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19 <h1>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
20 Database</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
24 times of day from the <a href="tz-link.html">tz database</a>
25 source files. It might be helpful, but not absolutely necessary,
26 for the reader to have already downloaded the
27 latest release of the database and become familiar with the basic layout
28 of the data files. The format is explained in the man page
29 for the zic compiler, <code>zic.8.txt</code>, in
30 the <code>code</code> subdirectory.
31 Although this guide covers many of the common cases, it is not a
32 complete summary of what zic accepts; the man page is the
33 authoritative reference.</p>
34
35 <p>Well begin by talking about the rules for changing between standard
36 and daylight saving time since well need that information when we talk
37 about the zones.</p>
38
39 <p>First, lets consider the special daylight saving time rules
40 for Chicago (from the <code>northamerica</code> file in
41 the <code>data</code> subdirectory):</p>
42
43 <pre>#Rule NAME FROM TO - IN ON AT SAVE LETTER
44 Rule Chicago 1920 only - Jun 13 2:00 1:00 D
45 Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S
46 Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D
47 Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D
48 Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S
49 Rule 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
98 first and last calendar years defining a contiguous range over which a specific
99 Rule 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
101 event that a rule should only apply to a single year. Often, the keyword
102 <code>max</code> is used to extend a rules application into the
103 indefinite future; it is a platform-agnostic stand-in for the largest
104 representable year.
105
106 <p>The next column, <code>-</code>, is reserved; for compatibility with earlier
107 releases, it always contains a hyphen, which acts as a kind of null value.
108 Prior to the 2020b release, it was called the <code>TYPE</code> field, though
109 it had not been used in the main data since the 2000e release.
110 An obsolescent supplementary file used the
111 field as a proof-of-concept to allow <code>zic</code> to apply a given Rule
112 line only to certain types of years within the specified range as
113 dictated by the output of a separate script, such as: only years which would
114 have a US presidential election, or only years which wouldnt.
115
116 <p>The <code>SAVE</code> column contains the local (wall clock) offset from
117 local standard time.
118 This is usually either zero for standard time or one hour for daylight
119 saving time; but theres no reason, in principle, why it cant
120 take on other values.
121
122 <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>)
123 column can contain a variable
124 part of the usual abbreviation of the time zones name, or it can just
125 be a hyphen if theres no variable part. For example, the abbreviation
126 used in the central time zone will be either CST or CDT.
127 The variable part is S or D;
128 and, sure enough, thats just what we find in
129 the <code>LETTER</code> column
130 in 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
134 want 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
137 daylight saving time; and any number of Rule lines can be in effect
138 during a given period (which will always be a non-empty set of
139 contiguous calendar years).</li>
140 <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code>
141 columns contain state that exists between transitions. More about this
142 when we talk about the US rules.</li>
143 </ul>
144
145 <p>In the example above, the transition to daylight saving time
146 happened on the 13<small><sup>th</sup></small> of June in 1920, and on
147 the last Sunday in March in 1921; but the return to standard time
148 happened on the last Sunday in October in both of those
149 years. Similarly, the rule for changing to daylight saving time was
150 the same from 1922 to 1966; but the rule for returning to standard
151 time 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
156 Rule US 1918 1919 - Mar lastSun 2:00 1:00 D
157 Rule US 1918 1919 - Oct lastSun 2:00 0 S
158 Rule US 1942 only - Feb 9 2:00 1:00 W # War
159 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
160 Rule US 1945 only - Sep 30 2:00 0 S
161 Rule US 1967 2006 - Oct lastSun 2:00 0 S
162 Rule US 1967 1973 - Apr lastSun 2:00 1:00 D
163 Rule US 1974 only - Jan 6 2:00 1:00 D
164 Rule US 1975 only - Feb 23 2:00 1:00 D
165 Rule US 1976 1986 - Apr lastSun 2:00 1:00 D
166 Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D
167 Rule US 2007 max - Mar Sun>=8 2:00 1:00 D
168 Rule US 2007 max - Nov Sun>=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 dont 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>
256 column) is not necessarily the local (wall clock) time. The time can be
257 suffixed with s (for standard) to mean
258 local standard time, different from local (wall clock) time when observing
259 daylight saving time; or it can be suffixed with g,
260 u, or z, all three of which mean the
261 standard time at the
262 <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>.
