tz-how-to.html revision 1.1
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 151.1Schristos<a href="http://www.iana.org/time-zones/repository/tzdata-latest.tar.gz">latest 161.1Schristosrelease of the database</a> and become familiar with the basic layout 171.1Schristosof the data files. The format is explained in the “man 181.1Schristospage” for the zic compiler, <code>zic.8.txt</code>, in 191.1Schristosthe <code>code</code> subdirectory.</p> 201.1Schristos 211.1Schristos<p>We’ll begin by talking about the rules for changing between standard 221.1Schristosand daylight saving time since we’ll need that information when we talk 231.1Schristosabout the zones.</p> 241.1Schristos 251.1Schristos<p>First, let’s consider the special daylight saving time rules 261.1Schristosfor Chicago (from the <code>northamerica</code> file in 271.1Schristosthe <code>data</code> subdirectory):</p> 281.1Schristos 291.1Schristos<table border="1"> 301.1Schristos<tr> 311.1Schristos <th colspan="6">From the Source File</th> 321.1Schristos</tr> 331.1Schristos<tr> 341.1Schristos <td colspan="6" align="center"><table><tr><td> 351.1Schristos<pre> 361.1Schristos#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 371.1SchristosRule Chicago 1920 only - Jun 13 2:00 1:00 D 381.1SchristosRule Chicago 1920 1921 - Oct lastSun 2:00 0 S 391.1SchristosRule Chicago 1921 only - Mar lastSun 2:00 1:00 D 401.1SchristosRule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 411.1SchristosRule Chicago 1922 1954 - Sep lastSun 2:00 0 S 421.1SchristosRule Chicago 1955 1966 - Oct lastSun 2:00 0 S 431.1Schristos</pre> 441.1Schristos </td></tr></table></td> 451.1Schristos</tr> 461.1Schristos<tr> 471.1Schristos <th colspan="6">Reformatted a Bit</th> 481.1Schristos</tr> 491.1Schristos<tr> 501.1Schristos <th>From</th> 511.1Schristos <th>To</th> 521.1Schristos <th colspan="2">On</th> 531.1Schristos <th>At</th> 541.1Schristos <th>Action</th> 551.1Schristos</tr> 561.1Schristos<tr align="center"> 571.1Schristos <td colspan="2">1920 only</td> 581.1Schristos <td colspan="2">June 13<small><sup>th</sup></small></td> 591.1Schristos <td rowspan="6">02:00 local</td> 601.1Schristos <td>go to daylight saving time</td> 611.1Schristos</tr> 621.1Schristos<tr align="center"> 631.1Schristos <td>1920</td> 641.1Schristos <td>1921</td> 651.1Schristos <td rowspan="5">last Sunday</td> 661.1Schristos <td>in October</td> 671.1Schristos <td>return to standard time</td> 681.1Schristos</tr> 691.1Schristos<tr align="center"> 701.1Schristos <td colspan="2">1921 only</td> 711.1Schristos <td>in March</td> 721.1Schristos <td rowspan="2">go to daylight saving time</td> 731.1Schristos</tr> 741.1Schristos<tr align="center"> 751.1Schristos <td rowspan="2">1922</td> 761.1Schristos <td>1966</td> 771.1Schristos <td>in April</td> 781.1Schristos</tr> 791.1Schristos<tr align="center"> 801.1Schristos <td>1954</td> 811.1Schristos <td>in September</td> 821.1Schristos <td rowspan="2">return to standard time</td> 831.1Schristos</tr> 841.1Schristos<tr align="center"> 851.1Schristos <td>1955</td> 861.1Schristos <td>1966</td> 871.1Schristos <td>in October</td> 881.1Schristos</tr> 891.1Schristos</table> 901.1Schristos 911.1Schristos<p>We’ll basically just ignore the <code>TYPE</code> column. 921.1SchristosIn the 2007j release, the most recent as of this writing, the 931.1Schristos<code>TYPE</code> column never contains anything but a hyphen, 941.1Schristosa kind of null value. (From the description in <code>zic.8.txt</code>, 951.1Schristosthis appears to be a mechanism for removing years from a set 961.1Schristosin some localizable way. It’s used in the file, <code>pacificnew</code>, 971.1Schristosto determine whether a given year will have a US presidential election; 981.1Schristosbut everything related to that use is commented out.) 991.1Schristos 1001.1Schristos<p>The <code>SAVE</code> column contains the wall clock offset from 1011.1Schristoslocal standard time. 1021.1SchristosThis is usually either zero for standard time or one hour for daylight 1031.1Schristossaving time; but there’s no reason, in principle, why it can’t 1041.1Schristostake on other values. 