos</tr> 6751.8Schristos<tr> 6761.1Schristos <td>−10:30</td> 6771.1Schristos <td>−1:00</td> 6781.1Schristos <td rowspan="2">HST</td> 6791.1Schristos <td rowspan="2">Hawaii standard time</td> 6801.1Schristos <td>1947-06-08</td> 6811.1Schristos</tr> 6821.8Schristos<tr> 6831.1Schristos <td>−10:00³</td> 6841.1Schristos <td>+0:30³</td> 6851.1Schristos <td colspan="2">—</td> 6861.1Schristos</tr> 6871.1Schristos<tr> 6881.8Schristos <td colspan="6" class="footnote"> 6891.1Schristos ¹Switching to US rules…most recent transition (in 1919) was to standard time 6901.1Schristos </td> 6911.1Schristos</tr> 6921.1Schristos<tr> 6931.8Schristos <td colspan="6" class="footnote"> 6941.1Schristos ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 6951.1Schristos + (−9:30) = 13:30 local 6961.1Schristos </td> 6971.1Schristos</tr> 6981.1Schristos<tr> 6991.8Schristos <td colspan="6" class="footnote"> 7001.1Schristos ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 7011.1Schristos the civil time in Hawaii has been 7021.1Schristos <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 7031.1Schristos − 10:00 year-round. 7041.1Schristos </td> 7051.1Schristos</tr> 7061.1Schristos</table> 7071.1Schristos 7081.1Schristos<p>There will be a short quiz later. <code>8-)</code></p> 7091.1Schristos 7101.1Schristos<hr> 7111.1Schristos<address> 7121.1SchristosThis web page is in the public domain, so clarified as of 7131.1Schristos2015-10-20 by Bill Seymour. 7141.1Schristos<br> 7151.1SchristosAll suggestions and corrections will be welcome; all flames will be amusing. 7161.1SchristosMail to was at pobox dot com. 7171.1Schristos</address> 7181.1Schristos</body> 7191.1Schristos</html> 720