tz-how-to.html revision 1.8
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> 201.1Schristos<p>This page 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 281.1Schristosthe <code>code</code> subdirectory.</p> 291.1Schristos 301.1Schristos<p>We’ll begin by talking about the rules for changing between standard 311.1Schristosand daylight saving time since we’ll need that information when we talk 321.1Schristosabout the zones.</p> 331.1Schristos 341.1Schristos<p>First, let’s consider the special daylight saving time rules 351.1Schristosfor Chicago (from the <code>northamerica</code> file in 361.1Schristosthe <code>data</code> subdirectory):</p> 371.1Schristos 381.8Schristos<table> 391.1Schristos<tr> 401.1Schristos <th colspan="6">From the Source File</th> 411.1Schristos</tr> 421.1Schristos<tr> 431.8Schristos <td colspan="6"> 441.8Schristos <table class="rule"> 451.8Schristos <tr><td style="border:none;text-align:left"> 461.8Schristos<pre class="td"> 471.8Schristos#Rule NAME FROM TO - IN ON AT SAVE LETTER 481.1SchristosRule Chicago 1920 only - Jun 13 2:00 1:00 D 491.1SchristosRule Chicago 1920 1921 - Oct lastSun 2:00 0 S 501.1SchristosRule Chicago 1921 only - Mar lastSun 2:00 1:00 D 511.1SchristosRule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 521.1SchristosRule Chicago 1922 1954 - Sep lastSun 2:00 0 S 531.1SchristosRule Chicago 1955 1966 - Oct lastSun 2:00 0 S 541.1Schristos</pre> 551.1Schristos </td></tr></table></td> 561.1Schristos</tr> 571.1Schristos<tr> 581.1Schristos <th colspan="6">Reformatted a Bit</th> 591.1Schristos</tr> 601.1Schristos<tr> 611.1Schristos <th>From</th> 621.1Schristos <th>To</th> 631.1Schristos <th colspan="2">On</th> 641.1Schristos <th>At</th> 651.1Schristos <th>Action</th> 661.1Schristos</tr> 671.8Schristos<tr> 681.1Schristos <td colspan="2">1920 only</td> 691.1Schristos <td colspan="2">June 13<small><sup>th</sup></small></td> 701.1Schristos <td rowspan="6">02:00 local</td> 711.1Schristos <td>go to daylight saving time</td> 721.1Schristos</tr> 731.8Schristos<tr> 741.1Schristos <td>1920</td> 751.1Schristos <td>1921</td> 761.1Schristos <td rowspan="5">last Sunday</td> 771.1Schristos <td>in October</td> 781.1Schristos <td>return to standard time</td> 791.1Schristos</tr> 801.8Schristos<tr> 811.1Schristos <td colspan="2">1921 only</td> 821.1Schristos <td>in March</td> 831.1Schristos <td rowspan="2">go to daylight saving time</td> 841.1Schristos</tr> 851.8Schristos<tr> 861.1Schristos <td rowspan="2">1922</td> 871.1Schristos <td>1966</td> 881.1Schristos <td>in April</td> 891.1Schristos</tr> 901.8Schristos<tr> 911.1Schristos <td>1954</td> 921.1Schristos <td>in September</td> 931.1Schristos <td rowspan="2">return to standard time</td> 941.1Schristos</tr> 951.8Schristos<tr> 961.1Schristos <td>1955</td> 971.1Schristos <td>1966</td> 981.1Schristos <td>in October</td> 991.1Schristos</tr> 1001.1Schristos</table> 1011.1Schristos 1021.8Schristos<p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the 1031.8Schristosfirst and last calendar years defining a contiguous range over which a specific 1041.8SchristosRule line is to apply. The keyword <code>only</code> can be used in the 1051.8Schristos<code>TO</code> field to repeat the value of the <code>FROM</code> field in the 1061.8Schristosevent that a rule should only apply to a single year. Often, the keyword 1071.8Schristos<code>max</code> is used to extend a rule’s application into the 1081.8Schristosindefinite future; it is a platform-agnostic stand-in for the largest 1091.8Schristosrepresentable year. 1101.8Schristos 1111.8Schristos<p>The next column, <code>-</code>, is reserved; for compatibility with earlier 1121.8Schristosreleases, it always contains a hyphen, which acts as a kind of null value. 