1051.1Schristos 1061.1Schristos<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 1071.1Schristoscolumn can contain a variable 1081.1Schristospart of the usual abbreviation of the time zone’s name, or it can just 1091.1Schristosbe a hyphen if there’s no variable part. For example, the abbreviation 1101.1Schristosused in the central time zone will be either “CST” or 1111.1Schristos“CDT”. The variable part is ‘S’ or ‘D’; 1121.1Schristosand, sure enough, that’s just what we find in 1131.1Schristosthe <code>LETTER</code> column 1141.1Schristosin the <code>Chicago</code> rules. More about this when we talk about 1151.1Schristos“Zone” lines. 1161.1Schristos 1171.1Schristos<p>One important thing to notice is that “Rule” lines 1181.1Schristoswant at once to be both <i>transitions</i> and <i>steady states</i>: 1191.1Schristos<ul> 1201.1Schristos<li>On the one hand, they represent transitions between standard and 1211.1Schristosdaylight saving time; and any number of Rule lines can be in effect 1221.1Schristosduring a given period (which will always be a non-empty set of 1231.1Schristoscontiguous calendar years).</li> 1241.1Schristos<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 1251.1Schristoscolumns contain state that exists between transitions. More about this 1261.1Schristoswhen we talk about the US rules.</li> 1271.1Schristos</ul> 1281.1Schristos 1291.1Schristos<p>In the example above, the transition to daylight saving time 1301.1Schristoshappened on the 13<small><sup>th</sup></small> of June in 1920, and on 1311.1Schristosthe last Sunday in March in 1921; but the return to standard time 1321.1Schristoshappened on the last Sunday in October in both of those 1331.1Schristosyears. Similarly, the rule for changing to daylight saving time was 1341.1Schristosthe same from 1922 to 1966; but the rule for returning to standard 1351.1Schristostime changed in 1955. Got it?</p> 1361.1Schristos 1371.1Schristos<p>OK, now for the somewhat more interesting “US” rules:</p> 1381.1Schristos 1391.1Schristos<table border="1"> 1401.1Schristos<tr> 1411.1Schristos <th colspan="6">From the Source File</th> 1421.1Schristos</tr> 1431.1Schristos<tr> 1441.1Schristos <td colspan="6" align="center"><table><tr><td> 1451.1Schristos<pre> 1461.1Schristos#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 1471.1SchristosRule US 1918 1919 - Mar lastSun 2:00 1:00 D 1481.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 1491.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 1501.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 1511.1SchristosRule US 1945 only - Sep 30 2:00 0 S 1521.1SchristosRule US 1967 2006 - Oct lastSun 2:00 0 S 1531.1SchristosRule US 1967 1973 - Apr lastSun 2:00 1:00 D 1541.1SchristosRule US 1974 only - Jan 6 2:00 1:00 D 1551.1SchristosRule US 1975 only - Feb 23 2:00 1:00 D 1561.1SchristosRule US 1976 1986 - Apr lastSun 2:00 1:00 D 1571.1SchristosRule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 1581.1SchristosRule US 2007 max - Mar Sun>=8 2:00 1:00 D 1591.1SchristosRule US 2007 max - Nov Sun>=1 2:00 0 S 1601.1Schristos</pre> 1611.1Schristos </td></tr></table></td> 1621.1Schristos</tr> 1631.1Schristos<tr> 1641.1Schristos <th colspan="6">Reformatted a Bit</th> 1651.1Schristos</tr> 1661.1Schristos<tr> 1671.1Schristos <th>From</th> 1681.1Schristos <th>To</th> 1691.1Schristos <th colspan="2">On</th> 1701.1Schristos <th>At</th> 1711.1Schristos <th>Action</th> 1721.1Schristos</tr> 1731.1Schristos<tr align="center"> 1741.1Schristos <td rowspan="2">1918</td> 1751.1Schristos <td rowspan="2">1919</td> 1761.1Schristos <td rowspan="2">last Sunday</td> 1771.1Schristos <td>in March</td> 1781.1Schristos <td rowspan="3">02:00 local</td> 1791.1Schristos <td>go to daylight saving time</td> 1801.1Schristos</tr> 1811.1Schristos<tr align="center"> 1821.1Schristos <td>in October</td> 1831.1Schristos <td>return to standard time</td> 1841.1Schristos</tr> 1851.1Schristos<tr align="center"> 1861.1Schristos <td colspan="2">1942 only</td> 1871.1Schristos <td colspan="2">February 9<small><sup>th</sup></small></td> 1881.1Schristos <td>go to “war time”</td> 1891.1Schristos</tr> 1901.1Schristos<tr align="center"> 1911.1Schristos <td colspan="2" rowspan="2">1945 only</td> 1921.1Schristos <td colspan="2">August 14<small><sup>th</sup></small></td> 1931.1Schristos <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 1941.1Schristos <td> 1951.1Schristos rename “war time” to “peace<br>time;” 1961.1Schristos clocks don’t change 1971.1Schristos </td> 1981.1Schristos</tr> 1991.1Schristos<tr align="center"> 2001.1Schristos <td colspan="2">September 30<small><sup>th</sup></small></td> 2011.1Schristos <td rowspan="9">02:00 local</td> 2021.1Schristos <td rowspan="2">return to standard time</td> 2031.1Schristos</tr> 2041.1Schristos<tr align="center"> 2051.1Schristos <td rowspan="2">1967</td> 2061.1Schristos <td>2006</td> 2071.1Schristos <td rowspan="2">last Sunday</td> 2081.1Schristos <td>in October</td> 2091.1Schristos</tr> 2101.1Schristos<tr align="center"> 2111.1Schristos <td>1973</td> 2121.1Schristos <td>in April</td> 2131.1Schristos <td rowspan="6">go to daylight saving time</td> 2141.1Schristos</tr> 2151.1Schristos<tr align="center"> 2161.1Schristos <td colspan="2">1974 only</td> 2171.1Schristos <td colspan="2">January 6<small><sup>th</sup></small></td> 2181.1Schristos</tr> 2191.1Schristos<tr align="center"> 2201.1Schristos <td colspan="2">1975 only</td> 2211.1Schristos <td colspan="2">February 23<small><sup>rd</sup></small></td> 2221.1Schristos</tr> 2231.1Schristos<tr align="center"> 2241.1Schristos <td>1976</td> 2251.1Schristos <td>1986</td> 2261.1Schristos <td>last Sunday</td> 2271.1Schristos <td rowspan="2">in April</td> 2281.1Schristos</tr> 2291.1Schristos<tr align="center"> 2301.1Schristos <td>1987</td> 2311.1Schristos <td>2006</td> 2321.1Schristos <td>first Sunday</td> 2331.1Schristos</tr> 2341.1Schristos<tr align="center"> 2351.1Schristos <td rowspan="2">2007</td> 2361.1Schristos <td rowspan="2">present</td> 2371.1Schristos <td colspan="2">second Sunday in March</td> 2381.1Schristos</tr> 2391.1Schristos<tr align="center"> 2401.1Schristos <td colspan="2">first Sunday in November</td> 2411.1Schristos <td>return to standard time</td> 2421.1Schristos</tr> 2431.1Schristos</table> 2441.1Schristos 2451.1Schristos<p>There are two interesting things to note here.</p> 2461.1Schristos 2471.1Schristos<p>First, the time that something happens (in the <code>AT</code> 2481.1Schristoscolumn) is not necessarily the local wall clock time. The time can be 2491.1Schristossuffixed with ‘s’ (for “standard”) to mean 2501.1Schristoslocal standard time (different from wall clock time when observing 2511.1Schristosdaylight saving time); or it can be suffixed with ‘g’, 2521.1Schristos‘u’, or ‘z’, all three of which mean the 2531.1Schristosstandard time at the 2541.1Schristos<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridan</a>. 2551.1Schristos‘g’ stands for “<a 2561.1Schristoshref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 2571.1Schristos‘u’ stands for “<a 2581.1Schristoshref="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 2591.1Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 2601.1Schristos(whichever was official at the time); ‘z’ stands for the 2611.1Schristos<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 2621.1SchristosZ (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 2631.1SchristosThe time can also be suffixed with ‘w’ meaning “wall 2641.1Schristosclock time;” but it usually isn’t because that’s the 2651.1Schristosdefault.