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19<h1>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
20Database</a> Source Files</h1>
21<h2>by Bill Seymour</h2>
22<p>This guide uses the <code>America/Chicago</code> and
23<code>Pacific/Honolulu</code> zones as examples of how to infer
24times of day from the <a href="tz-link.html">tz database</a>
25source files. It might be helpful, but not absolutely necessary,
26for the reader to have already downloaded the
27latest release of the database and become familiar with the basic layout
28of the data files. The format is explained in the “man page”
29for the zic compiler, <code>zic.8.txt</code>, in
30the <code>code</code> subdirectory.
31Although this guide covers many of the common cases, it is not a
32complete summary of what zic accepts; the man page is the
33authoritative reference.</p>
34
35<p>We’ll begin by talking about the rules for changing between standard
36and daylight saving time since we’ll need that information when we talk
37about the zones.</p>
38
39<p>First, let’s consider the special daylight saving time rules
40for Chicago (from the <code>northamerica</code> file in
41the <code>data</code> subdirectory):</p>
42
43<pre>#Rule NAME    FROM TO    -   IN  ON      AT   SAVE LETTER
44Rule  Chicago 1920 only  -   Jun 13      2:00 1:00 D
45Rule  Chicago 1920 1921  -   Oct lastSun 2:00 0    S
46Rule  Chicago 1921 only  -   Mar lastSun 2:00 1:00 D
47Rule  Chicago 1922 1966  -   Apr lastSun 2:00 1:00 D
48Rule  Chicago 1922 1954  -   Sep lastSun 2:00 0    S
49Rule  Chicago 1955 1966  -   Oct lastSun 2:00 0    S
50</pre>
51<table>
52<tr>
53  <th colspan="6">Reformatted a Bit</th>
54</tr>
55<tr>
56  <th>From</th>
57  <th>To</th>
58  <th colspan="2">On</th>
59  <th>At</th>
60  <th>Action</th>
61</tr>
62<tr>
63  <td colspan="2">1920 only</td>
64  <td colspan="2">June 13<small><sup>th</sup></small></td>
65  <td rowspan="6">02:00 local</td>
66  <td>go to daylight saving time</td>
67</tr>
68<tr>
69  <td>1920</td>
70  <td>1921</td>
71  <td rowspan="5">last Sunday</td>
72  <td>in October</td>
73  <td>return to standard time</td>
74</tr>
75<tr>
76  <td colspan="2">1921 only</td>
77  <td>in March</td>
78  <td rowspan="2">go to daylight saving time</td>
79</tr>
80<tr>
81  <td rowspan="2">1922</td>
82  <td>1966</td>
83  <td>in April</td>
84</tr>
85<tr>
86  <td>1954</td>
87  <td>in September</td>
88  <td rowspan="2">return to standard time</td>
89</tr>
90<tr>
91  <td>1955</td>
92  <td>1966</td>
93  <td>in October</td>
94</tr>
95</table>
96
97<p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the
98first and last calendar years defining a contiguous range over which a specific
99Rule line is to apply.  The keyword <code>only</code> can be used in the
100<code>TO</code> field to repeat the value of the <code>FROM</code> field in the
101event that a rule should only apply to a single year.  Often, the keyword
102<code>max</code> is used to extend a rule’s application into the
103indefinite future; it is a platform-agnostic stand-in for the largest
104representable year.
105
106<p>The next column, <code>-</code>, is reserved; for compatibility with earlier
107releases, it always contains a hyphen, which acts as a kind of null value.
108Prior to the 2020b release, it was called the <code>TYPE</code> field, though
109it had not been used in the main data since the 2000e release.
110An obsolescent supplementary file used the
111field as a proof-of-concept to allow <code>zic</code> to apply a given Rule
112line only to certain “types” of years within the specified range as
113dictated by the output of a separate script, such as: only years which would
114have a US presidential election, or only years which wouldn’t.
115
116<p>The <code>SAVE</code> column contains the local (wall clock) offset from
117local standard time.
118This is usually either zero for standard time or one hour for daylight
119saving time; but there’s no reason, in principle, why it can’t
120take on other values.