1131.8SchristosPrior to the 2020b release, it was called the <code>TYPE</code> field, though 1141.8Schristosit was never used in the main data. An obsolescent supplementary file used the 1151.8Schristosfield as a proof-of-concept to allow <code>zic</code> to apply a given Rule 1161.8Schristosline only to certain “types” of years within the specified range as 1171.8Schristosdictated by the output of a separate script, such as: only years which would 1181.8Schristoshave a US presidential election, or only years which wouldn’t. 1191.1Schristos 1201.7Schristos<p>The <code>SAVE</code> column contains the local (wall clock) offset from 1211.1Schristoslocal standard time. 1221.1SchristosThis is usually either zero for standard time or one hour for daylight 1231.1Schristossaving time; but there’s no reason, in principle, why it can’t 1241.1Schristostake on other values. 1251.1Schristos 1261.1Schristos<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 1271.1Schristoscolumn can contain a variable 1281.1Schristospart of the usual abbreviation of the time zone’s name, or it can just 1291.1Schristosbe a hyphen if there’s no variable part. For example, the abbreviation 1301.1Schristosused in the central time zone will be either “CST” or 1311.1Schristos“CDT”. The variable part is ‘S’ or ‘D’; 1321.1Schristosand, sure enough, that’s just what we find in 1331.1Schristosthe <code>LETTER</code> column 1341.1Schristosin the <code>Chicago</code> rules. More about this when we talk about 1351.1Schristos“Zone” lines. 1361.1Schristos 1371.1Schristos<p>One important thing to notice is that “Rule” lines 1381.1Schristoswant at once to be both <i>transitions</i> and <i>steady states</i>: 1391.1Schristos<ul> 1401.1Schristos<li>On the one hand, they represent transitions between standard and 1411.1Schristosdaylight saving time; and any number of Rule lines can be in effect 1421.1Schristosduring a given period (which will always be a non-empty set of 1431.1Schristoscontiguous calendar years).</li> 1441.1Schristos<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 1451.1Schristoscolumns contain state that exists between transitions. More about this 1461.1Schristoswhen we talk about the US rules.</li> 1471.1Schristos</ul> 1481.1Schristos 1491.1Schristos<p>In the example above, the transition to daylight saving time 1501.1Schristoshappened on the 13<small><sup>th</sup></small> of June in 1920, and on 1511.1Schristosthe last Sunday in March in 1921; but the return to standard time 1521.1Schristoshappened on the last Sunday in October in both of those 1531.1Schristosyears. Similarly, the rule for changing to daylight saving time was 1541.1Schristosthe same from 1922 to 1966; but the rule for returning to standard 1551.1Schristostime changed in 1955. Got it?</p> 1561.1Schristos 1571.1Schristos<p>OK, now for the somewhat more interesting “US” rules:</p> 1581.1Schristos 1591.8Schristos<table> 1601.1Schristos<tr> 1611.1Schristos <th colspan="6">From the Source File</th> 1621.1Schristos</tr> 1631.1Schristos<tr> 1641.8Schristos <td colspan="6"> 1651.8Schristos <table class="rule"> 1661.8Schristos <tr><td style="border:none;text-align:left"> 1671.8Schristos<pre class="td"> 1681.8Schristos#Rule NAME FROM TO - IN ON AT SAVE LETTER/S 1691.1SchristosRule US 1918 1919 - Mar lastSun 2:00 1:00 D 1701.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 1711.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 1721.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 1731.1SchristosRule US 1945 only - Sep 30 2:00 0 S 1741.1SchristosRule US 1967 2006 - Oct lastSun 2:00 0 S 1751.1SchristosRule US 1967 1973 - Apr lastSun 2:00 1:00 D 1761.1SchristosRule US 1974 only - Jan 6 2:00 1:00 D 1771.1SchristosRule US 1975 only - Feb 23 2:00 1:00 D 1781.1SchristosRule US 1976 1986 - Apr lastSun 2:00 1:00 D 1791.1SchristosRule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 1801.1SchristosRule US 2007 max - Mar Sun>=8 2:00 1:00 D 1811.1SchristosRule US 2007 max - Nov Sun>=1 2:00 0 S 1821.1Schristos</pre> 1831.1Schristos </td></tr></table></td> 1841.1Schristos</tr> 1851.1Schristos<tr> 1861.1Schristos <th colspan="6">Reformatted a Bit</th> 1871.1Schristos</tr> 1881.1Schristos<tr> 1891.1Schristos <th>From</th> 1901.1Schristos <th>To</th> 1911.1Schristos <th colspan="2">On</th> 1921.1Schristos <th>At</th> 1931.1Schristos <th>Action</th> 1941.1Schristos</tr> 1951.8Schristos<tr> 1961.1Schristos <td rowspan="2">1918</td> 1971.1Schristos <td rowspan="2">1919</td> 1981.1Schristos <td rowspan="2">last Sunday</td> 1991.1Schristos <td>in March</td> 2001.1Schristos <td rowspan="3">02:00 local</td> 2011.1Schristos <td>go to daylight saving time</td> 2021.1Schristos</tr> 2031.8Schristos<tr> 2041.1Schristos <td>in October</td> 2051.1Schristos <td>return to standard time</td> 2061.1Schristos</tr> 2071.8Schristos<tr> 2081.1Schristos <td colspan="2">1942 only</td> 2091.1Schristos <td colspan="2">February 9<small><sup>th</sup></small></td> 2101.1Schristos <td>go to “war time”</td> 2111.1Schristos</tr> 2121.8Schristos<tr> 2131.1Schristos <td colspan="2" rowspan="2">1945 only</td> 2141.1Schristos <td colspan="2">August 14<small><sup>th</sup></small></td> 2151.1Schristos <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 2161.1Schristos <td> 2171.1Schristos rename “war time” to “peace<br>time;” 2181.1Schristos clocks don’t change 2191.1Schristos </td> 2201.1Schristos</tr> 2211.8Schristos<tr> 2221.1Schristos <td colspan="2">September 30<small><sup>th</sup></small></td> 2231.1Schristos <td rowspan="9">02:00 local</td> 2241.1Schristos <td rowspan="2">return to standard time</td> 2251.1Schristos</tr> 2261.8Schristos<tr> 2271.1Schristos <td rowspan="2">1967</td> 2281.1Schristos <td>2006</td> 2291.1Schristos <td rowspan="2">last Sunday</td> 2301.1Schristos <td>in October</td> 2311.1Schristos</tr> 2321.8Schristos<tr> 2331.1Schristos <td>1973</td> 2341.1Schristos <td>in April</td> 2351.1Schristos <td rowspan="6">go to daylight saving time</td> 2361.1Schristos</tr> 2371.8Schristos<tr> 2381.1Schristos <td colspan="2">1974 only</td> 2391.1Schristos <td colspan="2">January 6<small><sup>th</sup></small></td> 2401.1Schristos</tr> 2411.8Schristos<tr> 2421.1Schristos <td colspan="2">1975 only</td> 2431.1Schristos <td colspan="2">February 23<small><sup>rd</sup></small></td> 2441.1Schristos</tr> 2451.8Schristos<tr> 2461.1Schristos <td>1976</td> 2471.1Schristos <td>1986</td> 2481.1Schristos <td>last Sunday</td> 2491.1Schristos <td rowspan="2">in April</td> 2501.1Schristos</tr> 2511.8Schristos<tr> 2521.1Schristos <td>1987</td> 2531.1Schristos <td>2006</td> 2541.1Schristos <td>first Sunday</td> 2551.1Schristos</tr> 2561.8Schristos<tr> 2571.1Schristos <td rowspan="2">2007</td> 2581.1Schristos <td rowspan="2">present</td> 2591.1Schristos <td colspan="2">second Sunday in March</td> 2601.1Schristos</tr> 2611.8Schristos<tr> 2621.1Schristos <td colspan="2">first Sunday in November</td> 2631.1Schristos <td>return to standard time</td> 2641.1Schristos</tr> 2651.1Schristos</table> 2661.1Schristos 2671.1Schristos<p>There are two interesting things to note here.</p> 2681.1Schristos 2691.1Schristos<p>First, the time that something happens (in the <code>AT</code> 2701.7Schristoscolumn) is not necessarily the local (wall clock) time. The time can be 2711.1Schristossuffixed with ‘s’ (for “standard”) to mean 2721.7Schristoslocal standard time, different from local (wall clock) time when observing 2731.7Schristosdaylight saving time; or it can be suffixed with ‘g’, 2741.1Schristos‘u’, or ‘z’, all three of which mean the 2751.1Schristosstandard time at the 2761.2Schristos<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 2771.1Schristos‘g’ stands for “<a 2781.1Schristoshref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 2791.1Schristos‘u’ stands for “<a 2801.1Schristoshref="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 2811.1Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 2821.1Schristos(whichever was official at the time); ‘z’ stands for the 2831.1Schristos<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 2841.1SchristosZ (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 2851.7SchristosThe time can also be suffixed with ‘w’ meaning local (wall 2861.7Schristosclock) time; but it usually isn’t because that’s the 2871.1Schristosdefault.