</p> 2661.1Schristos 2671.1Schristos<p>Second, the day in the <code>ON</code> column, in addition to 2681.1Schristos“<code>lastSun</code>” or a particular day of the month, 2691.1Schristoscan have the form, “<code>Sun>=</code><i>x</i>” or 2701.1Schristos“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 2711.1Schristosof the month. For example, “<code>Sun>=8</code>” means 2721.1Schristos“the first Sunday on or after the eighth of the month,” in 2731.1Schristosother words, the second Sunday of the month. Furthermore, although 2741.1Schristosthere are no examples above, the weekday needn’t be 2751.1Schristos“<code>Sun</code>” in either form, but can be the usual 2761.1Schristosthree-character English abbreviation for any day of the week.</p> 2771.1Schristos 2781.1Schristos<p>And the US rules give us more examples of a couple of things 2791.1Schristosalready mentioned:</p> 2801.1Schristos 2811.1Schristos<ul> 2821.1Schristos<li>The rules for changing to and from daylight saving time are 2831.1Schristosactually <i>different sets</i> of rules; and the two sets can change 2841.1Schristosindependently. Consider, for example, that the rule for the return to 2851.1Schristosstandard time stayed the same from 1967 to 2006; but the rule for the 2861.1Schristostransition to daylight saving time changed several times in the same 2871.1Schristosperiod. There can also be periods, 1946 to 1966 for example, when no 2881.1Schristosrule from this group is in effect, and so either no transition 2891.1Schristoshappened in those years, or some other rule is in effect (perhaps a 2901.1Schristosstate or other more local rule).</li> 2911.1Schristos 2921.1Schristos<li>The <code>SAVE</code> and <code>LETTER</code> columns 2931.1Schristoscontain <i>steady state</i>, not transitions. Consider, for example, 2941.1Schristosthe transition from “war time” to “peace time” 2951.1Schristosthat happened on August 14, 1945. The “1:00” in 2961.1Schristosthe <code>SAVE</code> column is <i>not</i> an instruction to advance 2971.1Schristosthe clock an hour. It means that clocks should <i>be</i> one hour 2981.1Schristosahead of standard time, which they already are because of the previous 2991.1Schristosrule, so there should be no change.</li> 3001.1Schristos 3011.1Schristos</ul> 3021.1Schristos 3031.1Schristos<p>OK, now let’s look at a Zone record:</p> 3041.1Schristos 3051.1Schristos<table border="1"> 3061.1Schristos<tr> 3071.1Schristos <th colspan="5">From the Source File</th> 3081.1Schristos</tr> 3091.1Schristos<tr> 3101.1Schristos <td colspan="6" align="center"><table><tr><td> 3111.1Schristos<pre> 3121.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 3131.1SchristosZone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 3141.1Schristos -6:00 US C%sT 1920 3151.1Schristos -6:00 Chicago C%sT 1936 Mar 1 2:00 3161.1Schristos -5:00 - EST 1936 Nov 15 2:00 3171.1Schristos -6:00 Chicago C%sT 1942 3181.1Schristos -6:00 US C%sT 1946 3191.1Schristos -6:00 Chicago C%sT 1967 3201.1Schristos -6:00 US C%sT 3211.1Schristos</pre> 3221.1Schristos </td></tr></table></td> 3231.1Schristos</tr> 3241.1Schristos<tr> 3251.1Schristos <th colspan="5">Columns Renamed</th> 3261.1Schristos</tr> 3271.1Schristos<tr> 3281.1Schristos <th rowspan="2">Standard Offset<br> 3291.1Schristos from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 3301.1Schristos Meridian</a></th> 3311.1Schristos <th rowspan="2">Daylight<br>Saving Time</th> 3321.1Schristos <th rowspan="2">Abbreviation(s)</th> 3331.1Schristos <th colspan="2">Ending at Local Time</th> 3341.1Schristos</tr> 3351.1Schristos<tr> 3361.1Schristos <th>Date</th> 3371.1Schristos <th>Time</th> 