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18 <h1>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
19 Database</a> Source Files</h1>
20 <h2>by Bill Seymour</h2>
21 <p>This guide uses the <code>America/Chicago</code> and
22 <code>Pacific/Honolulu</code> zones as examples of how to infer
23 times of day from the <a href="tz-link.html">tz database</a>
24 source files. It might be helpful, but not absolutely necessary,
25 for the reader to have already downloaded the
26 latest release of the database and become familiar with the basic layout
27 of the data files. The format is explained in the man page
28 for the zic compiler, <code>zic.8.txt</code>, in
29 the <code>code</code> subdirectory.
30 Although this guide covers many of the common cases, it is not a
31 complete summary of what zic accepts; the man page is the
32 authoritative reference.</p>
33
34 <p>Well begin by talking about the rules for changing between standard
35 and daylight saving time since well need that information when we talk
36 about the zones.</p>
37
38 <p>First, lets consider the special daylight saving time rules
39 for Chicago (from the <code>northamerica</code> file in
40 the <code>data</code> subdirectory):</p>
41
42 <pre>#Rule NAME FROM TO - IN ON AT SAVE LETTER
43 Rule Chicago 1920 only - Jun 13 2:00 1:00 D
44 Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S
45 Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D
46 Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D
47 Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S
48 Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S
49 </pre>
50 <table>
51 <tr>
52 <th colspan="6">Reformatted a Bit</th>
53 </tr>
54 <tr>
55 <th>From</th>
56 <th>To</th>
57 <th colspan="2">On</th>
58 <th>At</th>
59 <th>Action</th>
60 </tr>
61 <tr>
62 <td colspan="2">1920 only</td>
63 <td colspan="2">June 13<small><sup>th</sup></small></td>
64 <td rowspan="6">02:00 local</td>
65 <td>go to daylight saving time</td>
66 </tr>
67 <tr>
68 <td>1920</td>
69 <td>1921</td>
70 <td rowspan="5">last Sunday</td>
71 <td>in October</td>
72 <td>return to standard time</td>
73 </tr>
74 <tr>
75 <td colspan="2">1921 only</td>
76 <td>in March</td>
77 <td rowspan="2">go to daylight saving time</td>
78 </tr>
79 <tr>
80 <td rowspan="2">1922</td>
81 <td>1966</td>
82 <td>in April</td>
83 </tr>
84 <tr>
85 <td>1954</td>
86 <td>in September</td>
87 <td rowspan="2">return to standard time</td>
88 </tr>
89 <tr>
90 <td>1955</td>
91 <td>1966</td>
92 <td>in October</td>
93 </tr>
94 </table>
95
96 <p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the
97 first and last calendar years defining a contiguous range over which a specific
98 Rule line is to apply. The keyword <code>only</code> can be used in the
99 <code>TO</code> field to repeat the value of the <code>FROM</code> field in the
100 event that a rule should only apply to a single year. Often, the keyword
101 <code>max</code> is used to extend a rules application into the
102 indefinite future; it is a platform-agnostic stand-in for the largest
103 representable year.
104
105 <p>The next column, <code>-</code>, is reserved; for compatibility with earlier
106 releases, it always contains a hyphen, which acts as a kind of null value.
107 Prior to the 2020b release, it was called the <code>TYPE</code> field, though
108 it had not been used in the main data since the 2000e release.
109 An obsolescent supplementary file used the
110 field as a proof-of-concept to allow <code>zic</code> to apply a given Rule
111 line only to certain types of years within the specified range as
112 dictated by the output of a separate script, such as: only years which would
113 have a US presidential election, or only years which wouldnt.
114
115 <p>The <code>SAVE</code> column contains the local (wall clock) offset from
116 local standard time.
117 This is usually either zero for standard time or one hour for daylight
118 saving time; but theres no reason, in principle, why it cant
119 take on other values.
120
121 <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>)
122 column can contain a variable
123 part of the usual abbreviation of the time zones name, or it can just
124 be a hyphen if theres no variable part. For example, the abbreviation
125 used in the central time zone will be either CST or CDT.