263 g stands for <a
264 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>;
265 u stands for <a
266 href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> or <a
267 href="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>
270 Z (a.k.a. Zulu which, in turn, stands for Z).
271 The time can also be suffixed with w meaning local (wall
272 clock) time; but it usually isnt because thats the
273 default.</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,
277 can have the form, <code>Sun>=</code><i>x</i> or
278 <code>Sun<=</code><i>x</i>, where <i>x</i> is a day
279 of the month. For example, <code>Sun>=8</code> means
280 the first Sunday on or after the eighth of the month, in
281 other words, the second Sunday of the month. Furthermore, although
282 there are no examples above, the weekday neednt be
283 <code>Sun</code> in either form, but can be the usual
284 three-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
287 already mentioned:</p>
288
289 <ul>
290 <li>The rules for changing to and from daylight saving time are
291 actually <i>different sets</i> of rules; and the two sets can change
292 independently. Consider, for example, that the rule for the return to
293 standard time stayed the same from 1967 to 2006; but the rule for the
294 transition to daylight saving time changed several times in the same
295 period. There can also be periods, 1946 to 1966 for example, when no
296 rule from this group is in effect, and so either no transition
297 happened in those years, or some other rule is in effect (perhaps a
298 state or other more local rule).</li>
299
300 <li>The <code>SAVE</code> and <code>LETTER</code> columns
301 contain <i>steady state</i>, not transitions. Consider, for example,
302 the transition from war time to peace time
303 that happened on August 14, 1945. The 1:00 in
304 the <code>SAVE</code> column is <i>not</i> an instruction to advance
305 the clock an hour. It means that clocks should <i>be</i> one hour
306 ahead of standard time, which they already are because of the previous
307 rule, so there should be no change.</li>
308
309 </ul>
310
311 <p>OK, now lets look at a Zone record:</p>
312
313 <pre>
314 #Zone NAME STDOFF RULES FORMAT [UNTIL]
315 Zone 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
391 contain one or more records, a Zone is considered to be a single
392 record with zero or more <i>continuation lines</i>. Thus, the keyword,
393 <code>Zone</code>, and the zone name are not
394 repeated. The last line is the one without anything in
395 the <code>[UNTIL]</code> column.</p>
396
397 <p>Second, and more fundamentally, each line of a Zone represents a
398 steady state, not a transition between states. The state exists from
399 the date and time in the previous lines <code>[UNTIL]</code>
400 column up to the date and time in the current
401 lines <code>[UNTIL]</code> column. In other words, the date and
402 time in the <code>[UNTIL]</code> column is the instant that separates
403 this state from the next. Where that would be ambiguous because
404 were setting our clocks back, the <code>[UNTIL]</code> column
405 specifies the first occurrence of the instant. The state specified by
406 the last line, the one without anything in the <code>[UNTIL]</code>
407 column, continues to the present.</p>
408
409 <p>The first line typically specifies the mean solar time observed
410 before the introduction of standard time. Since theres no line before
411 that, it has no beginning. <code>8-) </code> For some places near the <a
412 href="https://en.wikipedia.org/wiki/International_Date_Line">International
413 Date Line</a>, the first <i>two</i> lines will show solar times
414 differing by 24 hours; this corresponds to a movement of the Date
415 Line. For example:</p>
416
417 <pre>
418 #Zone NAME STDOFF RULES FORMAT [UNTIL]
419 Zone 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
424 from the Alaska/Canada border to the Bering Strait; and the time in
425 Alaska was then 24 hours earlier than it had
426 been. <code><aside></code>(This also means that, although the sun had set
427 on Friday 6 October and risen on Saturday 7 October in the Julian calendar,
428 which Russia was still using then for religious reasons, upon the transfer
429 Saturday afternoon became Friday afternoon, and then the sun set and rose once
430 more on <i>second instances of the same days with different names</i>,
431 Friday 18 and Saturday 19 October in the Gregorian calendar. Isnt civil time
432 wonderful? <code>8-)</code>)<code></aside></code></p>
433
434 <p>The abbreviation, LMT stands for local mean
435 time, which is an invention of