3381.1Schristos</tr> 3391.1Schristos<tr align="center"> 3401.1Schristos <td>−5:50:36</td> 3411.1Schristos <td>not observed</td> 3421.1Schristos <td>LMT</td> 3431.1Schristos <td>1883-11-18</td> 3441.1Schristos <td>12:09:24</td> 3451.1Schristos</tr> 3461.1Schristos<tr align="center"> 3471.1Schristos <td rowspan="2">−6:00:00</td> 3481.1Schristos <td>US rules</td> 3491.1Schristos <td rowspan="2">CST or CDT</td> 3501.1Schristos <td>1920-01-01</td> 3511.1Schristos <td>00:00:00</td> 3521.1Schristos</tr> 3531.1Schristos<tr align="center"> 3541.1Schristos <td>Chicago rules</td> 3551.1Schristos <td>1936-03-01</td> 3561.1Schristos <td rowspan="2">02:00:00</td> 3571.1Schristos</tr> 3581.1Schristos<tr align="center"> 3591.1Schristos <td>−5:00:00</td> 3601.1Schristos <td>not observed</td> 3611.1Schristos <td>EST</td> 3621.1Schristos <td>1936-11-15</td> 3631.1Schristos</tr> 3641.1Schristos<tr align="center"> 3651.1Schristos <td rowspan="4">−6:00:00</td> 3661.1Schristos <td>Chicago rules</td> 3671.1Schristos <td>CST or CDT</td> 3681.1Schristos <td>1942-01-01</td> 3691.1Schristos <td rowspan="3">00:00:00</td> 3701.1Schristos</tr> 3711.1Schristos<tr align="center"> 3721.1Schristos <td>US rules</td> 3731.1Schristos <td>CST, CWT or CPT</td> 3741.1Schristos <td>1946-01-01</td> 3751.1Schristos</tr> 3761.1Schristos<tr align="center"> 3771.1Schristos <td>Chicago rules</td> 3781.1Schristos <td rowspan="2">CST or CDT</td> 3791.1Schristos <td>1967-01-01</td> 3801.1Schristos</tr> 3811.1Schristos<tr align="center"> 3821.1Schristos <td>US rules</td> 3831.1Schristos <td colspan="2">—</td> 3841.1Schristos</tr> 3851.1Schristos</table> 3861.1Schristos 3871.1Schristos<p>There are a couple of interesting differences between Zones and Rules.</p> 3881.1Schristos 3891.1Schristos<p>First, and somewhat trivially, whereas Rules are considered to 3901.1Schristoscontain one or more records, a Zone is considered to be a single 3911.1Schristosrecord with zero or more <i>continuation lines</i>. Thus, the keyword, 3921.1Schristos“<code>Zone</code>,” and the zone name are not 3931.1Schristosrepeated. The last line is the one without anything in 3941.1Schristosthe <code>[UNTIL]</code> column.</p> 3951.1Schristos 3961.1Schristos<p>Second, and more fundamentally, each line of a Zone represents a 3971.1Schristossteady state, not a transition between states. The state exists from 3981.1Schristosthe date and time in the previous line’s <code>[UNTIL]</code> 3991.1Schristoscolumn up to the date and time in the current 4001.1Schristosline’s <code>[UNTIL]</code> column. In other words, the date and 4011.1Schristostime in the <code>[UNTIL]</code> column is the instant that separates 4021.1Schristosthis state from the next. Where that would be ambiguous because 4031.1Schristoswe’re setting our clocks back, the <code>[UNTIL]</code> column 4041.1Schristosspecifies the first occurrence of the instant. The state specified by 4051.1Schristosthe last line, the one without anything in the <code>[UNTIL]</code> 4061.1Schristoscolumn, continues to the present.</p> 4071.1Schristos 4081.1Schristos<p>The first line typically specifies the mean solar time observed 4091.1Schristosbefore the introduction of standard time. Since there’s no line before 4101.1Schristosthat, it has no beginning. <code>8-) </code> For some places near the <a 4111.1Schristoshref="https://en.wikipedia.org/wiki/International_Date_Line">International 4121.1SchristosDate Line</a>, the first <i>two</i> lines will show solar times 4131.1Schristosdiffering by 24 hours; this corresponds to a movement of the Date 4141.1SchristosLine. For example:</p> 4151.1Schristos 4161.1Schristos<pre> 4171.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 4181.1SchristosZone America/Juneau 15:02:19 - LMT 1867 Oct 18 4191.1Schristos -8:57:41 - LMT ... 