tz-how-to.html revision 1.3
11.1Schristos<!DOCTYPE html 21.1Schristos PUBLIC "-//W3C//DTD HTML 4.01//EN" 31.1Schristos "http://www.w3.org/TR/html4/strict.dtd"> 41.1Schristos<html> 51.3Schristos<head> 61.3Schristos<title>How to Read the tz Database</title> 71.3Schristos<meta http-equiv="Content-type" content='text/html; charset="UTF-8"'> 81.3Schristos</head> 91.1Schristos<body> 101.1Schristos<h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 111.1SchristosDatabase</a> Source Files</h2> 121.1Schristos<h3>by Bill Seymour</h3> 131.1Schristos<p>This page uses the <code>America/Chicago</code> and 141.1Schristos<code>Pacific/Honolulu</code> zones as examples of how to infer 151.1Schristostimes of day from the <a href="tz-link.htm">tz database</a> 161.1Schristossource files. It might be helpful, but not absolutely necessary, 171.1Schristosfor the reader to have already downloaded the 181.2Schristoslatest release of the database and become familiar with the basic layout 191.1Schristosof the data files. The format is explained in the “man 201.1Schristospage” for the zic compiler, <code>zic.8.txt</code>, in 211.1Schristosthe <code>code</code> subdirectory.</p> 221.1Schristos 231.1Schristos<p>We’ll begin by talking about the rules for changing between standard 241.1Schristosand daylight saving time since we’ll need that information when we talk 251.1Schristosabout the zones.</p> 261.1Schristos 271.1Schristos<p>First, let’s consider the special daylight saving time rules 281.1Schristosfor Chicago (from the <code>northamerica</code> file in 291.1Schristosthe <code>data</code> subdirectory):</p> 301.1Schristos 311.1Schristos<table border="1"> 321.1Schristos<tr> 331.1Schristos <th colspan="6">From the Source File</th> 341.1Schristos</tr> 351.1Schristos<tr> 361.1Schristos <td colspan="6" align="center"><table><tr><td> 371.1Schristos<pre> 381.1Schristos#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 391.1SchristosRule Chicago 1920 only - Jun 13 2:00 1:00 D 401.1SchristosRule Chicago 1920 1921 - Oct lastSun 2:00 0 S 411.1SchristosRule Chicago 1921 only - Mar lastSun 2:00 1:00 D 421.1SchristosRule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 431.1SchristosRule Chicago 1922 1954 - Sep lastSun 2:00 0 S 441.1SchristosRule Chicago 1955 1966 - Oct lastSun 2:00 0 S 451.1Schristos</pre> 461.1Schristos </td></tr></table></td> 471.1Schristos</tr> 481.1Schristos<tr> 491.1Schristos <th colspan="6">Reformatted a Bit</th> 501.1Schristos</tr> 511.1Schristos<tr> 521.1Schristos <th>From</th> 531.1Schristos <th>To</th> 541.1Schristos <th colspan="2">On</th> 551.1Schristos <th>At</th> 561.1Schristos <th>Action</th> 571.1Schristos</tr> 581.1Schristos<tr align="center"> 591.1Schristos <td colspan="2">1920 only</td> 601.1Schristos <td colspan="2">June 13<small><sup>th</sup></small></td> 611.1Schristos <td rowspan="6">02:00 local</td> 621.1Schristos <td>go to daylight saving time</td> 631.1Schristos</tr> 641.1Schristos<tr align="center"> 651.1Schristos <td>1920</td> 661.1Schristos <td>1921</td> 671.1Schristos <td rowspan="5">last Sunday</td> 681.1Schristos <td>in October</td> 691.1Schristos <td>return to standard time</td> 701.1Schristos</tr> 711.1Schristos<tr align="center"> 721.1Schristos <td colspan="2">1921 only</td> 731.1Schristos <td>in March</td> 741.1Schristos <td rowspan="2">go to daylight saving time</td> 751.1Schristos</tr> 761.1Schristos<tr align="center"> 771.1Schristos <td rowspan="2">1922</td> 781.1Schristos <td>1966</td> 791.1Schristos <td>in April</td> 801.1Schristos</tr> 811.1Schristos<tr align="center"> 821.1Schristos <td>1954</td> 831.1Schristos <td>in September</td> 841.1Schristos <td rowspan="2">return to standard time</td> 851.1Schristos</tr> 861.1Schristos<tr align="center"> 871.1Schristos <td>1955</td> 881.1Schristos <td>1966</td> 891.1Schristos <td>in October</td> 901.1Schristos</tr> 911.1Schristos</table> 921.1Schristos 931.1Schristos<p>We’ll basically just ignore the <code>TYPE</code> column. 