tz-how-to.html revision 1.2
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> 51.1Schristos<head><title>How to Read the tz Database</title></head> 61.1Schristos<body> 71.1Schristos<h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 81.1SchristosDatabase</a> Source Files</h2> 91.1Schristos<h3>by Bill Seymour</h3> 101.1Schristos<p>This page uses the <code>America/Chicago</code> and 111.1Schristos<code>Pacific/Honolulu</code> zones as examples of how to infer 121.1Schristostimes of day from the <a href="tz-link.htm">tz database</a> 131.1Schristossource files. It might be helpful, but not absolutely necessary, 141.1Schristosfor the reader to have already downloaded the 151.2Schristoslatest release of the database and become familiar with the basic layout 161.1Schristosof the data files. The format is explained in the “man 171.1Schristospage” for the zic compiler, <code>zic.8.txt</code>, in 181.1Schristosthe <code>code</code> subdirectory.</p> 191.1Schristos 201.1Schristos<p>We’ll begin by talking about the rules for changing between standard 211.1Schristosand daylight saving time since we’ll need that information when we talk 221.1Schristosabout the zones.</p> 231.1Schristos 241.1Schristos<p>First, let’s consider the special daylight saving time rules 251.1Schristosfor Chicago (from the <code>northamerica</code> file in 261.1Schristosthe <code>data</code> subdirectory):</p> 271.1Schristos 281.1Schristos<table border="1"> 291.1Schristos<tr> 301.1Schristos <th colspan="6">From the Source File</th> 311.1Schristos</tr> 321.1Schristos<tr> 331.1Schristos <td colspan="6" align="center"><table><tr><td> 341.1Schristos<pre> 351.1Schristos#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 361.1SchristosRule Chicago 1920 only - Jun 13 2:00 1:00 D 371.1SchristosRule Chicago 1920 1921 - Oct lastSun 2:00 0 S 381.1SchristosRule Chicago 1921 only - Mar lastSun 2:00 1:00 D 391.1SchristosRule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 401.1SchristosRule Chicago 1922 1954 - Sep lastSun 2:00 0 S 411.1SchristosRule Chicago 1955 1966 - Oct lastSun 2:00 0 S 421.1Schristos</pre> 431.1Schristos </td></tr></table></td> 441.1Schristos</tr> 451.1Schristos<tr> 461.1Schristos <th colspan="6">Reformatted a Bit</th> 471.1Schristos</tr> 481.1Schristos<tr> 491.1Schristos <th>From</th> 501.1Schristos <th>To</th> 511.1Schristos <th colspan="2">On</th> 521.1Schristos <th>At</th> 531.1Schristos <th>Action</th> 541.1Schristos</tr> 551.1Schristos<tr align="center"> 561.1Schristos <td colspan="2">1920 only</td> 571.1Schristos <td colspan="2">June 13<small><sup>th</sup></small></td> 581.1Schristos <td rowspan="6">02:00 local</td> 591.1Schristos <td>go to daylight saving time</td> 601.1Schristos</tr> 611.1Schristos<tr align="center"> 621.1Schristos <td>1920</td> 631.1Schristos <td>1921</td> 641.1Schristos <td rowspan="5">last Sunday</td> 651.1Schristos <td>in October</td> 661.1Schristos <td>return to standard time</td> 671.1Schristos</tr> 681.1Schristos<tr align="center"> 691.1Schristos <td colspan="2">1921 only</td> 701.1Schristos <td>in March</td> 711.1Schristos <td rowspan="2">go to daylight saving time</td> 721.1Schristos</tr> 731.1Schristos<tr align="center"> 741.1Schristos <td rowspan="2">1922</td> 751.1Schristos <td>1966</td> 761.1Schristos <td>in April</td> 771.1Schristos</tr> 781.1Schristos<tr align="center"> 791.1Schristos <td>1954</td> 801.1Schristos <td>in September</td> 811.1Schristos <td rowspan="2">return to standard time</td> 821.1Schristos</tr> 831.1Schristos<tr align="center"> 841.1Schristos <td>1955</td> 851.1Schristos <td>1966</td> 861.1Schristos <td>in October</td> 871.1Schristos</tr> 881.1Schristos</table> 891.1Schristos 901.1Schristos<p>We’ll basically just ignore the <code>TYPE</code> column. 