4201.1Schristos</pre> 4211.1Schristos 4221.1Schristos<p>When Alaska was purchased from Russia in 1867, the Date Line moved 4231.1Schristosfrom the Alaska/Canada border to the Bering Strait; and the time in 4241.1SchristosAlaska was then 24 hours earlier than it had 4251.1Schristosbeen. <code><aside></code>(6 October in the Julian calendar, 4261.1Schristoswhich Russia was still using then for religious reasons, was followed 4271.1Schristosby <i>a second instance of the same day with a different name</i>, 18 4281.1SchristosOctober in the Gregorian calendar. Isn’t civil time 4291.1Schristoswonderful? <code>8-)</code>)<code></aside></code></p> 4301.1Schristos 4311.1Schristos<p>The abbreviation, “LMT” stands for “local mean 4321.1Schristostime”, which is an invention of 4331.1Schristosthe <a href="https://en.wikipedia.org/wiki/Tz_database">tz 4341.1Schristosdatabase</a> and was probably never actually used during the 4351.1Schristosperiod. Furthermore, the value is almost certainly wrong except in the 4361.1Schristosarchetypal place after which the zone is named. (The tz database 4371.1Schristosusually doesn’t provide a separate Zone record for places where 4381.1Schristosnothing significant happened after 1970.)</p> 4391.1Schristos 4401.1Schristos<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 4411.1Schristos<ul> 4421.1Schristos<li>A hyphen, a kind of null value, means that we have not set our 4431.1Schristosclocks ahead of standard time.</li> 4441.1Schristos 4451.1Schristos<li>An amount of time (usually but not necessarily “1:00” 4461.1Schristosmeaning one hour) means that we have set our clocks ahead by that 4471.1Schristosamount.</li> 4481.1Schristos 4491.1Schristos<li>Some alphabetic string means that we <i>might have</i> set our 4501.1Schristosclocks ahead; and we need to check the rule the name of which is the 4511.1Schristosgiven alphabetic string.</li> 4521.1Schristos</ul> 4531.1Schristos 4541.1Schristos<p>An example of a specific amount of time is:</p> 4551.1Schristos<pre> 4561.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 4571.1SchristosZone Pacific/Honolulu ... 1933 Apr 30 2:00 4581.1Schristos -10:30 1:00 HDT 1933 May 21 2:00 4591.1Schristos ... 4601.1Schristos</pre> 4611.1Schristos 4621.1Schristos<p>Hawaii tried daylight saving time for three weeks in 1933 and 4631.1Schristosdecided they didn’t like it. <code>8-) </code>Note that 4641.1Schristosthe <code>GMTOFF</code> column always contains the standard time 4651.1Schristosoffset, so the wall clock time during this period was GMT − 4661.1Schristos10:30 + 1:00 = GMT − 9:30.</p> 4671.1Schristos 4681.1Schristos<p>The <code>FORMAT</code> column specifies the usual abbreviation of 4691.1Schristosthe time zone name. It can have one of three forms:</p> 4701.1Schristos<ul> 4711.1Schristos 4721.1Schristos<li>a string of three or more characters that are either ASCII alphanumerics, 4731.1Schristos“<code>+</code>”, or “<code>-</code>”, 4741.1Schristosin which case that’s the abbreviation</li> 4751.1Schristos 4761.1Schristos<li>a pair of strings separated by a slash 4771.1Schristos(‘<code>/</code>’), in which case the first string is the 4781.1Schristosabbreviation for the standard time name and the second string is the 4791.1Schristosabbreviation for the daylight saving time name</li> 4801.1Schristos 4811.1Schristos<li>a string containing “<code>%s</code>,” in which case 4821.1Schristosthe “<code>%s</code>” will be replaced by the text in the 4831.1Schristosappropriate Rule’s <code>LETTER</code> column</li> 4841.1Schristos</ul> 4851.1Schristos 4861.1Schristos<p>The last two make sense only if there’s a named rule in effect.</p> 4871.1Schristos 4881.1Schristos<p>An example of a slash is:</p> 4891.1Schristos<pre> 4901.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 4911.1SchristosZone Europe/London ... 