</p> 2881.1Schristos 2891.1Schristos<p>Second, the day in the <code>ON</code> column, in addition to 2901.1Schristos“<code>lastSun</code>” or a particular day of the month, 2911.1Schristoscan have the form, “<code>Sun>=</code><i>x</i>” or 2921.1Schristos“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 2931.1Schristosof the month. For example, “<code>Sun>=8</code>” means 2941.1Schristos“the first Sunday on or after the eighth of the month,” in 2951.1Schristosother words, the second Sunday of the month. Furthermore, although 2961.1Schristosthere are no examples above, the weekday needn’t be 2971.1Schristos“<code>Sun</code>” in either form, but can be the usual 2981.1Schristosthree-character English abbreviation for any day of the week.</p> 2991.1Schristos 3001.1Schristos<p>And the US rules give us more examples of a couple of things 3011.1Schristosalready mentioned:</p> 3021.1Schristos 3031.1Schristos<ul> 3041.1Schristos<li>The rules for changing to and from daylight saving time are 3051.1Schristosactually <i>different sets</i> of rules; and the two sets can change 3061.1Schristosindependently. Consider, for example, that the rule for the return to 3071.1Schristosstandard time stayed the same from 1967 to 2006; but the rule for the 3081.1Schristostransition to daylight saving time changed several times in the same 3091.1Schristosperiod. There can also be periods, 1946 to 1966 for example, when no 3101.1Schristosrule from this group is in effect, and so either no transition 3111.1Schristoshappened in those years, or some other rule is in effect (perhaps a 3121.1Schristosstate or other more local rule).</li> 3131.1Schristos 3141.1Schristos<li>The <code>SAVE</code> and <code>LETTER</code> columns 3151.1Schristoscontain <i>steady state</i>, not transitions. Consider, for example, 3161.1Schristosthe transition from “war time” to “peace time” 3171.1Schristosthat happened on August 14, 1945. The “1:00” in 3181.1Schristosthe <code>SAVE</code> column is <i>not</i> an instruction to advance 3191.1Schristosthe clock an hour. It means that clocks should <i>be</i> one hour 3201.1Schristosahead of standard time, which they already are because of the previous 3211.1Schristosrule, so there should be no change.</li> 3221.1Schristos 3231.1Schristos</ul> 3241.1Schristos 3251.1Schristos<p>OK, now let’s look at a Zone record:</p> 3261.1Schristos 3271.8Schristos<table> 3281.1Schristos<tr> 3291.1Schristos <th colspan="5">From the Source File</th> 3301.1Schristos</tr> 3311.1Schristos<tr> 3321.8Schristos <td colspan="5"> 3331.8Schristos <table class="rule"> 3341.8Schristos <tr><td style="border:none;text-align:left"> 3351.8Schristos<pre class="td"> 3361.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 3371.1SchristosZone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 3381.1Schristos -6:00 US C%sT 1920 3391.1Schristos -6:00 Chicago C%sT 1936 Mar 1 2:00 3401.1Schristos -5:00 - EST 1936 Nov 15 2:00 3411.1Schristos -6:00 Chicago C%sT 1942 3421.1Schristos -6:00 US C%sT 1946 3431.1Schristos -6:00 Chicago C%sT 1967 3441.1Schristos -6:00 US C%sT 3451.1Schristos</pre> 3461.1Schristos </td></tr></table></td> 3471.1Schristos</tr> 3481.1Schristos<tr> 3491.1Schristos <th colspan="5">Columns Renamed</th> 3501.1Schristos</tr> 3511.1Schristos<tr> 3521.1Schristos <th rowspan="2">Standard Offset<br> 3531.1Schristos from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 3541.1Schristos Meridian</a></th> 3551.1Schristos <th rowspan="2">Daylight<br>Saving Time</th> 