126 The variable part is S or D;
127 and, sure enough, thats just what we find in
128 the <code>LETTER</code> column
129 in the <code>Chicago</code> rules. More about this when we talk about
130 Zone lines.
131
132 <p>One important thing to notice is that Rule lines
133 want at once to be both <i>transitions</i> and <i>steady states</i>:
134 <ul>
135 <li>On the one hand, they represent transitions between standard and
136 daylight saving time; and any number of Rule lines can be in effect
137 during a given period (which will always be a non-empty set of
138 contiguous calendar years).</li>
139 <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code>
140 columns contain state that exists between transitions. More about this
141 when we talk about the US rules.</li>
142 </ul>
143
144 <p>In the example above, the transition to daylight saving time
145 happened on the 13<small><sup>th</sup></small> of June in 1920, and on
146 the last Sunday in March in 1921; but the return to standard time
147 happened on the last Sunday in October in both of those
148 years. Similarly, the rule for changing to daylight saving time was
149 the same from 1922 to 1966; but the rule for returning to standard
150 time changed in 1955. Got it?</p>
151
152 <p>OK, now for the somewhat more interesting US rules:</p>
153
154 <pre>#Rule NAME FROM TO - IN ON AT SAVE LETTER/S
155 Rule US 1918 1919 - Mar lastSun 2:00 1:00 D
156 Rule US 1918 1919 - Oct lastSun 2:00 0 S
157 Rule US 1942 only - Feb 9 2:00 1:00 W # War
158 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
159 Rule US 1945 only - Sep 30 2:00 0 S
160 Rule US 1967 2006 - Oct lastSun 2:00 0 S
161 Rule US 1967 1973 - Apr lastSun 2:00 1:00 D
162 Rule US 1974 only - Jan 6 2:00 1:00 D
163 Rule US 1975 only - Feb 23 2:00 1:00 D
164 Rule US 1976 1986 - Apr lastSun 2:00 1:00 D
165 Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D
166 Rule US 2007 max - Mar Sun>=8 2:00 1:00 D
167 Rule US 2007 max - Nov Sun>=1 2:00 0 S
168 </pre>
169 <table>
170 <tr>
171 <th colspan="6">Reformatted a Bit</th>
172 </tr>
173 <tr>
174 <th>From</th>
175 <th>To</th>
176 <th colspan="2">On</th>
177 <th>At</th>
178 <th>Action</th>
179 </tr>
180 <tr>
181 <td rowspan="2">1918</td>
182 <td rowspan="2">1919</td>
183 <td rowspan="2">last Sunday</td>
184 <td>in March</td>
185 <td rowspan="3">02:00 local</td>
186 <td>go to daylight saving time</td>
187 </tr>
188 <tr>
189 <td>in October</td>
190 <td>return to standard time</td>
191 </tr>
192 <tr>
193 <td colspan="2">1942 only</td>
194 <td colspan="2">February 9<small><sup>th</sup></small></td>
195 <td>go to war time</td>
196 </tr>
197 <tr>
198 <td colspan="2" rowspan="2">1945 only</td>
199 <td colspan="2">August 14<small><sup>th</sup></small></td>
200 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td>
201 <td>
202 rename war time to peace<br>time;
203 clocks dont change
204 </td>
205 </tr>
206 <tr>
207 <td colspan="2">September 30<small><sup>th</sup></small></td>
208 <td rowspan="9">02:00 local</td>
209 <td rowspan="2">return to standard time</td>
210 </tr>
211 <tr>
212 <td rowspan="2">1967</td>
213 <td>2006</td>
214 <td rowspan="2">last Sunday</td>
215 <td>in October</td>
216 </tr>
217 <tr>
218 <td>1973</td>
219 <td>in April</td>
220 <td rowspan="6">go to daylight saving time</td>
221 </tr>
222 <tr>
223 <td colspan="2">1974 only</td>
224 <td colspan="2">January 6<small><sup>th</sup></small></td>
225 </tr>
226 <tr>
227 <td colspan="2">1975 only</td>
228 <td colspan="2">February 23<small><sup>rd</sup></small></td>
229 </tr>
230 <tr>
231 <td>1976</td>
232 <td>1986</td>
233 <td>last Sunday</td>
234 <td rowspan="2">in April</td>
235 </tr>
236 <tr>
237 <td>1987</td>
238 <td>2006</td>
239 <td>first Sunday</td>
240 </tr>
241 <tr>
242 <td rowspan="2">2007</td>
243 <td rowspan="2">present</td>
244 <td colspan="2">second Sunday in March</td>
245 </tr>
246 <tr>
247 <td colspan="2">first Sunday in November</td>
248 <td>return to standard time</td>
249 </tr>
250 </table>
251
252 <p>There are two interesting things to note here.</p>
253
254 <p>First, the time that something happens (in the <code>AT</code>
255 column) is not necessarily the local (wall clock) time. The time can be
256 suffixed with s (for standard) to mean
257 local standard time, different from local (wall clock) time when observing
258 daylight saving time; or it can be suffixed with g,
259 u, or z, all three of which mean the
260 standard time at the
261 <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>.