911.1SchristosIn the 2007j release, the most recent as of this writing, the 921.1Schristos<code>TYPE</code> column never contains anything but a hyphen, 931.1Schristosa kind of null value. (From the description in <code>zic.8.txt</code>, 941.1Schristosthis appears to be a mechanism for removing years from a set 951.1Schristosin some localizable way. It’s used in the file, <code>pacificnew</code>, 961.1Schristosto determine whether a given year will have a US presidential election; 971.1Schristosbut everything related to that use is commented out.) 981.1Schristos 991.1Schristos<p>The <code>SAVE</code> column contains the wall clock offset from 1001.1Schristoslocal standard time. 1011.1SchristosThis is usually either zero for standard time or one hour for daylight 1021.1Schristossaving time; but there’s no reason, in principle, why it can’t 1031.1Schristostake on other values. 1041.1Schristos 1051.1Schristos<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 1061.1Schristoscolumn can contain a variable 1071.1Schristospart of the usual abbreviation of the time zone’s name, or it can just 1081.1Schristosbe a hyphen if there’s no variable part. For example, the abbreviation 1091.1Schristosused in the central time zone will be either “CST” or 1101.1Schristos“CDT”. The variable part is ‘S’ or ‘D’; 1111.1Schristosand, sure enough, that’s just what we find in 1121.1Schristosthe <code>LETTER</code> column 1131.1Schristosin the <code>Chicago</code> rules. More about this when we talk about 1141.1Schristos“Zone” lines. 1151.1Schristos 1161.1Schristos<p>One important thing to notice is that “Rule” lines 1171.1Schristoswant at once to be both <i>transitions</i> and <i>steady states</i>: 1181.1Schristos<ul> 1191.1Schristos<li>On the one hand, they represent transitions between standard and 1201.1Schristosdaylight saving time; and any number of Rule lines can be in effect 1211.1Schristosduring a given period (which will always be a non-empty set of 1221.1Schristoscontiguous calendar years).</li> 1231.1Schristos<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 1241.1Schristoscolumns contain state that exists between transitions. More about this 1251.1Schristoswhen we talk about the US rules.</li> 1261.1Schristos</ul> 1271.1Schristos 1281.1Schristos<p>In the example above, the transition to daylight saving time 1291.1Schristoshappened on the 13<small><sup>th</sup></small> of June in 1920, and on 1301.1Schristosthe last Sunday in March in 1921; but the return to standard time 1311.1Schristoshappened on the last Sunday in October in both of those 1321.1Schristosyears. Similarly, the rule for changing to daylight saving time was 1331.1Schristosthe same from 1922 to 1966; but the rule for returning to standard 1341.1Schristostime changed in 1955. Got it?</p> 1351.1Schristos 1361.1Schristos<p>OK, now for the somewhat more interesting “US” rules:</p> 1371.1Schristos 1381.1Schristos<table border="1"> 1391.1Schristos<tr> 1401.1Schristos <th colspan="6">From the Source File</th> 1411.1Schristos</tr> 1421.1Schristos<tr> 1431.1Schristos <td colspan="6" align="center"><table><tr><td> 1441.1Schristos<pre> 1451.1Schristos#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 1461.1SchristosRule US 1918 1919 - Mar lastSun 2:00 1:00 D 1471.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 1481.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 1491.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 1501.1SchristosRule US 1945 only - Sep 30 2:00 0 S 1511.1SchristosRule US 1967 2006 - Oct lastSun 2:00 0 S 1521.1SchristosRule US 1967 1973 - Apr lastSun 2:00 1:00 D 1531.1SchristosRule US 1974 only - Jan 6 2:00 1:00 D 1541.1SchristosRule US 1975 only - Feb 23 2:00 1:00 D 1551.1SchristosRule US 1976 1986 - Apr lastSun 2:00 1:00 D 1561.1SchristosRule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 1571.1SchristosRule US 2007 max - Mar Sun>=8 2:00 1:00 D 1581.1SchristosRule US 2007 max - Nov Sun>=1 2:00 0 S 1591.1Schristos</pre> 1601.1Schristos </td></tr></table></td> 1611.1Schristos</tr> 