3561.1Schristos <th rowspan="2">Abbreviation(s)</th> 3571.1Schristos <th colspan="2">Ending at Local Time</th> 3581.1Schristos</tr> 3591.1Schristos<tr> 3601.1Schristos <th>Date</th> 3611.1Schristos <th>Time</th> 3621.1Schristos</tr> 3631.8Schristos<tr> 3641.1Schristos <td>−5:50:36</td> 3651.1Schristos <td>not observed</td> 3661.1Schristos <td>LMT</td> 3671.1Schristos <td>1883-11-18</td> 3681.1Schristos <td>12:09:24</td> 3691.1Schristos</tr> 3701.8Schristos<tr> 3711.1Schristos <td rowspan="2">−6:00:00</td> 3721.1Schristos <td>US rules</td> 3731.1Schristos <td rowspan="2">CST or CDT</td> 3741.1Schristos <td>1920-01-01</td> 3751.1Schristos <td>00:00:00</td> 3761.1Schristos</tr> 3771.8Schristos<tr> 3781.1Schristos <td>Chicago rules</td> 3791.1Schristos <td>1936-03-01</td> 3801.1Schristos <td rowspan="2">02:00:00</td> 3811.1Schristos</tr> 3821.8Schristos<tr> 3831.1Schristos <td>−5:00:00</td> 3841.1Schristos <td>not observed</td> 3851.1Schristos <td>EST</td> 3861.1Schristos <td>1936-11-15</td> 3871.1Schristos</tr> 3881.8Schristos<tr> 3891.1Schristos <td rowspan="4">−6:00:00</td> 3901.1Schristos <td>Chicago rules</td> 3911.1Schristos <td>CST or CDT</td> 3921.1Schristos <td>1942-01-01</td> 3931.1Schristos <td rowspan="3">00:00:00</td> 3941.1Schristos</tr> 3951.8Schristos<tr> 3961.1Schristos <td>US rules</td> 3971.1Schristos <td>CST, CWT or CPT</td> 3981.1Schristos <td>1946-01-01</td> 3991.1Schristos</tr> 4001.8Schristos<tr> 4011.1Schristos <td>Chicago rules</td> 4021.1Schristos <td rowspan="2">CST or CDT</td> 4031.1Schristos <td>1967-01-01</td> 4041.1Schristos</tr> 4051.8Schristos<tr> 4061.1Schristos <td>US rules</td> 4071.1Schristos <td colspan="2">—</td> 4081.1Schristos</tr> 4091.1Schristos</table> 4101.1Schristos 4111.1Schristos<p>There are a couple of interesting differences between Zones and Rules.</p> 4121.1Schristos 4131.1Schristos<p>First, and somewhat trivially, whereas Rules are considered to 4141.1Schristoscontain one or more records, a Zone is considered to be a single 4151.1Schristosrecord with zero or more <i>continuation lines</i>. Thus, the keyword, 4161.1Schristos“<code>Zone</code>,” and the zone name are not 4171.1Schristosrepeated. The last line is the one without anything in 4181.1Schristosthe <code>[UNTIL]</code> column.</p> 4191.1Schristos 4201.1Schristos<p>Second, and more fundamentally, each line of a Zone represents a 4211.1Schristossteady state, not a transition between states. The state exists from 4221.1Schristosthe date and time in the previous line’s <code>[UNTIL]</code> 4231.1Schristoscolumn up to the date and time in the current 4241.1Schristosline’s <code>[UNTIL]</code> column. In other words, the date and 4251.1Schristostime in the <code>[UNTIL]</code> column is the instant that separates 4261.1Schristosthis state from the next. Where that would be ambiguous because 4271.1Schristoswe’re setting our clocks back, the <code>[UNTIL]</code> column 4281.1Schristosspecifies the first occurrence of the instant. The state specified by 4291.1Schristosthe last line, the one without anything in the <code>[UNTIL]</code> 4301.1Schristoscolumn, continues to the present.</p> 4311.1Schristos 4321.1Schristos<p>The first line typically specifies the mean solar time observed 4331.1Schristosbefore the introduction of standard time. Since there’s no line before 4341.1Schristosthat, it has no beginning. <code>8-) </code> For some places near the <a 4351.1Schristoshref="https://en.wikipedia.org/wiki/International_Date_Line">International 4361.1SchristosDate Line</a>, the first <i>two</i> lines will show solar times 4371.1Schristosdiffering by 24 hours; this corresponds to a movement of the Date 4381.1SchristosLine. For example:</p> 4391.1Schristos 4401.1Schristos<pre> 4411.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 4421.1SchristosZone America/Juneau 15:02:19 - LMT 1867 Oct 18 4431.1Schristos -8:57:41 - LMT ... 