121
122<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>)
123column can contain a variable
124part of the usual abbreviation of the time zone’s name, or it can just
125be a hyphen if there’s no variable part. For example, the abbreviation
126used in the central time zone will be either “CST” or “CDT”.
127The variable part is ‘S’ or ‘D’;
128and, sure enough, that’s just what we find in
129the <code>LETTER</code> column
130in the <code>Chicago</code> rules. More about this when we talk about
131“Zone” lines.
132
133<p>One important thing to notice is that “Rule” lines
134want at once to be both <i>transitions</i> and <i>steady states</i>:
135<ul>
136<li>On the one hand, they represent transitions between standard and
137daylight saving time; and any number of Rule lines can be in effect
138during a given period (which will always be a non-empty set of
139contiguous calendar years).</li>
140<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code>
141columns contain state that exists between transitions. More about this
142when we talk about the US rules.</li>
143</ul>
144
145<p>In the example above, the transition to daylight saving time
146happened on the 13<small><sup>th</sup></small> of June in 1920, and on
147the last Sunday in March in 1921; but the return to standard time
148happened on the last Sunday in October in both of those
149years. Similarly, the rule for changing to daylight saving time was
150the same from 1922 to 1966; but the rule for returning to standard
151time changed in 1955. Got it?</p>
152
153<p>OK, now for the somewhat more interesting “US” rules:</p>
154
155<pre>#Rule NAME FROM TO    -   IN  ON        AT   SAVE LETTER/S
156Rule  US   1918 1919  -   Mar lastSun  2:00  1:00 D
157Rule  US   1918 1919  -   Oct lastSun  2:00  0    S
158Rule  US   1942 only  -   Feb 9        2:00  1:00 W # War
159Rule  US   1945 only  -   Aug 14      23:00u 1:00 P # Peace
160Rule  US   1945 only  -   Sep 30       2:00  0    S
161Rule  US   1967 2006  -   Oct lastSun  2:00  0    S
162Rule  US   1967 1973  -   Apr lastSun  2:00  1:00 D
163Rule  US   1974 only  -   Jan 6        2:00  1:00 D
164Rule  US   1975 only  -   Feb 23       2:00  1:00 D
165Rule  US   1976 1986  -   Apr lastSun  2:00  1:00 D
166Rule  US   1987 2006  -   Apr Sun&gt;=1   2:00  1:00 D
167Rule  US   2007 max   -   Mar Sun&gt;=8   2:00  1:00 D
168Rule  US   2007 max   -   Nov Sun&gt;=1   2:00  0    S
169</pre>
170<table>
171<tr>
172  <th colspan="6">Reformatted a Bit</th>
173</tr>
174<tr>
175  <th>From</th>
176  <th>To</th>
177  <th colspan="2">On</th>
178  <th>At</th>
179  <th>Action</th>
180</tr>
181<tr>
182  <td rowspan="2">1918</td>
183  <td rowspan="2">1919</td>
184  <td rowspan="2">last Sunday</td>
185  <td>in March</td>
186  <td rowspan="3">02:00 local</td>
187  <td>go to daylight saving time</td>
188</tr>
189<tr>
190  <td>in October</td>
191  <td>return to standard time</td>
192</tr>
193<tr>
194  <td colspan="2">1942 only</td>
195  <td colspan="2">February 9<small><sup>th</sup></small></td>
196  <td>go to “war time”</td>
197</tr>
198<tr>
199  <td colspan="2" rowspan="2">1945 only</td>
200  <td colspan="2">August 14<small><sup>th</sup></small></td>
201  <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td>
202  <td>
203    rename “war time” to “peace<br>time;”
204    clocks don’t change
205  </td>
206</tr>
207<tr>
208  <td colspan="2">September 30<small><sup>th</sup></small></td>
209  <td rowspan="9">02:00 local</td>
210  <td rowspan="2">return to standard time</td>
211</tr>
212<tr>
213  <td rowspan="2">1967</td>
214  <td>2006</td>
215  <td rowspan="2">last Sunday</td>
216  <td>in October</td>
217</tr>
218<tr>
219  <td>1973</td>
220  <td>in April</td>
221  <td rowspan="6">go to daylight saving time</td>
222</tr>
223<tr>
224  <td colspan="2">1974 only</td>
225  <td colspan="2">January 6<small><sup>th</sup></small></td>
226</tr>
227<tr>
228  <td colspan="2">1975 only</td>
229  <td colspan="2">February 23<small><sup>rd</sup></small></td>
230</tr>
231<tr>
232  <td>1976</td>
233  <td>1986</td>
234  <td>last Sunday</td>
235  <td rowspan="2">in April</td>
236</tr>
237<tr>
238  <td>1987</td>
239  <td>2006</td>
240  <td>first Sunday</td>
241</tr>
242<tr>
243  <td rowspan="2">2007</td>
244  <td rowspan="2">present</td>
245  <td colspan="2">second Sunday in March</td>
246</tr>
247<tr>
248  <td colspan="2">first Sunday in November</td>
249  <td>return to standard time</td>
250</tr>
251</table>
252
253<p>There are two interesting things to note here.</p>
254
255<p>First, the time that something happens (in the <code>AT</code>
256column) is not necessarily the local (wall clock) time. The time can be
257suffixed with ‘s’ (for “standard”) to mean
258local standard time, different from local (wall clock) time when observing
259daylight saving time; or it can be suffixed with ‘g’,
260‘u’, or ‘z’, all three of which mean the
261standard time at the
262<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>.