262 g stands for <a
263 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>;
264 u stands for <a
265 href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> or <a
266 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
267 (whichever was official at the time); z stands for the
268 <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a>
269 Z (a.k.a. Zulu which, in turn, stands for Z).
270 The time can also be suffixed with w meaning local (wall
271 clock) time; but it usually isnt because thats the
272 default.</p>
273
274 <p>Second, the day in the <code>ON</code> column, in addition to
275 <code>lastSun</code> or a particular day of the month,
276 can have the form, <code>Sun>=</code><i>x</i> or
277 <code>Sun<=</code><i>x</i>, where <i>x</i> is a day
278 of the month. For example, <code>Sun>=8</code> means
279 the first Sunday on or after the eighth of the month, in
280 other words, the second Sunday of the month. Furthermore, although
281 there are no examples above, the weekday neednt be
282 <code>Sun</code> in either form, but can be the usual
283 three-character English abbreviation for any day of the week.</p>
284
285 <p>And the US rules give us more examples of a couple of things
286 already mentioned:</p>
287
288 <ul>
289 <li>The rules for changing to and from daylight saving time are
290 actually <i>different sets</i> of rules; and the two sets can change
291 independently. Consider, for example, that the rule for the return to
292 standard time stayed the same from 1967 to 2006; but the rule for the
293 transition to daylight saving time changed several times in the same
294 period. There can also be periods, 1946 to 1966 for example, when no
295 rule from this group is in effect, and so either no transition
296 happened in those years, or some other rule is in effect (perhaps a
297 state or other more local rule).</li>
298
299 <li>The <code>SAVE</code> and <code>LETTER</code> columns
300 contain <i>steady state</i>, not transitions. Consider, for example,
301 the transition from war time to peace time
302 that happened on August 14, 1945. The 1:00 in
303 the <code>SAVE</code> column is <i>not</i> an instruction to advance
304 the clock an hour. It means that clocks should <i>be</i> one hour
305 ahead of standard time, which they already are because of the previous
306 rule, so there should be no change.</li>
307
308 </ul>
309
310 <p>OK, now lets look at a Zone record:</p>
311
312 <pre>
313 #Zone NAME STDOFF RULES FORMAT [UNTIL]
314 Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24
315 -6:00 US C%sT 1920
316 -6:00 Chicago C%sT 1936 Mar 1 2:00
317 -5:00 - EST 1936 Nov 15 2:00
318 -6:00 Chicago C%sT 1942
319 -6:00 US C%sT 1946
320 -6:00 Chicago C%sT 1967
321 -6:00 US C%sT
322 </pre>
323 <table>
324 <tr>
325 <th colspan="5">Columns Renamed</th>
326 </tr>
327 <tr>
328 <th rowspan="2">Standard Offset<br>
329 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime
330 Meridian</a></th>
331 <th rowspan="2">Daylight<br>Saving Time</th>
332 <th rowspan="2">Abbreviation(s)</th>
333 <th colspan="2">Ending at Local Time</th>
334 </tr>
335 <tr>
336 <th>Date</th>
337 <th>Time</th>
338 </tr>
339 <tr>
340 <td>5:50:36</td>
341 <td>not observed</td>
342 <td>LMT</td>
343 <td>1883-11-18</td>
344 <td>12:09:24</td>
345 </tr>
346 <tr>
347 <td rowspan="2">6:00:00</td>
348 <td>US rules</td>
349 <td rowspan="2">CST or CDT</td>
350 <td>1920-01-01</td>
351 <td>00:00:00</td>
352 </tr>
353 <tr>
354 <td>Chicago rules</td>
355 <td>1936-03-01</td>
356 <td rowspan="2">02:00:00</td>
357 </tr>
358 <tr>
359 <td>5:00:00</td>