1621.1Schristos<tr> 1631.1Schristos <th colspan="6">Reformatted a Bit</th> 1641.1Schristos</tr> 1651.1Schristos<tr> 1661.1Schristos <th>From</th> 1671.1Schristos <th>To</th> 1681.1Schristos <th colspan="2">On</th> 1691.1Schristos <th>At</th> 1701.1Schristos <th>Action</th> 1711.1Schristos</tr> 1721.1Schristos<tr align="center"> 1731.1Schristos <td rowspan="2">1918</td> 1741.1Schristos <td rowspan="2">1919</td> 1751.1Schristos <td rowspan="2">last Sunday</td> 1761.1Schristos <td>in March</td> 1771.1Schristos <td rowspan="3">02:00 local</td> 1781.1Schristos <td>go to daylight saving time</td> 1791.1Schristos</tr> 1801.1Schristos<tr align="center"> 1811.1Schristos <td>in October</td> 1821.1Schristos <td>return to standard time</td> 1831.1Schristos</tr> 1841.1Schristos<tr align="center"> 1851.1Schristos <td colspan="2">1942 only</td> 1861.1Schristos <td colspan="2">February 9<small><sup>th</sup></small></td> 1871.1Schristos <td>go to “war time”</td> 1881.1Schristos</tr> 1891.1Schristos<tr align="center"> 1901.1Schristos <td colspan="2" rowspan="2">1945 only</td> 1911.1Schristos <td colspan="2">August 14<small><sup>th</sup></small></td> 1921.1Schristos <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 1931.1Schristos <td> 1941.1Schristos rename “war time” to “peace<br>time;” 1951.1Schristos clocks don’t change 1961.1Schristos </td> 1971.1Schristos</tr> 1981.1Schristos<tr align="center"> 1991.1Schristos <td colspan="2">September 30<small><sup>th</sup></small></td> 2001.1Schristos <td rowspan="9">02:00 local</td> 2011.1Schristos <td rowspan="2">return to standard time</td> 2021.1Schristos</tr> 2031.1Schristos<tr align="center"> 2041.1Schristos <td rowspan="2">1967</td> 2051.1Schristos <td>2006</td> 2061.1Schristos <td rowspan="2">last Sunday</td> 2071.1Schristos <td>in October</td> 2081.1Schristos</tr> 2091.1Schristos<tr align="center"> 2101.1Schristos <td>1973</td> 2111.1Schristos <td>in April</td> 2121.1Schristos <td rowspan="6">go to daylight saving time</td> 2131.1Schristos</tr> 2141.1Schristos<tr align="center"> 2151.1Schristos <td colspan="2">1974 only</td> 2161.1Schristos <td colspan="2">January 6<small><sup>th</sup></small></td> 2171.1Schristos</tr> 2181.1Schristos<tr align="center"> 2191.1Schristos <td colspan="2">1975 only</td> 2201.1Schristos <td colspan="2">February 23<small><sup>rd</sup></small></td> 2211.1Schristos</tr> 2221.1Schristos<tr align="center"> 2231.1Schristos <td>1976</td> 2241.1Schristos <td>1986</td> 2251.1Schristos <td>last Sunday</td> 2261.1Schristos <td rowspan="2">in April</td> 2271.1Schristos</tr> 2281.1Schristos<tr align="center"> 2291.1Schristos <td>1987</td> 2301.1Schristos <td>2006</td> 2311.1Schristos <td>first Sunday</td> 2321.1Schristos</tr> 2331.1Schristos<tr align="center"> 2341.1Schristos <td rowspan="2">2007</td> 2351.1Schristos <td rowspan="2">present</td> 2361.1Schristos <td colspan="2">second Sunday in March</td> 2371.1Schristos</tr> 2381.1Schristos<tr align="center"> 2391.1Schristos <td colspan="2">first Sunday in November</td> 2401.1Schristos <td>return to standard time</td> 2411.1Schristos</tr> 2421.1Schristos</table> 2431.1Schristos 2441.1Schristos<p>There are two interesting things to note here.</p> 2451.1Schristos 2461.1Schristos<p>First, the time that something happens (in the <code>AT</code> 2471.1Schristoscolumn) is not necessarily the local wall clock time. The time can be 2481.1Schristossuffixed with ‘s’ (for “standard”) to mean 2491.1Schristoslocal standard time (different from wall clock time when observing 2501.1Schristosdaylight saving time); or it can be suffixed with ‘g’, 2511.1Schristos‘u’, or ‘z’, all three of which mean the 2521.1Schristosstandard time at the 2531.2Schristos<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 2541.1Schristos‘g’ stands for “<a 2551.1Schristoshref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 2561.1Schristos‘u’ stands for “<a 2571.1Schristoshref="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 2581.1Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 2591.1Schristos(whichever was official at the time); ‘z’ stands for the 2601.1Schristos<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 2611.1SchristosZ (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 2621.1SchristosThe time can also be suffixed with ‘w’ meaning “wall 2631.1Schristosclock time;” but it usually isn’t because that’s the 2641.1Schristosdefault.