941.1SchristosIn the 2007j release, the most recent as of this writing, the 951.1Schristos<code>TYPE</code> column never contains anything but a hyphen, 961.1Schristosa kind of null value. (From the description in <code>zic.8.txt</code>, 971.1Schristosthis appears to be a mechanism for removing years from a set 981.1Schristosin some localizable way. It’s used in the file, <code>pacificnew</code>, 991.1Schristosto determine whether a given year will have a US presidential election; 1001.1Schristosbut everything related to that use is commented out.) 1011.1Schristos 1021.1Schristos<p>The <code>SAVE</code> column contains the wall clock offset from 1031.1Schristoslocal standard time. 1041.1SchristosThis is usually either zero for standard time or one hour for daylight 1051.1Schristossaving time; but there’s no reason, in principle, why it can’t 1061.1Schristostake on other values. 1071.1Schristos 1081.1Schristos<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 1091.1Schristoscolumn can contain a variable 1101.1Schristospart of the usual abbreviation of the time zone’s name, or it can just 1111.1Schristosbe a hyphen if there’s no variable part. For example, the abbreviation 1121.1Schristosused in the central time zone will be either “CST” or 1131.1Schristos“CDT”. The variable part is ‘S’ or ‘D’; 1141.1Schristosand, sure enough, that’s just what we find in 1151.1Schristosthe <code>LETTER</code> column 1161.1Schristosin the <code>Chicago</code> rules. More about this when we talk about 1171.1Schristos“Zone” lines. 1181.1Schristos 1191.1Schristos<p>One important thing to notice is that “Rule” lines 1201.1Schristoswant at once to be both <i>transitions</i> and <i>steady states</i>: 1211.1Schristos<ul> 1221.1Schristos<li>On the one hand, they represent transitions between standard and 1231.1Schristosdaylight saving time; and any number of Rule lines can be in effect 1241.1Schristosduring a given period (which will always be a non-empty set of 1251.1Schristoscontiguous calendar years).</li> 1261.1Schristos<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 1271.1Schristoscolumns contain state that exists between transitions. More about this 1281.1Schristoswhen we talk about the US rules.</li> 1291.1Schristos</ul> 1301.1Schristos 1311.1Schristos<p>In the example above, the transition to daylight saving time 1321.1Schristoshappened on the 13<small><sup>th</sup></small> of June in 1920, and on 1331.1Schristosthe last Sunday in March in 1921; but the return to standard time 1341.1Schristoshappened on the last Sunday in October in both of those 1351.1Schristosyears. Similarly, the rule for changing to daylight saving time was 1361.1Schristosthe same from 1922 to 1966; but the rule for returning to standard 1371.1Schristostime changed in 1955. Got it?</p> 1381.1Schristos 1391.1Schristos<p>OK, now for the somewhat more interesting “US” rules:</p> 1401.1Schristos 1411.1Schristos<table border="1"> 1421.1Schristos<tr> 1431.1Schristos <th colspan="6">From the Source File</th> 1441.1Schristos</tr> 1451.1Schristos<tr> 1461.1Schristos <td colspan="6" align="center"><table><tr><td> 1471.1Schristos<pre> 1481.1Schristos#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 1491.1SchristosRule US 1918 1919 - Mar lastSun 2:00 1:00 D 1501.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 1511.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 1521.