263‘g’ stands for “<a
264href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”;
265‘u’ stands for “<a
266href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a
267href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>”
268(whichever was official at the time); ‘z’ stands for the
269<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a>
270Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’).
271The time can also be suffixed with ‘w’ meaning local (wall
272clock) time; but it usually isn’t because that’s the
273default.</p>
274
275<p>Second, the day in the <code>ON</code> column, in addition to
276“<code>lastSun</code>” or a particular day of the month,
277can have the form, “<code>Sun&gt;=</code><i>x</i>” or
278“<code>Sun&lt;=</code><i>x</i>,” where <i>x</i> is a day
279of the month. For example, “<code>Sun&gt;=8</code>” means
280“the first Sunday on or after the eighth of the month,” in
281other words, the second Sunday of the month. Furthermore, although
282there are no examples above, the weekday needn’t be
283“<code>Sun</code>” in either form, but can be the usual
284three-character English abbreviation for any day of the week.</p>
285
286<p>And the US rules give us more examples of a couple of things
287already mentioned:</p>
288
289<ul>
290<li>The rules for changing to and from daylight saving time are
291actually <i>different sets</i> of rules; and the two sets can change
292independently. Consider, for example, that the rule for the return to
293standard time stayed the same from 1967 to 2006; but the rule for the
294transition to daylight saving time changed several times in the same
295period.  There can also be periods, 1946 to 1966 for example, when no
296rule from this group is in effect, and so either no transition
297happened in those years, or some other rule is in effect (perhaps a
298state or other more local rule).</li>
299
300<li>The <code>SAVE</code> and <code>LETTER</code> columns
301contain <i>steady state</i>, not transitions. Consider, for example,
302the transition from “war time” to “peace time”
303that happened on August 14, 1945. The “1:00” in
304the <code>SAVE</code> column is <i>not</i> an instruction to advance
305the clock an hour. It means that clocks should <i>be</i> one hour
306ahead of standard time, which they already are because of the previous
307rule, so there should be no change.</li>
308
309</ul>
310
311<p>OK, now let’s look at a Zone record:</p>
312
313<pre>
314#Zone       NAME      STDOFF   RULES FORMAT [UNTIL]
315Zone  America/Chicago -5:50:36 -       LMT  1883 Nov 18 12:09:24
316                      -6:00    US      C%sT 1920
317                      -6:00    Chicago C%sT 1936 Mar  1  2:00
318                      -5:00    -       EST  1936 Nov 15  2:00
319                      -6:00    Chicago C%sT 1942
320                      -6:00    US      C%sT 1946
321                      -6:00    Chicago C%sT 1967
322                      -6:00    US      C%sT
323</pre>
324<table>
325<tr>
326  <th colspan="5">Columns Renamed</th>
327</tr>
328<tr>
329  <th rowspan="2">Standard Offset<br>