</p> 2651.1Schristos 2661.1Schristos<p>Second, the day in the <code>ON</code> column, in addition to 2671.1Schristos“<code>lastSun</code>” or a particular day of the month, 2681.1Schristoscan have the form, “<code>Sun>=</code><i>x</i>” or 2691.1Schristos“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 2701.1Schristosof the month. For example, “<code>Sun>=8</code>” means 2711.1Schristos“the first Sunday on or after the eighth of the month,” in 2721.1Schristosother words, the second Sunday of the month. Furthermore, although 2731.1Schristosthere are no examples above, the weekday needn’t be 2741.1Schristos“<code>Sun</code>” in either form, but can be the usual 2751.1Schristosthree-character English abbreviation for any day of the week.</p> 2761.1Schristos 2771.1Schristos<p>And the US rules give us more examples of a couple of things 2781.1Schristosalready mentioned:</p> 2791.1Schristos 2801.1Schristos<ul> 2811.1Schristos<li>The rules for changing to and from daylight saving time are 2821.1Schristosactually <i>different sets</i> of rules; and the two sets can change 2831.1Schristosindependently. Consider, for example, that the rule for the return to 2841.1Schristosstandard time stayed the same from 1967 to 2006; but the rule for the 2851.1Schristostransition to daylight saving time changed several times in the same 2861.1Schristosperiod. There can also be periods, 1946 to 1966 for example, when no 2871.1Schristosrule from this group is in effect, and so either no transition 2881.1Schristoshappened in those years, or some other rule is in effect (perhaps a 2891.1Schristosstate or other more local rule).</li> 2901.1Schristos 2911.1Schristos<li>The <code>SAVE</code> and <code>LETTER</code> columns 2921.1Schristoscontain <i>steady state</i>, not transitions. Consider, for example, 2931.1Schristosthe transition from “war time” to “peace time” 2941.1Schristosthat happened on August 14, 1945. The “1:00” in 2951.1Schristosthe <code>SAVE</code> column is <i>not</i> an instruction to advance 2961.1Schristosthe clock an hour. It means that clocks should <i>be</i> one hour 2971.1Schristosahead of standard time, which they already are because of the previous 2981.1Schristosrule, so there should be no change.</li> 2991.1Schristos 3001.1Schristos</ul> 3011.1Schristos 3021.1Schristos<p>OK, now let’s look at a Zone record:</p> 3031.1Schristos 3041.1Schristos<table border="1"> 3051.1Schristos<tr> 3061.1Schristos <th colspan="5">From the Source File</th> 3071.1Schristos</tr> 3081.1Schristos<tr> 3091.1Schristos <td colspan="6" align="center"><table><tr><td> 3101.1Schristos<pre> 3111.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 3121.1SchristosZone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 3131.1Schristos -6:00 US C%sT 1920 3141.1Schristos -6:00 Chicago C%sT 1936 Mar 1 2:00 3151.1Schristos -5:00 - EST 1936 Nov 15 2:00 3161.1Schristos -6:00 Chicago C%sT 1942 3171.1Schristos -6:00 US C%sT 1946 3181.1Schristos -6:00 Chicago C%sT 1967 3191.1Schristos -6:00 US C%sT 3201.1Schristos</pre> 3211.1Schristos </td></tr></table></td> 3221.1Schristos</tr> 3231.1Schristos<tr> 3241.1Schristos <th colspan="5">Columns Renamed</th> 3251.1Schristos</tr> 3261.1Schristos<tr> 3271.1Schristos <th rowspan="2">Standard Offset<br> 3281.1Schristos from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 3291.1Schristos Meridian</a></th> 3301.1Schristos <th