1996 4921.1Schristos 0:00 EU GMT/BST 4931.1Schristos</pre> 4941.1Schristos 4951.1Schristos<p>The current time in the UK is called either Greenwich mean time or 4961.1SchristosBritish summer time.</p> 4971.1Schristos 4981.1Schristos<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 4991.1Schristoshappens when switching to a named rule. To what values should 5001.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 5011.1Schristos 5021.1Schristos<ul> 5031.1Schristos<li>If at least one transition has happened, use 5041.1Schristosthe <code>SAVE</code> and <code>LETTER</code> data from the most 5051.1Schristosrecent.</li> 5061.1Schristos 5071.1Schristos<li>If switching to a named rule before any transition has happened, 5081.1Schristosassume standard time (<code>SAVE</code> zero), and use 5091.1Schristosthe <code>LETTER</code> data from the earliest transition with 5101.1Schristosa <code>SAVE</code> of zero. 5111.1Schristos 5121.1Schristos</ul> 5131.1Schristos 5141.1Schristos<p>And three last things about the <code>FORMAT</code> column:</p> 5151.1Schristos<ul> 5161.1Schristos 5171.1Schristos<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 5181.1Schristosdatabase</a> gives abbreviations for time zone names in <i>popular 5191.1Schristosusage</i>, which is not necessarily “correct” by law. For 5201.1Schristosexample, the last line in 5211.1Schristos<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 5221.1Schristos“HST” for “Hawaii standard time” even though the 5231.1Schristos<a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a> 5241.1Schristosname for that time zone is “Hawaii-Aleutian standard time.” 5251.1SchristosThis author has read that there are also some places in Australia where 5261.1Schristospopular time zone names differ from the legal ones. 5271.1Schristos 5281.1Schristos<li>No attempt is made to <a 5291.1Schristoshref="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 5301.1Schristosthe abbreviations. They are intended to be the values returned through the 5311.1Schristos<code>"%Z"</code> format specifier to 5321.1Schristos<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 5331.1Schristos<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 5341.1Schristosfunction in the 5351.1Schristos<a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 5361.1Schristos 5371.1Schristos<li>If there is no generally-accepted abbreviation for a time zone, 5381.1Schristosa numeric offset is used instead, e.g., <code>+07</code> for 7 hours 5391.1Schristosahead of Greenwich. By convention, <code>-00</code> is used in a 5401.1Schristoszone while uninhabited, where the offset is zero but in some sense 5411.1Schristosthe true offset is undefined. 5421.1Schristos</ul> 5431.1Schristos 5441.1Schristos<p>As a final example, here’s the complete history for Hawaii:</p> 5451.1Schristos 5461.1Schristos<table border="1"> 5471.1Schristos<tr> 5481.1Schristos <th colspan="6">Relevant Excerpts from the US Rules</th> 5491.1Schristos</tr> 5501.1Schristos<tr> 5511.1Schristos <td colspan="6" align="center"><table><tr><td> 5521.1Schristos<pre> 5531.1Schristos#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 5541.1SchristosRule US 1918 1919 - Oct lastSun 2:00 0 S 5551.1SchristosRule US 1942 only - Feb 9 2:00 1:00 W # War 5561.1SchristosRule US 1945 only - Aug 14 23:00u 1:00 P # Peace 5571.1SchristosRule US 1945 only - Sep 30 2:00 0 S 5581.1Schristos</pre> 5591.1Schristos </td></tr></table></td> 5601.1Schristos</tr> 5611.1Schristos<tr> 5621.1Schristos <th colspan="6">The Zone Record</th> 5631.1Schristos</tr> 5641.1Schristos<tr> 5651.1Schristos <td colspan="6" align="center"><table><tr><td> 5661.1Schristos<pre> 5671.1Schristos#Zone NAME GMTOFF RULES FORMAT [UNTIL] 5681.1SchristosZone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 5691.1Schristos -10:30 - HST 1933 Apr 30 2:00 5701.1Schristos -10:30 1:00 HDT 1933 May 21 2:00 5711.1Schristos -10:30 US H%sT 1947 Jun 8 2:00 5721.1Schristos -10:00 - HST 5731.1Schristos</pre> 5741.1Schristos </td></tr></table></td> 5751.1Schristos</tr> 5761.1Schristos<tr> 5771.1Schristos <th colspan="6">What We Infer</th> 5781.1Schristos</tr> 5791.1Schristos<tr> 5801.1Schristos <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 5811.1Schristos <th rowspan="2">Adjust<br>Clocks</th> 5821.1Schristos <th colspan="2">Time Zone</th> 5831.1Schristos <th colspan="2">Ending at Local Time</th> 5841.1Schristos</tr> 5851.1Schristos<tr> 5861.1Schristos <th>Abbrv.