330    from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime
331    Meridian</a></th>
332  <th rowspan="2">Daylight<br>Saving Time</th>
333  <th rowspan="2">Abbreviation(s)</th>
334  <th colspan="2">Ending at Local Time</th>
335</tr>
336<tr>
337  <th>Date</th>
338  <th>Time</th>
339</tr>
340<tr>
341  <td>−5:50:36</td>
342  <td>not observed</td>
343  <td>LMT</td>
344  <td>1883-11-18</td>
345  <td>12:09:24</td>
346</tr>
347<tr>
348  <td rowspan="2">−6:00:00</td>
349  <td>US rules</td>
350  <td rowspan="2">CST or CDT</td>
351  <td>1920-01-01</td>
352  <td>00:00:00</td>
353</tr>
354<tr>
355  <td>Chicago rules</td>
356  <td>1936-03-01</td>
357  <td rowspan="2">02:00:00</td>
358</tr>
359<tr>
360  <td>−5:00:00</td>
361  <td>not observed</td>
362  <td>EST</td>
363  <td>1936-11-15</td>
364</tr>
365<tr>
366  <td rowspan="4">−6:00:00</td>
367  <td>Chicago rules</td>
368  <td>CST or CDT</td>
369  <td>1942-01-01</td>
370  <td rowspan="3">00:00:00</td>
371</tr>
372<tr>
373  <td>US rules</td>
374  <td>CST, CWT or CPT</td>
375  <td>1946-01-01</td>
376</tr>
377<tr>
378  <td>Chicago rules</td>
379  <td rowspan="2">CST or CDT</td>
380  <td>1967-01-01</td>
381</tr>
382<tr>
383  <td>US rules</td>
384  <td colspan="2">–</td>
385</tr>
386</table>
387
388<p>There are a couple of interesting differences between Zones and Rules.</p>
389
390<p>First, and somewhat trivially, whereas Rules are considered to
391contain one or more records, a Zone is considered to be a single
392record with zero or more <i>continuation lines</i>. Thus, the keyword,
393“<code>Zone</code>,” and the zone name are not
394repeated. The last line is the one without anything in
395the <code>[UNTIL]</code> column.</p>
396
397<p>Second, and more fundamentally, each line of a Zone represents a
398steady state, not a transition between states. The state exists from
399the date and time in the previous line’s <code>[UNTIL]</code>
400column up to the date and time in the current
401line’s <code>[UNTIL]</code> column. In other words, the date and
402time in the <code>[UNTIL]</code> column is the instant that separates
403this state from the next. Where that would be ambiguous because
404we’re setting our clocks back, the <code>[UNTIL]</code> column
405specifies the first occurrence of the instant. The state specified by
406the last line, the one without anything in the <code>[UNTIL]</code>
407column, continues to the present.</p>
408
409<p>The first line typically specifies the mean solar time observed
410before the introduction of standard time. Since there’s no line before
411that, it has no beginning. <code>8-) </code> For some places near the <a
412href="https://en.wikipedia.org/wiki/International_Date_Line">International
413Date Line</a>, the first <i>two</i> lines will show solar times
414differing by 24 hours; this corresponds to a movement of the Date
415Line.  For example:</p>
416
417<pre>
418#Zone NAME          STDOFF   RULES FORMAT [UNTIL]
419Zone America/Juneau 15:02:19 -     LMT    1867 Oct 19  0:31:13u
420                    -8:57:41 -     LMT    ...