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 1531.1SchristosRule US 1945 only - Sep 30 2:00 0 S 1541.1SchristosRule US 1967 2006 - Oct lastSun 2:00 0 S 1551.1SchristosRule US 1967 1973 - Apr lastSun 2:00 1:00 D 1561.1SchristosRule US 1974 only - Jan 6 2:00 1:00 D 1571.1SchristosRule US 1975 only - Feb 23 2:00 1:00 D 1581.1SchristosRule US 1976 1986 - Apr lastSun 2:00 1:00 D 1591.1SchristosRule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 1601.1SchristosRule US 2007 max - Mar Sun>=8 2:00 1:00 D 1611.1SchristosRule US 2007 max - Nov Sun>=1 2:00 0 S 1621.1Schristos</pre> 1631.1Schristos </td></tr></table></td> 1641.1Schristos</tr> 1651.1Schristos<tr> 1661.1Schristos <th colspan="6">Reformatted a Bit</th> 1671.1Schristos</tr> 1681.1Schristos<tr> 1691.1Schristos <th>From</th> 1701.1Schristos <th>To</th> 1711.1Schristos <th colspan="2">On</th> 1721.1Schristos <th>At</th> 1731.1Schristos <th>Action</th> 1741.1Schristos</tr> 1751.1Schristos<tr align="center"> 1761.1Schristos <td rowspan="2">1918</td> 1771.1Schristos <td rowspan="2">1919</td> 1781.1Schristos <td rowspan="2">last Sunday</td> 1791.1Schristos <td>in March</td> 1801.1Schristos <td rowspan="3">02:00 local</td> 1811.1Schristos <td>go to daylight saving time</td> 1821.1Schristos</tr> 1831.1Schristos<tr align="center"> 1841.1Schristos <td>in October</td> 1851.1Schristos <td>return to standard time</td> 1861.1Schristos</tr> 1871.1Schristos<tr align="center"> 1881.1Schristos <td colspan="2">1942 only</td> 1891.1Schristos <td colspan="2">February 9<small><sup>th</sup></small></td> 1901.1Schristos <td>go to “war time”</td> 1911.1Schristos</tr> 1921.1Schristos<tr align="center"> 1931.1Schristos <td colspan="2" rowspan="2">1945 only</td> 1941.1Schristos <td colspan="2">August 14<small><sup>th</sup></small></td> 1951.1Schristos <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 1961.1Schristos <td> 1971.1Schristos rename “war time” to “peace<br>time;” 1981.1Schristos clocks don’t change 1991.1Schristos </td> 2001.1Schristos</tr> 2011.1Schristos<tr align="center"> 2021.1Schristos <td colspan="2">September 30<small><sup>th</sup></small></td> 2031.1Schristos <td rowspan="9">02:00 local</td> 2041.1Schristos <td rowspan="2">return to standard time</td> 2051.1Schristos</tr> 2061.1Schristos<tr align="center"> 2071.1Schristos <td rowspan="2">1967</td> 2081.1Schristos <td>2006</td> 2091.1Schristos <td rowspan="2">last Sunday</td> 2101.1Schristos <td>in October</td> 2111.1Schristos</tr> 2121.1Schristos<tr align="center"> 2131.1Schristos <td>1973</td> 2141.1Schristos <td>in April</td> 2151.1Schristos <td rowspan="6">go to daylight saving time</td> 2161.1Schristos</tr> 2171.1Schristos<tr align="center"> 2181.1Schristos <td colspan="2">1974 only</td> 2191.1Schristos <td colspan="2">January 6<small><sup>th</sup></small></td> 2201.1Schristos</tr> 2211.1Schristos<tr align="center"> 2221.1Schristos <td colspan="2">1975 only</td> 2231.1Schristos <td colspan="2">February 23<small><sup>rd</sup></small></td> 2241.1Schristos</tr> 2251.1Schristos<tr align="center"> 2261.1Schristos <td>1976</td> 2271.1Schristos <td>1986</td> 2281.1Schristos <td>last Sunday</td> 2291.1Schristos <td rowspan="2">in April</td> 2301.1Schristos</tr> 2311.1Schristos<tr align="center"> 2321.1Schristos <td>1987</td> 2331.1Schristos <td>2006</td> 2341.1Schristos <td>first Sunday</td> 2351.1Schristos</tr> 2361.1Schristos<tr align="center"> 2371.1Schristos <td rowspan="2">2007</td> 2381.1Schristos <td rowspan="2">present</td> 2391.1Schristos <td colspan="2">second Sunday in March</td> 2401.1Schristos</tr> 2411.1Schristos<tr align="center"> 2421.1Schristos <td colspan="2">first Sunday in November</td> 2431.1Schristos <td>return to standard time</td> 2441.1Schristos</tr> 2451.1Schristos</table> 2461.1Schristos 2471.1Schristos<p>There are two interesting things to note here.