rowspan="2">Daylight<br>Saving Time</th> 3311.1Schristos <th rowspan="2">Abbreviation(s)</th> 3321.1Schristos <th colspan="2">Ending at Local Time</th> 3331.1Schristos</tr> 3341.1Schristos<tr> 3351.1Schristos <th>Date</th> 3361.1Schristos <th>Time</th> 3371.1Schristos</tr> 3381.1Schristos<tr align="center"> 3391.1Schristos <td>−5:50:36</td> 3401.1Schristos <td>not observed</td> 3411.1Schristos <td>LMT</td> 3421.1Schristos <td>1883-11-18</td> 3431.1Schristos <td>12:09:24</td> 3441.1Schristos</tr> 3451.1Schristos<tr align="center"> 3461.1Schristos <td rowspan="2">−6:00:00</td> 3471.1Schristos <td>US rules</td> 3481.1Schristos <td rowspan="2">CST or CDT</td> 3491.1Schristos <td>1920-01-01</td> 3501.1Schristos <td>00:00:00</td> 3511.1Schristos</tr> 3521.1Schristos<tr align="center"> 3531.1Schristos <td>Chicago rules</td> 3541.1Schristos <td>1936-03-01</td> 3551.1Schristos <td rowspan="2">02:00:00</td> 3561.1Schristos</tr> 3571.1Schristos<tr align="center"> 3581.1Schristos <td>−5:00:00</td> 3591.1Schristos <td>not observed</td> 3601.1Schristos <td>EST</td> 3611.1Schristos <td>1936-11-15</td> 3621.1Schristos</tr> 3631.1Schristos<tr align="center"> 3641.1Schristos <td rowspan="4">−6:00:00</td> 3651.1Schristos <td>Chicago rules</td> 3661.1Schristos <td>CST or CDT</td> 3671.1Schristos <td>1942-01-01</td> 3681.1Schristos <td rowspan="3">00:00:00</td> 3691.1Schristos</tr> 3701.1Schristos<tr align="center"> 3711.1Schristos <td>US rules</td> 3721.1Schristos <td>CST, CWT or CPT</td> 3731.1Schristos <td>1946-01-01</td> 3741.1Schristos</tr> 3751.1Schristos<tr align="center"> 3761.1Schristos <td>Chicago rules</td> 3771.1Schristos <td rowspan="2">CST or CDT</td> 3781.1Schristos <td>1967-01-01</td> 3791.1Schristos</tr> 3801.1Schristos<tr align="center"> 3811.1Schristos <td>US rules</td> 3821.1Schristos <td colspan="2">—</td> 3831.1Schristos</tr> 3841.1Schristos</table> 3851.1Schristos 3861.1Schristos<p>There are a couple of interesting differences between Zones and Rules.</p> 3871.1Schristos 3881.1Schristos<p>First, and somewhat trivially, whereas Rules are considered to 3891.1Schristoscontain one or more records, a Zone is considered to be a single 3901.1Schristosrecord with zero or more <i>continuation lines</i>. Thus, the keyword, 3911.1Schristos“<code>Zone</code>,” and the zone name are not 3921.1Schristosrepeated. The last line is the one without anything in 3931.1Schristosthe <code>[UNTIL]</code> column.</p> 3941.1Schristos 3951.1Schristos<p>Second, and more fundamentally, each line of a Zone represents a 3961.1Schristossteady state, not a transition between states. The state exists from 3971.1Schristosthe date and time in the previous line’s <code>[UNTIL]</code> 3981.1Schristoscolumn up to the date and time in the current 3991.1Schristosline’s <code>[UNTIL]</code> column. In other words, the date and 4001.1Schristostime in the <code>[UNTIL]</code> column is the instant that separates 4011.1Schristosthis state from the next. Where that would be ambiguous because 4021.1Schristoswe’re setting our clocks back, the <code>[UNTIL]</code> column 4031.1Schristosspecifies the first occurrence of the instant. The state specified by 4041.1Schristosthe last line, the one without anything in the <code>[UNTIL]</code> 4051.1Schristoscolumn, continues to the present.</p> 4061.1Schristos 4071.1Schristos<p>The first line typically specifies the mean solar time observed 4081.1Schristosbefore the introduction of standard time. Since there’s no line before 4091.1Schristosthat, it has no beginning. <code>8-) </code> For some places near the <a 4101.1Schristoshref="https://en.wikipedia.org/wiki/International_Date_Line">International 4111.1SchristosDate Line</a>, the first <i>two</i> lines will show solar times 4121.1Schristosdiffering by 24 hours; this corresponds to a movement of the Date 4131.1SchristosLine. For example:</p> 4141.1Schristos 4151.1Schristos<pre> 4161.