421</pre>
422
423<p>When Alaska was purchased from Russia in 1867, the Date Line moved
424from the Alaska/Canada border to the Bering Strait; and the time in
425Alaska was then 24 hours earlier than it had
426been. <code>&lt;aside&gt;</code>(This also means that, although the sun had set
427on Friday 6 October and risen on Saturday 7 October in the Julian calendar,
428which Russia was still using then for religious reasons, upon the transfer
429Saturday afternoon became Friday afternoon, and then the sun set and rose once
430more on <i>second instances of the same days with different names</i>,
431Friday 18 and Saturday 19 October in the Gregorian calendar. Isn’t civil time
432wonderful? <code>8-)</code>)<code>&lt;/aside&gt;</code></p>
433
434<p>The abbreviation, “LMT” stands for “local mean
435time”, which is an invention of
436the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
437database</a> and was probably never actually used during the
438period. Furthermore, the value is almost certainly wrong except in the
439archetypal place after which the zone is named. (The tz database
440usually doesn’t provide a separate Zone record for places where
441nothing significant happened after 1970.)</p>
442
443<p>The <code>RULES</code> column tells us whether daylight saving time is being observed:
444<ul>
445<li>A hyphen, a kind of null value, means that we have not set our
446clocks ahead of standard time.</li>
447
448<li>An amount of time (usually but not necessarily “1:00”
449meaning one hour) means that we have set our clocks ahead by that
450amount.</li>
451
452<li>Some alphabetic string means that we <i>might have</i> set our
453clocks ahead; and we need to check the rule the name of which is the
454given alphabetic string.</li>
455</ul>
456
457<p>An example of a specific amount of time is:</p>
458<pre>
459#Zone NAME            STDOFF RULES FORMAT [UNTIL]
460Zone Pacific/Honolulu ...                 1933 Apr 30  2:00
461                      -10:30 1:00  HDT    1933 May 21 12:00
462                      ...
463</pre>
464
465<p>Hawaii tried daylight saving time for three weeks in 1933 and
466decided they didn’t like it. <code>8-) </code>Note that
467the <code>STDOFF</code> column always contains the standard time
468offset, so the local (wall clock) time during this period was GMT −
46910:30 + 1:00 = GMT − 9:30.</p>
470
471<p>The <code>FORMAT</code> column specifies the usual abbreviation of
472the time zone name. It should have one of four forms:</p>
473<ul>
474
475<li>a time zone abbreviation that is a string of three or more
476characters that are either ASCII alphanumerics,
477“<code>+</code>”, or “<code>-</code>”</li>
478
479<li>the string “%z”, in which case the
480“<code>%z</code>” will be replaced by a numeric time zone
481abbreviation</li>
482
483<li>a pair of time zone abbreviations separated by a slash
484(‘<code>/</code>’), in which case the first string is the
485abbreviation for the standard time name and the second string is the
486abbreviation for the daylight saving time name</li>
487
488<li>a string containing “<code>%s</code>”, in which case
489the “<code>%s</code>” will be replaced by the text in the
490appropriate Rule’s <code>LETTER</code> column, and the resulting
491string should be a time zone abbreviation</li>
492</ul>
493
494<p>The last two make sense only if there’s a named rule in effect.</p>
495
496<p>An example of a slash is:</p>
497<pre>
498#Zone NAME          STDOFF RULES FORMAT  [UNTIL]
499Zone  Europe/London ...                  1996
500                    0:00   EU    GMT/BST
501</pre>
502
503<p>The current time in the UK is called either Greenwich mean time or
504British summer time.</p>
505
506<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what
507happens when switching to a named rule. To what values should
508the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p>
509
510<ul>
511<li>If at least one transition has happened, use
512the <code>SAVE</code> and <code>LETTER</code> data from the most
513recent.</li>
514
515<li>If switching to a named rule before any transition has happened,
516assume standard time (<code>SAVE</code> zero), and use
517the <code>LETTER</code> data from the earliest transition with
518a <code>SAVE</code> of zero.
519
520</ul>
521
522<p>And three last things about the <code>FORMAT</code> column:</p>
523<ul>
524
525<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz
526database</a> gives abbreviations for time zones
527in popular English-language usage. For
528example, the last line in
529<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives
530“HST” for “Hawaii standard time” even though the
531<a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a>
532name for that time zone is “Hawaii–Aleutian standard time”.
533This author has read that there are also some places in Australia where
534popular time zone names differ from the legal ones.
535
536<li>No attempt is made to <a
537href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a>
538the abbreviations. They are intended to be the values returned through the
539<code>"%Z"</code> format specifier to
540<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s
541<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/strftime.html"><code>strftime</code></a>
542function in the
543<a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>.
544
545<li>If there is no generally accepted abbreviation for a time zone,
546a numeric offset is used instead, e.g., <code>+07</code> for 7 hours
547ahead of Greenwich. By convention, <code>-00</code> is used in a
548zone while uninhabited, where the offset is zero but in some sense
549the true offset is undefined.