</p> 2481.1Schristos 2491.1Schristos<p>First, the time that something happens (in the <code>AT</code> 2501.1Schristoscolumn) is not necessarily the local wall clock time. The time can be 2511.1Schristossuffixed with ‘s’ (for “standard”) to mean 2521.1Schristoslocal standard time (different from wall clock time when observing 2531.1Schristosdaylight saving time); or it can be suffixed with ‘g’, 2541.1Schristos‘u’, or ‘z’, all three of which mean the 2551.1Schristosstandard time at the 2561.2Schristos<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 2571.1Schristos‘g’ stands for “<a 2581.1Schristoshref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 2591.1Schristos‘u’ stands for “<a 2601.1Schristoshref="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 2611.1Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 2621.1Schristos(whichever was official at the time); ‘z’ stands for the 2631.1Schristos<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 2641.1SchristosZ (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 2651.1SchristosThe time can also be suffixed with ‘w’ meaning “wall 2661.1Schristosclock time;” but it usually isn’t because that’s the 2671.1Schristosdefault.</p> 2681.1Schristos 2691.1Schristos<p>Second, the day in the <code>ON</code> column, in addition to 2701.1Schristos“<code>lastSun</code>” or a particular day of the month, 2711.1Schristoscan have the form, “<code>Sun>=</code><i>x</i>” or 2721.1Schristos“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 2731.1Schristosof the month. For example, “<code>Sun>=8</code>” means 2741.1Schristos“the first Sunday on or after the eighth of the month,” in 2751.1Schristosother words, the second Sunday of the month. Furthermore, although 2761.1Schristosthere are no examples above, the weekday needn’t be 2771.1Schristos“<code>Sun</code>” in either form, but can be the usual 2781.1Schristosthree-character English abbreviation for any day of the week.</p> 2791.1Schristos 2801.1Schristos<p>And the US rules give us more examples of a couple of things 2811.1Schristosalready mentioned:</p> 2821.1Schristos 2831.1Schristos<ul> 2841.1Schristos<li>The rules for changing to and from daylight saving time are 2851.1Schristosactually <i>different sets</i> of rules; and the two sets can change 2861.1Schristosindependently. Consider, for example, that the rule for the return to 2871.1Schristosstandard time stayed the same from 1967 to 2006; but the rule for the 2881.1Schristostransition to daylight saving time changed several times in the same 2891.1Schristosperiod. There can also be periods, 1946 to 1966 for example, when no 2901.1Schristosrule from this group is in effect, and so either no transition 2911.1Schristoshappened in those years, or some other rule is in effect (perhaps a 2921.1Schristosstate or other more local rule).</li> 2931.1Schristos 2941.1Schristos<li>The <code>SAVE</code> and <code>LETTER</code> columns 2951.1Schristoscontain <i>steady state</i>, not transitions. Consider, for example, 2961.1Schristosthe transition from “war time” to “peace time” 2971.1Schristosthat happened on August 14, 1945. The “1:00” in 2981.1Schristosthe <code>SAVE</code> column is <i>not</i> an instruction to advance 2991.1Schristosthe clock an hour. It means that clocks should <i>be</i> one hour 3001.1Schristosahead of standard time, which they already are because of the previous 3011.1Schristosrule, so there should be no change.</li> 3021.1Schristos 3031.1Schristos</ul> 3041.1Schristos 3051.1Schristos<p>OK, now let’s look at a Zone record:</p> 3061.1Schristos 3071.1Schristos<table border="1"> 3081.1Schristos<tr> 3091.1Schristos <th colspan="5">From the Source File</th> 3101.1Schristos</tr> 3111.1Schristos<tr> 3121.1Schristos <td colspan="6" align="center"><table><tr><td> 3131.1Schristos<pre> 3141.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 3151.1SchristosZone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 3161.1Schristos -6:00 US C%sT 1920 3171.1Schristos -6:00 Chicago C%sT 1936 Mar 1 2:00 3181.1Schristos -5:00 - EST 1936 Nov 15 2:00 3191.1Schristos -6:00 Chicago C%sT 1942 3201.1Schristos -6:00 US C%sT 1946 3211.1Schristos -6:00 Chicago C%sT 1967 3221.1Schristos -6:00 US C%sT 3231.1Schristos</pre> 3241.1Schristos </td></tr></table></td> 3251.1Schristos</tr> 3261.1Schristos<tr> 3271.1Schristos <th colspan="5">Columns Renamed</th> 3281.1Schristos</tr> 3291.1Schristos<tr> 3301.1Schristos <th rowspan="2">Standard Offset<br> 3311.1Schristos