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 4171.1SchristosZone America/Juneau 15:02:19 - LMT 1867 Oct 18 4181.1Schristos -8:57:41 - LMT ... 4191.1Schristos</pre> 4201.1Schristos 4211.1Schristos<p>When Alaska was purchased from Russia in 1867, the Date Line moved 4221.1Schristosfrom the Alaska/Canada border to the Bering Strait; and the time in 4231.1SchristosAlaska was then 24 hours earlier than it had 4241.1Schristosbeen. <code><aside></code>(6 October in the Julian calendar, 4251.1Schristoswhich Russia was still using then for religious reasons, was followed 4261.1Schristosby <i>a second instance of the same day with a different name</i>, 18 4271.1SchristosOctober in the Gregorian calendar. Isn’t civil time 4281.1Schristoswonderful? <code>8-)</code>)<code></aside></code></p> 4291.1Schristos 4301.1Schristos<p>The abbreviation, “LMT” stands for “local mean 4311.1Schristostime”, which is an invention of 4321.1Schristosthe <a href="https://en.wikipedia.org/wiki/Tz_database">tz 4331.1Schristosdatabase</a> and was probably never actually used during the 4341.1Schristosperiod. Furthermore, the value is almost certainly wrong except in the 4351.1Schristosarchetypal place after which the zone is named. (The tz database 4361.1Schristosusually doesn’t provide a separate Zone record for places where 4371.1Schristosnothing significant happened after 1970.)</p> 4381.1Schristos 4391.1Schristos<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 4401.1Schristos<ul> 4411.1Schristos<li>A hyphen, a kind of null value, means that we have not set our 4421.1Schristosclocks ahead of standard time.</li> 4431.1Schristos 4441.1Schristos<li>An amount of time (usually but not necessarily “1:00” 4451.1Schristosmeaning one hour) means that we have set our clocks ahead by that 4461.1Schristosamount.</li> 4471.1Schristos 4481.1Schristos<li>Some alphabetic string means that we <i>might have</i> set our 4491.1Schristosclocks ahead; and we need to check the rule the name of which is the 4501.1Schristosgiven alphabetic string.</li> 4511.1Schristos</ul> 4521.1Schristos 4531.1Schristos<p>An example of a specific amount of time is:</p> 4541.1Schristos<pre> 4551.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 4561.1SchristosZone Pacific/Honolulu ... 1933 Apr 30 2:00 4571.1Schristos -10:30 1:00 HDT 1933 May 21 2:00 4581.1Schristos ... 4591.1Schristos</pre> 4601.1Schristos 4611.1Schristos<p>Hawaii tried daylight saving time for three weeks in 1933 and 4621.1Schristosdecided they didn’t like it. <code>8-) </code>Note that 4631.1Schristosthe <code>GMTOFF</code> column always contains the standard time 4641.1Schristosoffset, so the wall clock time during this period was GMT − 4651.1Schristos10:30 + 1:00 = GMT − 9:30.</p> 4661.1Schristos 4671.1Schristos<p>The <code>FORMAT</code> column specifies the usual abbreviation of 4681.1Schristosthe time zone name. It can have one of three forms:</p> 4691.1Schristos<ul> 4701.1Schristos 4711.1Schristos<li>a string of three or more characters that are either ASCII alphanumerics, 4721.1Schristos“<code>+</code>”, or “<code>-</code>”, 4731.1Schristosin which case that’s the abbreviation</li> 4741.1Schristos 4751.1Schristos<li>a pair of strings separated by a slash 4761.1Schristos(‘<code>/</code>’), in which case the first string is the 4771.1Schristosabbreviation for the standard time name and the second string is the 4781.1Schristosabbreviation for the daylight saving time name</li> 4791.1Schristos 4801.1Schristos<li>a string containing “<code>%s</code>,” in which case 4811.1Schristosthe “<code>%s</code>” will be replaced by the text in the 4821.1Schristosappropriate Rule’s <code>LETTER</code> column</li> 4831.1Schristos</ul> 4841.1Schristos 4851.1Schristos<p>The last two make sense only if there’s a named rule in effect.</p> 4861.1Schristos 4871.1Schristos<p>An example of a slash is:</p> 4881.1Schristos<pre> 4891.