550</ul>
551
552<p>As a final example, here’s the complete history for Hawaii:</p>
553
554<pre># Relevant Excerpts from the US Rules
555#Rule NAME FROM TO   -    IN  ON      AT     SAVE LETTER/S
556Rule  US   1918 1919 -    Oct lastSun  2:00  0    S
557Rule  US   1942 only -    Feb  9       2:00  1:00 W # War
558Rule  US   1945 only -    Aug 14      23:00u 1:00 P # Peace
559Rule  US   1945 only -    Sep lastSun  2:00  0    S
560
561# The Zone Record
562#Zone NAME            STDOFF    RULES FORMAT [UNTIL]
563Zone Pacific/Honolulu -10:31:26 -     LMT    1896 Jan 13 12:00
564                      -10:30    -     HST    1933 Apr 30  2:00
565                      -10:30    1:00  HDT    1933 May 21  2:00
566                      -10:30    US    H%sT   1947 Jun  8  2:00
567                      -10:00    -     HST
568</pre>
569<table>
570<tr>
571  <th colspan="6">What We Infer</th>
572</tr>
573<tr>
574  <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th>
575  <th rowspan="2">Adjust<br>Clocks</th>
576  <th colspan="2">Time Zone</th>
577  <th colspan="2">Ending at Local Time</th>
578</tr>
579<tr>
580  <th>Abbrv.</th>
581  <th>Name</th>
582  <th>Date</th>
583  <th>Time</th>
584</tr>
585<tr>
586  <td>−10:31:26</td>
587  <td>–</td>
588  <td>LMT</td>
589  <td>local mean time</td>
590  <td>1896-01-13</td>
591  <td>12:00</td>
592</tr>
593<tr>
594  <td>−10:30</td>
595  <td>+0:01:26</td>
596  <td>HST</td>
597  <td>Hawaii standard time</td>
598  <td>1933-04-30</td>
599  <td>02:00</td>
600</tr>
601<tr>
602  <td>−9:30</td>
603  <td>+1:00</td>
604  <td>HDT</td>
605  <td>Hawaii daylight time</td>
606  <td>1933-05-21</td>
607  <td>12:00</td>
608</tr>
609<tr>
610  <td>−10:30&sup1;</td>
611  <td>−1:00&sup1;</td>
612  <td>HST&sup1;</td>
613  <td>Hawaii standard time</td>
614  <td>1942-02-09</td>
615  <td>02:00</td>
616</tr>
617<tr>
618  <td rowspan="2">−9:30</td>
619  <td>+1:00</td>
620  <td>HWT</td>
621  <td>Hawaii war time</td>
622  <td>1945-08-14</td>
623  <td>13:30&sup2;</td>
624</tr>
625<tr>
626  <td>0</td>
627  <td>HPT</td>
628  <td>Hawaii peace time</td>
629  <td>1945-09-30</td>
630  <td rowspan="2">02:00</td>
631</tr>
632<tr>
633  <td>−10:30</td>
634  <td>−1:00</td>
635  <td rowspan="2">HST</td>
636  <td rowspan="2">Hawaii standard time</td>
637  <td>1947-06-08</td>
638</tr>
639<tr>
640  <td>−10:00&sup3;</td>
641  <td>+0:30&sup3;</td>
642  <td colspan="2">–</td>
643</tr>
644<tr>
645  <td colspan="6" class="footnote">
646    &sup1;Switching to US rules...most recent transition (in 1919) was to standard time
647  </td>
648</tr>
649<tr>
650  <td colspan="6" class="footnote">
651    &sup2;23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>
652    + (−9:30) = 13:30 local
653  </td>
654</tr>
655<tr>
656  <td colspan="6" class="footnote">
657    &sup3;Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947-06-08T12:30Z</a>,
658    the civil time in Hawaii has been
659    <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
660    −10:00 year-round.
661  </td>
662</tr>
663</table>
664
665<p>There will be a short quiz later. <code>8-)</code></p>
666</main>
667
668<footer>
669<hr>
670This web page is in the public domain, so clarified as of
6712015-10-20 by Bill Seymour.
672<br>
673All suggestions and corrections will be welcome; all flames will be amusing.
674Mail to was at pobox dot com.
675</footer>
676</body>
677</html>
678