from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 3321.1Schristos Meridian</a></th> 3331.1Schristos <th rowspan="2">Daylight<br>Saving Time</th> 3341.1Schristos <th rowspan="2">Abbreviation(s)</th> 3351.1Schristos <th colspan="2">Ending at Local Time</th> 3361.1Schristos</tr> 3371.1Schristos<tr> 3381.1Schristos <th>Date</th> 3391.1Schristos <th>Time</th> 3401.1Schristos</tr> 3411.1Schristos<tr align="center"> 3421.1Schristos <td>−5:50:36</td> 3431.1Schristos <td>not observed</td> 3441.1Schristos <td>LMT</td> 3451.1Schristos <td>1883-11-18</td> 3461.1Schristos <td>12:09:24</td> 3471.1Schristos</tr> 3481.1Schristos<tr align="center"> 3491.1Schristos <td rowspan="2">−6:00:00</td> 3501.1Schristos <td>US rules</td> 3511.1Schristos <td rowspan="2">CST or CDT</td> 3521.1Schristos <td>1920-01-01</td> 3531.1Schristos <td>00:00:00</td> 3541.1Schristos</tr> 3551.1Schristos<tr align="center"> 3561.1Schristos <td>Chicago rules</td> 3571.1Schristos <td>1936-03-01</td> 3581.1Schristos <td rowspan="2">02:00:00</td> 3591.1Schristos</tr> 3601.1Schristos<tr align="center"> 3611.1Schristos <td>−5:00:00</td> 3621.1Schristos <td>not observed</td> 3631.1Schristos <td>EST</td> 3641.1Schristos <td>1936-11-15</td> 3651.1Schristos</tr> 3661.1Schristos<tr align="center"> 3671.1Schristos <td rowspan="4">−6:00:00</td> 3681.1Schristos <td>Chicago rules</td> 3691.1Schristos <td>CST or CDT</td> 3701.1Schristos <td>1942-01-01</td> 3711.1Schristos <td rowspan="3">00:00:00</td> 3721.1Schristos</tr> 3731.1Schristos<tr align="center"> 3741.1Schristos <td>US rules</td> 3751.1Schristos <td>CST, CWT or CPT</td> 3761.1Schristos <td>1946-01-01</td> 3771.1Schristos</tr> 3781.1Schristos<tr align="center"> 3791.1Schristos <td>Chicago rules</td> 3801.1Schristos <td rowspan="2">CST or CDT</td> 3811.1Schristos <td>1967-01-01</td> 3821.1Schristos</tr> 3831.1Schristos<tr align="center"> 3841.1Schristos <td>US rules</td> 3851.1Schristos <td colspan="2">—</td> 3861.1Schristos</tr> 3871.1Schristos</table> 3881.1Schristos 3891.1Schristos<p>There are a couple of interesting differences between Zones and Rules.</p> 3901.1Schristos 3911.1Schristos<p>First, and somewhat trivially, whereas Rules are considered to 3921.1Schristoscontain one or more records, a Zone is considered to be a single 3931.1Schristosrecord with zero or more <i>continuation lines</i>. Thus, the keyword, 3941.1Schristos“<code>Zone</code>,” and the zone name are not 3951.1Schristosrepeated. The last line is the one without anything in 3961.1Schristosthe <code>[UNTIL]</code> column.</p> 3971.1Schristos 3981.1Schristos<p>Second, and more fundamentally, each line of a Zone represents a 3991.1Schristossteady state, not a transition between states. The state exists from 4001.1Schristosthe date and time in the previous line’s <code>[UNTIL]</code> 4011.1Schristoscolumn up to the date and time in the current 4021.1Schristosline’s <code>[UNTIL]</code> column. In other words, the date and 4031.1Schristostime in the <code>[UNTIL]</code> column is the instant that separates 4041.1Schristosthis state from the next. Where that would be ambiguous because 4051.1Schristoswe’re setting our clocks back, the <code>[UNTIL]</code> column 4061.1Schristosspecifies the first occurrence of the instant. The state specified by 4071.1Schristosthe last line, the one without anything in the <code>[UNTIL]</code> 4081.1Schristoscolumn, continues to the present.</p> 4091.1Schristos 4101.1Schristos<p>The first line typically specifies the mean solar time observed 4111.1Schristosbefore the introduction of standard time. Since there’s no line before 4121.1Schristosthat, it has no beginning. <code>8-) </code> For some places near the <a 4131.1Schristoshref="https://en.wikipedia.org/wiki/International_Date_Line">International 4141.1SchristosDate Line</a>, the first <i>two</i> lines will show solar times 4151.1Schristosdiffering by 24 hours; this corresponds to a movement of the Date 4161.1SchristosLine. For example:</p> 4171.1Schristos 4181.1Schristos<pre> 4191.