436 the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
437 database</a> and was probably never actually used during the
438 period. Furthermore, the value is almost certainly wrong except in the
439 archetypal place after which the zone is named. (The tz database
440 usually doesnt provide a separate Zone record for places where
441 nothing 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
446 clocks ahead of standard time.</li>
447
448 <li>An amount of time (usually but not necessarily 1:00
449 meaning one hour) means that we have set our clocks ahead by that
450 amount.</li>
451
452 <li>Some alphabetic string means that we <i>might have</i> set our
453 clocks ahead; and we need to check the rule the name of which is the
454 given 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]
460 Zone 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
466 decided they didnt like it. <code>8-) </code>Note that
467 the <code>STDOFF</code> column always contains the standard time
468 offset, so the local (wall clock) time during this period was GMT
469 10:30 + 1:00 = GMT 9:30.</p>
470
471 <p>The <code>FORMAT</code> column specifies the usual abbreviation of
472 the 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
476 characters 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
481 abbreviation</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
485 abbreviation for the standard time name and the second string is the
486 abbreviation for the daylight saving time name</li>
487
488 <li>a string containing <code>%s</code>, in which case
489 the <code>%s</code> will be replaced by the text in the
490 appropriate Rules <code>LETTER</code> column, and the resulting
491 string should be a time zone abbreviation</li>
492 </ul>
493
494 <p>The last two make sense only if theres 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]
499 Zone 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
504 British summer time.</p>
505
506 <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what
507 happens when switching to a named rule. To what values should
508 the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p>
509
510 <ul>
511 <li>If at least one transition has happened, use
512 the <code>SAVE</code> and <code>LETTER</code> data from the most
513 recent.</li>
514
515 <li>If switching to a named rule before any transition has happened,
516 assume standard time (<code>SAVE</code> zero), and use
517 the <code>LETTER</code> data from the earliest transition with
518 a <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
526 database</a> gives abbreviations for time zones
527 in popular English-language usage. For
528 example, 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>
532 name for that time zone is HawaiiAleutian standard time.
533 This author has read that there are also some places in Australia where
534 popular time zone names differ from the legal ones.
535
536 <li>No attempt is made to <a
537 href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a>
538 the 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>
542 function 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,
546 a numeric offset is used instead, e.g., <code>+07</code> for 7 hours
547 ahead of Greenwich. By convention, <code>-00</code> is used in a
548 zone while uninhabited, where the offset is zero but in some sense
549 the true offset is undefined.
550 </ul>
551
552 <p>As a final example, heres 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
556 Rule US 1918 1919 - Oct lastSun 2:00 0 S
557 Rule US 1942 only - Feb 9 2:00 1:00 W # War
558 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
559 Rule US 1945 only - Sep lastSun 2:00 0 S
560
561 # The Zone Record
562 #Zone NAME STDOFF RULES FORMAT [UNTIL]
563 Zone 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>10:30¹</td>
611 <td>1:00¹</td>
612 <td>HST¹</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>13:30²</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>10:00³</td>
641 <td>+0:30³</td>
642 <td colspan="2"></td>
643 </tr>
644 <tr>
645 <td colspan="6" class="footnote">
646 ¹Switching to US rules...most recent transition (in 1919) was to standard time
647 </td>
648 </tr>
649 <tr>
650 <td colspan="6" class="footnote">
651 ²23:00 <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 ³Since <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>
670 This web page is in the public domain, so clarified as of
671 2015-10-20 by Bill Seymour.
672 <br>
673 All suggestions and corrections will be welcome; all flames will be amusing.
674 Mail to was at pobox dot com.
675 </footer>
676 </body>
677 </html>
678