4441.1Schristos</pre> 4451.1Schristos 4461.1Schristos<p>When Alaska was purchased from Russia in 1867, the Date Line moved 4471.1Schristosfrom the Alaska/Canada border to the Bering Strait; and the time in 4481.1SchristosAlaska was then 24 hours earlier than it had 4491.1Schristosbeen. <code><aside></code>(6 October in the Julian calendar, 4501.1Schristoswhich Russia was still using then for religious reasons, was followed 4511.1Schristosby <i>a second instance of the same day with a different name</i>, 18 4521.1SchristosOctober in the Gregorian calendar. Isn’t civil time 4531.1Schristoswonderful? <code>8-)</code>)<code></aside></code></p> 4541.1Schristos 4551.1Schristos<p>The abbreviation, “LMT” stands for “local mean 4561.1Schristostime”, which is an invention of 4571.1Schristosthe <a href="https://en.wikipedia.org/wiki/Tz_database">tz 4581.1Schristosdatabase</a> and was probably never actually used during the 4591.1Schristosperiod. Furthermore, the value is almost certainly wrong except in the 4601.1Schristosarchetypal place after which the zone is named. (The tz database 4611.1Schristosusually doesn’t provide a separate Zone record for places where 4621.1Schristosnothing significant happened after 1970.)</p> 4631.1Schristos 4641.1Schristos<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 4651.1Schristos<ul> 4661.1Schristos<li>A hyphen, a kind of null value, means that we have not set our 4671.1Schristosclocks ahead of standard time.</li> 4681.1Schristos 4691.1Schristos<li>An amount of time (usually but not necessarily “1:00” 4701.1Schristosmeaning one hour) means that we have set our clocks ahead by that 4711.1Schristosamount.</li> 4721.1Schristos 4731.1Schristos<li>Some alphabetic string means that we <i>might have</i> set our 4741.1Schristosclocks ahead; and we need to check the rule the name of which is the 4751.1Schristosgiven alphabetic string.</li> 4761.1Schristos</ul> 4771.1Schristos 4781.1Schristos<p>An example of a specific amount of time is:</p> 4791.1Schristos<pre> 4801.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 4811.6SchristosZone Pacific/Honolulu ... 1933 Apr 30 2:00 4821.6Schristos -10:30 1:00 HDT 1933 May 21 12:00 4831.1Schristos ... 4841.1Schristos</pre> 4851.1Schristos 4861.1Schristos<p>Hawaii tried daylight saving time for three weeks in 1933 and 4871.1Schristosdecided they didn’t like it. <code>8-) </code>Note that 4881.7Schristosthe <code>STDOFF</code> column always contains the standard time 4891.7Schristosoffset, so the local (wall clock) time during this period was GMT − 4901.1Schristos10:30 + 1:00 = GMT − 9:30.</p> 4911.1Schristos 4921.1Schristos<p>The <code>FORMAT</code> column specifies the usual abbreviation of 4931.1Schristosthe time zone name. It can have one of three forms:</p> 4941.1Schristos<ul> 4951.1Schristos 4961.1Schristos<li>a string of three or more characters that are either ASCII alphanumerics, 4971.1Schristos“<code>+</code>”, or “<code>-</code>”, 4981.1Schristosin which case that’s the abbreviation</li> 4991.1Schristos 5001.1Schristos<li>a pair of strings separated by a slash 5011.1Schristos(‘<code>/</code>’), in which case the first string is the 5021.1Schristosabbreviation for the standard time name and the second string is the 5031.1Schristosabbreviation for the daylight saving time name</li> 5041.1Schristos 5051.1Schristos<li>a string containing “<code>%s</code>,” in which case 5061.1Schristosthe “<code>%s</code>” will be replaced by the text in the 5071.1Schristosappropriate Rule’s <code>LETTER</code> column</li> 5081.1Schristos</ul> 5091.1Schristos 5101.1Schristos<p>The last two make sense only if there’s a named rule in effect.</p> 5111.1Schristos 5121.1Schristos<p>An example of a slash is:</p> 5131.1Schristos<pre> 5141.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 5151.1SchristosZone Europe/London ... 