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 4201.1SchristosZone America/Juneau 15:02:19 - LMT 1867 Oct 18 4211.1Schristos -8:57:41 - LMT ... 4221.1Schristos</pre> 4231.1Schristos 4241.1Schristos<p>When Alaska was purchased from Russia in 1867, the Date Line moved 4251.1Schristosfrom the Alaska/Canada border to the Bering Strait; and the time in 4261.1SchristosAlaska was then 24 hours earlier than it had 4271.1Schristosbeen. <code><aside></code>(6 October in the Julian calendar, 4281.1Schristoswhich Russia was still using then for religious reasons, was followed 4291.1Schristosby <i>a second instance of the same day with a different name</i>, 18 4301.1SchristosOctober in the Gregorian calendar. Isn’t civil time 4311.1Schristoswonderful? <code>8-)</code>)<code></aside></code></p> 4321.1Schristos 4331.1Schristos<p>The abbreviation, “LMT” stands for “local mean 4341.1Schristostime”, which is an invention of 4351.1Schristosthe <a href="https://en.wikipedia.org/wiki/Tz_database">tz 4361.1Schristosdatabase</a> and was probably never actually used during the 4371.1Schristosperiod. Furthermore, the value is almost certainly wrong except in the 4381.1Schristosarchetypal place after which the zone is named. (The tz database 4391.1Schristosusually doesn’t provide a separate Zone record for places where 4401.1Schristosnothing significant happened after 1970.)</p> 4411.1Schristos 4421.1Schristos<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 4431.1Schristos<ul> 4441.1Schristos<li>A hyphen, a kind of null value, means that we have not set our 4451.1Schristosclocks ahead of standard time.</li> 4461.1Schristos 4471.1Schristos<li>An amount of time (usually but not necessarily “1:00” 4481.1Schristosmeaning one hour) means that we have set our clocks ahead by that 4491.1Schristosamount.</li> 4501.1Schristos 4511.1Schristos<li>Some alphabetic string means that we <i>might have</i> set our 4521.1Schristosclocks ahead; and we need to check the rule the name of which is the 4531.1Schristosgiven alphabetic string.</li> 4541.1Schristos</ul> 4551.1Schristos 4561.1Schristos<p>An example of a specific amount of time is:</p> 4571.1Schristos<pre> 4581.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 4591.1SchristosZone Pacific/Honolulu ... 1933 Apr 30 2:00 4601.1Schristos -10:30 1:00 HDT 1933 May 21 2:00 4611.1Schristos ... 4621.1Schristos</pre> 4631.1Schristos 4641.1Schristos<p>Hawaii tried daylight saving time for three weeks in 1933 and 4651.1Schristosdecided they didn’t like it. <code>8-) </code>Note that 4661.1Schristosthe <code>GMTOFF</code> column always contains the standard time 4671.1Schristosoffset, so the wall clock time during this period was GMT − 4681.1Schristos10:30 + 1:00 = GMT − 9:30.</p> 4691.1Schristos 4701.1Schristos<p>The <code>FORMAT</code> column specifies the usual abbreviation of 4711.1Schristosthe time zone name. It can have one of three forms:</p> 4721.1Schristos<ul> 4731.1Schristos 4741.1Schristos<li>a string of three or more characters that are either ASCII alphanumerics, 4751.1Schristos“<code>+</code>”, or “<code>-</code>”, 4761.1Schristosin which case that’s the abbreviation</li> 4771.1Schristos 4781.1Schristos<li>a pair of strings separated by a slash 4791.1Schristos(‘<code>/</code>’), in which case the first string is the 4801.1Schristosabbreviation for the standard time name and the second string is the 4811.1Schristosabbreviation for the daylight saving time name</li> 4821.1Schristos 4831.1Schristos<li>a string containing “<code>%s</code>,” in which case 4841.1Schristosthe “<code>%s</code>” will be replaced by the text in the 4851.1Schristosappropriate Rule’s <code>LETTER</code> column</li> 4861.1Schristos</ul> 4871.1Schristos 4881.1Schristos<p>The last two make sense only if there’s a named rule in effect.</p> 4891.1Schristos 4901.1Schristos<p>An example of a slash is:</p> 4911.1Schristos<pre> 4921.