</th> 5871.1Schristos <th>Name</th> 5881.1Schristos <th>Date</th> 5891.1Schristos <th>Time</th> 5901.1Schristos</tr> 5911.1Schristos<tr align="center"> 5921.1Schristos <td>−10:31:26</td> 5931.1Schristos <td>—</td> 5941.1Schristos <td>LMT</td> 5951.1Schristos <td>local mean time</td> 5961.1Schristos <td>1900-01-01</td> 5971.1Schristos <td>12:00</td> 5981.1Schristos</tr> 5991.1Schristos<tr align="center"> 6001.1Schristos <td>−10:30</td> 6011.1Schristos <td>+0:01:26</td> 6021.1Schristos <td>HST</td> 6031.1Schristos <td>Hawaii standard time</td> 6041.1Schristos <td>1933-04-30</td> 6051.1Schristos <td rowspan="3">02:00</td> 6061.1Schristos</tr> 6071.1Schristos<tr align="center"> 6081.1Schristos <td>−9:30</td> 6091.1Schristos <td>+1:00</td> 6101.1Schristos <td>HDT</td> 6111.1Schristos <td>Hawaii daylight time</td> 6121.1Schristos <td>1933-05-21</td> 6131.1Schristos</tr> 6141.1Schristos<tr align="center"> 6151.1Schristos <td>−10:30¹</td> 6161.1Schristos <td>−1:00¹</td> 6171.1Schristos <td>HST¹</td> 6181.1Schristos <td>Hawaii standard time</td> 6191.1Schristos <td>1942-02-09</td> 6201.1Schristos</tr> 6211.1Schristos<tr align="center"> 6221.1Schristos <td rowspan="2">−9:30</td> 6231.1Schristos <td>+1:00</td> 6241.1Schristos <td>HWT</td> 6251.1Schristos <td>Hawaii war time</td> 6261.1Schristos <td>1945-08-14</td> 6271.1Schristos <td>13:30²</td> 6281.1Schristos</tr> 6291.1Schristos<tr align="center"> 6301.1Schristos <td>0</td> 6311.1Schristos <td>HPT</td> 6321.1Schristos <td>Hawaii peace time</td> 6331.1Schristos <td>1945-09-30</td> 6341.1Schristos <td rowspan="2">02:00</td> 6351.1Schristos</tr> 6361.1Schristos<tr align="center"> 6371.1Schristos <td>−10:30</td> 6381.1Schristos <td>−1:00</td> 6391.1Schristos <td rowspan="2">HST</td> 6401.1Schristos <td rowspan="2">Hawaii standard time</td> 6411.1Schristos <td>1947-06-08</td> 6421.1Schristos</tr> 6431.1Schristos<tr align="center"> 6441.1Schristos <td>−10:00³</td> 6451.1Schristos <td>+0:30³</td> 6461.1Schristos <td colspan="2">—</td> 6471.1Schristos</tr> 6481.1Schristos<tr> 6491.1Schristos <td colspan="6"> 6501.1Schristos ¹Switching to US rules…most recent transition (in 1919) was to standard time 6511.1Schristos </td> 6521.1Schristos</tr> 6531.1Schristos<tr> 6541.1Schristos <td colspan="6"> 6551.1Schristos ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 6561.1Schristos + (−9:30) = 13:30 local 6571.1Schristos </td> 6581.1Schristos</tr> 6591.1Schristos<tr> 6601.1Schristos <td colspan="6"> 6611.1Schristos ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 6621.1Schristos the civil time in Hawaii has been 6631.1Schristos <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 6641.1Schristos − 10:00 year-round. 6651.1Schristos </td> 6661.1Schristos</tr> 6671.1Schristos</table> 6681.1Schristos 6691.1Schristos<p>There will be a short quiz later. <code>8-)</code></p> 6701.1Schristos 6711.1Schristos<hr> 6721.1Schristos<address> 6731.1SchristosThis web page is in the public domain, so clarified as of 6741.1Schristos2015-10-20 by Bill Seymour. 6751.1Schristos<br> 6761.1SchristosAll suggestions and corrections will be welcome; all flames will be amusing. 6771.1SchristosMail to was at pobox dot com. 6781.1Schristos</address> 6791.1Schristos</body> 6801.1Schristos</html> 681