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 4901.1SchristosZone Europe/London ... 1996 4911.1Schristos 0:00 EU GMT/BST 4921.1Schristos</pre> 4931.1Schristos 4941.1Schristos<p>The current time in the UK is called either Greenwich mean time or 4951.1SchristosBritish summer time.</p> 4961.1Schristos 4971.1Schristos<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 4981.1Schristoshappens when switching to a named rule. To what values should 4991.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 5001.1Schristos 5011.1Schristos<ul> 5021.1Schristos<li>If at least one transition has happened, use 5031.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data from the most 5041.1Schristosrecent.</li> 5051.1Schristos 5061.1Schristos<li>If switching to a named rule before any transition has happened, 5071.1Schristosassume standard time (<code>SAVE</code> zero), and use 5081.1Schristosthe <code>LETTER</code> data from the earliest transition with 5091.1Schristosa <code>SAVE</code> of zero. 5101.1Schristos 5111.1Schristos</ul> 5121.1Schristos 5131.1Schristos<p>And three last things about the <code>FORMAT</code> column:</p> 5141.1Schristos<ul> 5151.1Schristos 5161.1Schristos<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 5171.1Schristosdatabase</a> gives abbreviations for time zone names in <i>popular 5181.1Schristosusage</i>, which is not necessarily “correct” by law. For 5191.1Schristosexample, the last line in 5201.1Schristos<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 5211.1Schristos“HST” for “Hawaii standard time” even though the 5221.1Schristos<a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a> 5231.1Schristosname for that time zone is “Hawaii-Aleutian standard time.” 5241.1SchristosThis author has read that there are also some places in Australia where 5251.1Schristospopular time zone names differ from the legal ones. 5261.1Schristos 5271.1Schristos<li>No attempt is made to <a 5281.1Schristoshref="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 5291.1Schristosthe abbreviations. They are intended to be the values returned through the 5301.1Schristos<code>"%Z"</code> format specifier to 5311.1Schristos<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 5321.1Schristos<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 5331.1Schristosfunction in the 5341.1Schristos<a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 5351.1Schristos 5361.1Schristos<li>If there is no generally-accepted abbreviation for a time zone, 5371.1Schristosa numeric offset is used instead, e.g., <code>+07</code> for 7 hours 5381.1Schristosahead of Greenwich. By convention, <code>-00</code> is used in a 5391.1Schristoszone while uninhabited, where the offset is zero but in some sense 5401.1Schristosthe true offset is undefined. 5411.1Schristos</ul> 5421.1Schristos 5431.1Schristos<p>As a final example, here’s the complete history for Hawaii:</p> 5441.1Schristos 5451.1Schristos<table border="1"> 5461.1Schristos<tr> 5471.1Schristos <th colspan="6">Relevant Excerpts from the US Rules</th> 5481.1Schristos</tr> 5491.1Schristos<tr> 5501.1Schristos <td colspan="6" align="center"><table><tr><td> 5511.1Schristos<pre> 5521.1Schristos#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 5531.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 5541.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 5551.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 5561.1SchristosRule US 1945 only - Sep 30 2:00 0 S 5571.1Schristos</pre> 5581.1Schristos </td></tr></table></td> 5591.1Schristos</tr> 5601.1Schristos<tr> 5611.1Schristos <th colspan="6">The Zone Record</th> 5621.1Schristos</tr> 5631.1Schristos<tr> 5641.1Schristos <td colspan="6" align="center"><table><tr><td> 5651.1Schristos<pre> 5661.