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 4931.1SchristosZone Europe/London ... 1996 4941.1Schristos 0:00 EU GMT/BST 4951.1Schristos</pre> 4961.1Schristos 4971.1Schristos<p>The current time in the UK is called either Greenwich mean time or 4981.1SchristosBritish summer time.</p> 4991.1Schristos 5001.1Schristos<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 5011.1Schristoshappens when switching to a named rule. To what values should 5021.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 5031.1Schristos 5041.1Schristos<ul> 5051.1Schristos<li>If at least one transition has happened, use 5061.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data from the most 5071.1Schristosrecent.</li> 5081.1Schristos 5091.1Schristos<li>If switching to a named rule before any transition has happened, 5101.1Schristosassume standard time (<code>SAVE</code> zero), and use 5111.1Schristosthe <code>LETTER</code> data from the earliest transition with 5121.1Schristosa <code>SAVE</code> of zero. 5131.1Schristos 5141.1Schristos</ul> 5151.1Schristos 5161.1Schristos<p>And three last things about the <code>FORMAT</code> column:</p> 5171.1Schristos<ul> 5181.1Schristos 5191.1Schristos<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 5201.1Schristosdatabase</a> gives abbreviations for time zone names in <i>popular 5211.1Schristosusage</i>, which is not necessarily “correct” by law. For 5221.1Schristosexample, the last line in 5231.1Schristos<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 5241.1Schristos“HST” for “Hawaii standard time” even though the 5251.3Schristos<a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 5261.1Schristosname for that time zone is “Hawaii-Aleutian standard time.” 5271.1SchristosThis author has read that there are also some places in Australia where 5281.1Schristospopular time zone names differ from the legal ones. 5291.1Schristos 5301.1Schristos<li>No attempt is made to <a 5311.1Schristoshref="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 5321.1Schristosthe abbreviations. They are intended to be the values returned through the 5331.1Schristos<code>"%Z"</code> format specifier to 5341.1Schristos<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 5351.1Schristos<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 5361.1Schristosfunction in the 5371.3Schristos<a href="http://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 5381.1Schristos 5391.1Schristos<li>If there is no generally-accepted abbreviation for a time zone, 5401.1Schristosa numeric offset is used instead, e.g., <code>+07</code> for 7 hours 5411.1Schristosahead of Greenwich. By convention, <code>-00</code> is used in a 5421.1Schristoszone while uninhabited, where the offset is zero but in some sense 5431.1Schristosthe true offset is undefined. 5441.1Schristos</ul> 5451.1Schristos 5461.1Schristos<p>As a final example, here’s the complete history for Hawaii:</p> 5471.1Schristos 5481.1Schristos<table border="1"> 5491.1Schristos<tr> 5501.1Schristos <th colspan="6">Relevant Excerpts from the US Rules</th> 5511.1Schristos</tr> 5521.1Schristos<tr> 5531.1Schristos <td colspan="6" align="center"><table><tr><td> 5541.1Schristos<pre> 5551.1Schristos#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 5561.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 5571.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 5581.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 5591.1SchristosRule US 1945 only - Sep 30 2:00 0 S 5601.1Schristos</pre> 5611.1Schristos </td></tr></table></td> 5621.1Schristos</tr> 5631.1Schristos<tr> 5641.1Schristos <th colspan="6">The Zone Record</th> 5651.1Schristos</tr> 5661.1Schristos<tr> 5671.1Schristos <td colspan="6" align="center"><table><tr><td> 5681.1Schristos<pre> 5691.