1996 5161.1Schristos 0:00 EU GMT/BST 5171.1Schristos</pre> 5181.1Schristos 5191.1Schristos<p>The current time in the UK is called either Greenwich mean time or 5201.1SchristosBritish summer time.</p> 5211.1Schristos 5221.1Schristos<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 5231.1Schristoshappens when switching to a named rule. To what values should 5241.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 5251.1Schristos 5261.1Schristos<ul> 5271.1Schristos<li>If at least one transition has happened, use 5281.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data from the most 5291.1Schristosrecent.</li> 5301.1Schristos 5311.1Schristos<li>If switching to a named rule before any transition has happened, 5321.1Schristosassume standard time (<code>SAVE</code> zero), and use 5331.1Schristosthe <code>LETTER</code> data from the earliest transition with 5341.1Schristosa <code>SAVE</code> of zero. 5351.1Schristos 5361.1Schristos</ul> 5371.1Schristos 5381.1Schristos<p>And three last things about the <code>FORMAT</code> column:</p> 5391.1Schristos<ul> 5401.1Schristos 5411.1Schristos<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 5421.5Schristosdatabase</a> gives abbreviations for time zones in <i>popular 5431.1Schristosusage</i>, which is not necessarily “correct” by law. For 5441.1Schristosexample, the last line in 5451.1Schristos<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 5461.1Schristos“HST” for “Hawaii standard time” even though the 5471.3Schristos<a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 5481.1Schristosname for that time zone is “Hawaii-Aleutian standard time.” 5491.1SchristosThis author has read that there are also some places in Australia where 5501.1Schristospopular time zone names differ from the legal ones. 5511.1Schristos 5521.1Schristos<li>No attempt is made to <a 5531.1Schristoshref="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 5541.1Schristosthe abbreviations. They are intended to be the values returned through the 5551.1Schristos<code>"%Z"</code> format specifier to 5561.1Schristos<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 5571.7Schristos<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 5581.1Schristosfunction in the 5591.7Schristos<a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 5601.1Schristos 5611.1Schristos<li>If there is no generally-accepted abbreviation for a time zone, 5621.1Schristosa numeric offset is used instead, e.g., <code>+07</code> for 7 hours 5631.1Schristosahead of Greenwich. By convention, <code>-00</code> is used in a 5641.1Schristoszone while uninhabited, where the offset is zero but in some sense 5651.1Schristosthe true offset is undefined. 5661.1Schristos</ul> 5671.1Schristos 5681.1Schristos<p>As a final example, here’s the complete history for Hawaii:</p> 5691.1Schristos 5701.8Schristos<table> 5711.1Schristos<tr> 5721.1Schristos <th colspan="6">Relevant Excerpts from the US Rules</th> 5731.1Schristos</tr> 5741.1Schristos<tr> 5751.8Schristos <td colspan="6"> 5761.8Schristos <table class="rule"> 5771.8Schristos <tr><td style="border:none;text-align:left"> 5781.8Schristos<pre class="td"> 5791.8Schristos#Rule NAME FROM TO - IN ON AT SAVE LETTER/S 5801.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 5811.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 5821.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 5831.6SchristosRule US 1945 only - Sep lastSun 2:00 0 S 5841.1Schristos</pre> 5851.1Schristos </td></tr></table></td> 5861.1Schristos</tr> 5871.1Schristos<tr> 5881.1Schristos <th colspan="6">The Zone Record</th> 5891.1Schristos</tr> 5901.1Schristos<tr> 5911.8Schristos <td colspan="6"> 5921.8Schristos <table class="rule"> 5931.8Schristos <tr><td style="border:none;text-align:left"> 5941.8Schristos<pre class="td"> 5951.7Schristos#Zone NAME STDOFF RULES FORMAT [UNTIL] 5961.6SchristosZone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 5971.1Schristos -10:30 - HST 1933 Apr 30 2:00 5981.1Schristos -10:30 1:00 HDT 1933 May 21 2:00 5991.1Schristos -10:30 US H%sT 1947 Jun 8 2:00 6001.1Schristos -10:00 - HST 6011.1Schristos</pre> 6021.1Schristos </td></tr></table></td> 6031.1Schristos</tr> 6041.1Schristos<tr> 6051.1Schristos <th colspan="6">What We Infer</th> 6061.1Schristos</tr> 6071.1Schristos<tr> 6081.1Schristos <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 6091.1Schristos <th rowspan="2">Adjust<br>Clocks</th> 6101.1Schristos <th colspan="2">Time Zone</th> 6111.1Schristos <th colspan="2">Ending at Local Time</th> 6121.1Schristos</tr> 6131.1Schristos<tr> 6141.1Schristos <th>Abbrv.</th> 6151.1Schristos <th>Name</th> 6161.1Schristos <th>Date</th> 6171.1Schristos <th>Time</th> 6181.1Schristos</tr> 6191.8Schristos<tr> 6201.1Schristos <td>−10:31:26</td> 6211.1Schristos <td>—</td> 6221.1Schristos <td>LMT</td> 6231.1Schristos <td>local mean time</td> 6241.6Schristos <td>1896-01-13</td> 6251.1Schristos <td>12:00</td> 6261.1Schristos</tr> 6271.8Schristos<tr> 6281.1Schristos <td>−10:30</td> 6291.1Schristos <td>+0:01:26</td> 6301.1Schristos <td>HST</td> 6311.1Schristos <td>Hawaii standard time</td> 6321.1Schristos <td>1933-04-30</td> 6331.6Schristos <td>02:00</td> 6341.1Schristos</tr> 6351.8Schristos<tr> 6361.1Schristos <td>−9:30</td> 6371.1Schristos <td>+1:00</td> 6381.1Schristos <td>HDT</td> 6391.1Schristos <td>Hawaii daylight time</td> 6401.1Schristos <td>1933-05-21</td> 6411.6Schristos <td>12:00</td> 6421.1Schristos</tr> 6431.8Schristos<tr> 6441.1Schristos <td>−10:30¹</td> 6451.1Schristos <td>−1:00¹</td> 6461.1Schristos <td>HST¹</td> 6471.1Schristos <td>Hawaii standard time</td> 6481.1Schristos <td>1942-02-09</td> 6491.6Schristos <td>02:00</td> 6501.1Schristos</tr> 6511.8Schristos<tr> 6521.1Schristos <td rowspan="2">−9:30</td> 6531.1Schristos <td>+1:00</td> 6541.1Schristos <td>HWT</td> 6551.1Schristos <td>Hawaii war time</td> 6561.1Schristos <td>1945-08-14</td> 6571.1Schristos <td>13:30²</td> 6581.1Schristos</tr> 6591.8Schristos<tr> 6601.1Schristos <td>0</td> 6611.1Schristos <td>HPT</td> 6621.1Schristos <td>Hawaii peace time</td> 6631.1Schristos <td>1945-09-30</td> 6641.1Schristos <td rowspan="2">02:00</td> 6651.1Schristos</tr> 6661.8Schristos<tr> 6671.1Schristos <td>−10:30</td> 6681.1Schristos <td>−1:00</td> 6691.1Schristos <td rowspan="2">HST</td> 6701.1Schristos <td rowspan="2">Hawaii standard time</td> 6711.1Schristos <td>1947-06-08</td> 6721.1Schristos</tr> 6731.8Schristos<tr> 6741.1Schristos <td>−10:00³</td> 6751.1Schristos <td>+0:30³</td> 6761.1Schristos <td colspan="2">—</td> 6771.1Schristos</tr> 6781.1Schristos<tr> 6791.8Schristos <td colspan="6" class="footnote"> 6801.1Schristos ¹Switching to US rules…most recent transition (in 1919) was to standard time 6811.1Schristos </td> 6821.1Schristos</tr> 6831.1Schristos<tr> 6841.8Schristos <td colspan="6" class="footnote"> 6851.1Schristos ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 6861.1Schristos + (−9:30) = 13:30 local 6871.1Schristos </td> 6881.1Schristos</tr> 6891.1Schristos<tr> 6901.8Schristos <td colspan="6" class="footnote"> 6911.1Schristos ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 6921.1Schristos the civil time in Hawaii has been 6931.1Schristos <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 6941.1Schristos − 10:00 year-round. 6951.1Schristos </td> 6961.1Schristos</tr> 6971.1Schristos</table> 6981.1Schristos 6991.1Schristos<p>There will be a short quiz later. <code>8-)</code></p> 7001.1Schristos 7011.1Schristos<hr> 7021.1Schristos<address> 7031.1SchristosThis web page is in the public domain, so clarified as of 7041.1Schristos2015-10-20 by Bill Seymour. 7051.1Schristos<br> 7061.1SchristosAll suggestions and corrections will be welcome; all flames will be amusing. 7071.1SchristosMail to was at pobox dot com. 7081.1Schristos</address> 7091.1Schristos</body> 7101.1Schristos</html> 711