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 5671.1SchristosZone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 5681.1Schristos -10:30 - HST 1933 Apr 30 2:00 5691.1Schristos -10:30 1:00 HDT 1933 May 21 2:00 5701.1Schristos -10:30 US H%sT 1947 Jun 8 2:00 5711.1Schristos -10:00 - HST 5721.1Schristos</pre> 5731.1Schristos </td></tr></table></td> 5741.1Schristos</tr> 5751.1Schristos<tr> 5761.1Schristos <th colspan="6">What We Infer</th> 5771.1Schristos</tr> 5781.1Schristos<tr> 5791.1Schristos <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 5801.1Schristos <th rowspan="2">Adjust<br>Clocks</th> 5811.1Schristos <th colspan="2">Time Zone</th> 5821.1Schristos <th colspan="2">Ending at Local Time</th> 5831.1Schristos</tr> 5841.1Schristos<tr> 5851.1Schristos <th>Abbrv.</th> 5861.1Schristos <th>Name</th> 5871.1Schristos <th>Date</th> 5881.1Schristos <th>Time</th> 5891.1Schristos</tr> 5901.1Schristos<tr align="center"> 5911.1Schristos <td>−10:31:26</td> 5921.1Schristos <td>—</td> 5931.1Schristos <td>LMT</td> 5941.1Schristos <td>local mean time</td> 5951.1Schristos <td>1900-01-01</td> 5961.1Schristos <td>12:00</td> 5971.1Schristos</tr> 5981.1Schristos<tr align="center"> 5991.1Schristos <td>−10:30</td> 6001.1Schristos <td>+0:01:26</td> 6011.1Schristos <td>HST</td> 6021.1Schristos <td>Hawaii standard time</td> 6031.1Schristos <td>1933-04-30</td> 6041.1Schristos <td rowspan="3">02:00</td> 6051.1Schristos</tr> 6061.1Schristos<tr align="center"> 6071.1Schristos <td>−9:30</td> 6081.1Schristos <td>+1:00</td> 6091.1Schristos <td>HDT</td> 6101.1Schristos <td>Hawaii daylight time</td> 6111.1Schristos <td>1933-05-21</td> 6121.1Schristos</tr> 6131.1Schristos<tr align="center"> 6141.1Schristos <td>−10:30¹</td> 6151.1Schristos <td>−1:00¹</td> 6161.1Schristos <td>HST¹</td> 6171.1Schristos <td>Hawaii standard time</td> 6181.1Schristos <td>1942-02-09</td> 6191.1Schristos</tr> 6201.1Schristos<tr align="center"> 6211.1Schristos <td rowspan="2">−9:30</td> 6221.1Schristos <td>+1:00</td> 6231.1Schristos <td>HWT</td> 6241.1Schristos <td>Hawaii war time</td> 6251.1Schristos <td>1945-08-14</td> 6261.1Schristos <td>13:30²</td> 6271.1Schristos</tr> 6281.1Schristos<tr align="center"> 6291.1Schristos <td>0</td> 6301.1Schristos <td>HPT</td> 6311.1Schristos <td>Hawaii peace time</td> 6321.1Schristos <td>1945-09-30</td> 6331.1Schristos <td rowspan="2">02:00</td> 6341.1Schristos</tr> 6351.1Schristos<tr align="center"> 6361.1Schristos <td>−10:30</td> 6371.1Schristos <td>−1:00</td> 6381.1Schristos <td rowspan="2">HST</td> 6391.1Schristos <td rowspan="2">Hawaii standard time</td> 6401.1Schristos <td>1947-06-08</td> 6411.1Schristos</tr> 6421.1Schristos<tr align="center"> 6431.1Schristos <td>−10:00³</td> 6441.1Schristos <td>+0:30³</td> 6451.1Schristos <td colspan="2">—</td> 6461.1Schristos</tr> 6471.1Schristos<tr> 6481.1Schristos <td colspan="6"> 6491.1Schristos ¹Switching to US rules…most recent transition (in 1919) was to standard time 6501.1Schristos </td> 6511.1Schristos</tr> 6521.1Schristos<tr> 6531.1Schristos <td colspan="6"> 6541.1Schristos ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 6551.1Schristos + (−9:30) = 13:30 local 6561.1Schristos </td> 6571.1Schristos</tr> 6581.1Schristos<tr> 6591.1Schristos <td colspan="6"> 6601.1Schristos ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 6611.1Schristos the civil time in Hawaii has been 6621.1Schristos <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 6631.1Schristos − 10:00 year-round. 6641.1Schristos </td> 6651.1Schristos</tr> 6661.1Schristos</table> 6671.1Schristos 6681.1Schristos<p>There will be a short quiz later. <code>8-)</code></p> 6691.1Schristos 6701.1Schristos<hr> 6711.1Schristos<address> 6721.1SchristosThis web page is in the public domain, so clarified as of 6731.1Schristos2015-10-20 by Bill Seymour. 6741.1Schristos<br> 6751.1SchristosAll suggestions and corrections will be welcome; all flames will be amusing. 6761.1SchristosMail to was at pobox dot com. 6771.1Schristos</address> 6781.1Schristos</body> 6791.1Schristos</html> 680