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 5701.1SchristosZone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 5711.1Schristos -10:30 - HST 1933 Apr 30 2:00 5721.1Schristos -10:30 1:00 HDT 1933 May 21 2:00 5731.1Schristos -10:30 US H%sT 1947 Jun 8 2:00 5741.1Schristos -10:00 - HST 5751.1Schristos</pre> 5761.1Schristos </td></tr></table></td> 5771.1Schristos</tr> 5781.1Schristos<tr> 5791.1Schristos <th colspan="6">What We Infer</th> 5801.1Schristos</tr> 5811.1Schristos<tr> 5821.1Schristos <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 5831.1Schristos <th rowspan="2">Adjust<br>Clocks</th> 5841.1Schristos <th colspan="2">Time Zone</th> 5851.1Schristos <th colspan="2">Ending at Local Time</th> 5861.1Schristos</tr> 5871.1Schristos<tr> 5881.1Schristos <th>Abbrv.</th> 5891.1Schristos <th>Name</th> 5901.1Schristos <th>Date</th> 5911.1Schristos <th>Time</th> 5921.1Schristos</tr> 5931.1Schristos<tr align="center"> 5941.1Schristos <td>−10:31:26</td> 5951.1Schristos <td>—</td> 5961.1Schristos <td>LMT</td> 5971.1Schristos <td>local mean time</td> 5981.1Schristos <td>1900-01-01</td> 5991.1Schristos <td>12:00</td> 6001.1Schristos</tr> 6011.1Schristos<tr align="center"> 6021.1Schristos <td>−10:30</td> 6031.1Schristos <td>+0:01:26</td> 6041.1Schristos <td>HST</td> 6051.1Schristos <td>Hawaii standard time</td> 6061.1Schristos <td>1933-04-30</td> 6071.1Schristos <td rowspan="3">02:00</td> 6081.1Schristos</tr> 6091.1Schristos<tr align="center"> 6101.1Schristos <td>−9:30</td> 6111.1Schristos <td>+1:00</td> 6121.1Schristos <td>HDT</td> 6131.1Schristos <td>Hawaii daylight time</td> 6141.1Schristos <td>1933-05-21</td> 6151.1Schristos</tr> 6161.1Schristos<tr align="center"> 6171.1Schristos <td>−10:30¹</td> 6181.1Schristos <td>−1:00¹</td> 6191.1Schristos <td>HST¹</td> 6201.1Schristos <td>Hawaii standard time</td> 6211.1Schristos <td>1942-02-09</td> 6221.1Schristos</tr> 6231.1Schristos<tr align="center"> 6241.1Schristos <td rowspan="2">−9:30</td> 6251.1Schristos <td>+1:00</td> 6261.1Schristos <td>HWT</td> 6271.1Schristos <td>Hawaii war time</td> 6281.1Schristos <td>1945-08-14</td> 6291.1Schristos <td>13:30²</td> 6301.1Schristos</tr> 6311.1Schristos<tr align="center"> 6321.1Schristos <td>0</td> 6331.1Schristos <td>HPT</td> 6341.1Schristos <td>Hawaii peace time</td> 6351.1Schristos <td>1945-09-30</td> 6361.1Schristos <td rowspan="2">02:00</td> 6371.1Schristos</tr> 6381.1Schristos<tr align="center"> 6391.1Schristos <td>−10:30</td> 6401.1Schristos <td>−1:00</td> 6411.1Schristos <td rowspan="2">HST</td> 6421.1Schristos <td rowspan="2">Hawaii standard time</td> 6431.1Schristos <td>1947-06-08</td> 6441.1Schristos</tr> 6451.1Schristos<tr align="center"> 6461.1Schristos <td>−10:00³</td> 6471.1Schristos <td>+0:30³</td> 6481.1Schristos <td colspan="2">—</td> 6491.1Schristos</tr> 6501.1Schristos<tr> 6511.1Schristos <td colspan="6"> 6521.1Schristos ¹Switching to US rules…most recent transition (in 1919) was to standard time 6531.1Schristos </td> 6541.1Schristos</tr> 6551.1Schristos<tr> 6561.1Schristos <td colspan="6"> 6571.1Schristos ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 6581.1Schristos + (−9:30) = 13:30 local 6591.1Schristos </td> 6601.1Schristos</tr> 6611.1Schristos<tr> 6621.1Schristos <td colspan="6"> 6631.1Schristos ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 6641.1Schristos the civil time in Hawaii has been 6651.1Schristos <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 6661.1Schristos − 10:00 year-round. 6671.1Schristos </td> 6681.1Schristos</tr> 6691.1Schristos</table> 6701.1Schristos 6711.1Schristos<p>There will be a short quiz later. <code>8-)</code></p> 6721.1Schristos 6731.1Schristos<hr> 6741.1Schristos<address> 6751.1SchristosThis web page is in the public domain, so clarified as of 6761.1Schristos2015-10-20 by Bill Seymour. 6771.1Schristos<br> 6781.1SchristosAll suggestions and corrections will be welcome; all flames will be amusing. 6791.1SchristosMail to was at pobox dot com. 6801.1Schristos</address> 6811.1Schristos</body> 6821.1Schristos</html> 683