360 <td>not observed</td>
361 <td>EST</td>
362 <td>1936-11-15</td>
363 </tr>
364 <tr>
365 <td rowspan="4">6:00:00</td>
366 <td>Chicago rules</td>
367 <td>CST or CDT</td>
368 <td>1942-01-01</td>
369 <td rowspan="3">00:00:00</td>
370 </tr>
371 <tr>
372 <td>US rules</td>
373 <td>CST, CWT or CPT</td>
374 <td>1946-01-01</td>
375 </tr>
376 <tr>
377 <td>Chicago rules</td>
378 <td rowspan="2">CST or CDT</td>
379 <td>1967-01-01</td>
380 </tr>
381 <tr>
382 <td>US rules</td>
383 <td colspan="2"></td>
384 </tr>
385 </table>
386
387 <p>There are a couple of interesting differences between Zones and Rules.</p>
388
389 <p>First, and somewhat trivially, whereas Rules are considered to
390 contain one or more records, a Zone is considered to be a single
391 record with zero or more <i>continuation lines</i>. Thus, the keyword,
392 <code>Zone</code>, and the zone name are not
393 repeated. The last line is the one without anything in
394 the <code>[UNTIL]</code> column.</p>
395
396 <p>Second, and more fundamentally, each line of a Zone represents a
397 steady state, not a transition between states. The state exists from
398 the date and time in the previous lines <code>[UNTIL]</code>
399 column up to the date and time in the current
400 lines <code>[UNTIL]</code> column. In other words, the date and
401 time in the <code>[UNTIL]</code> column is the instant that separates
402 this state from the next. Where that would be ambiguous because
403 were setting our clocks back, the <code>[UNTIL]</code> column
404 specifies the first occurrence of the instant. The state specified by
405 the last line, the one without anything in the <code>[UNTIL]</code>
406 column, continues to the present.</p>
407
408 <p>The first line typically specifies the mean solar time observed
409 before the introduction of standard time. Since theres no line before
410 that, it has no beginning. <code>8-) </code> For some places near the <a
411 href="https://en.wikipedia.org/wiki/International_Date_Line">International
412 Date Line</a>, the first <i>two</i> lines will show solar times
413 differing by 24 hours; this corresponds to a movement of the Date
414 Line. For example:</p>
415
416 <pre>
417 #Zone NAME STDOFF RULES FORMAT [UNTIL]
418 Zone America/Juneau 15:02:19 - LMT 1867 Oct 18
419 -8:57:41 - LMT ...
420 </pre>
421
422 <p>When Alaska was purchased from Russia in 1867, the Date Line moved
423 from the Alaska/Canada border to the Bering Strait; and the time in
424 Alaska was then 24 hours earlier than it had
425 been. <code><aside></code>(6 October in the Julian calendar,
426 which Russia was still using then for religious reasons, was followed
427 by <i>a second instance of the same day with a different name</i>, 18
428 October in the Gregorian calendar. Isnt civil time
429 wonderful? <code>8-)</code>)<code></aside></code></p>
430
431 <p>The abbreviation, LMT stands for local mean
432 time, which is an invention of
433 the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
434 database</a> and was probably never actually used during the
435 period. Furthermore, the value is almost certainly wrong except in the
436 archetypal place after which the zone is named. (The tz database
437 usually doesnt provide a separate Zone record for places where
438 nothing significant happened after 1970.)</p>
439
440 <p>The <code>RULES</code> column tells us whether daylight saving time is being observed:
441 <ul>
442 <li>A hyphen, a kind of null value, means that we have not set our
443 clocks ahead of standard time.</li>
444
445 <li>An amount of time (usually but not necessarily 1:00
446 meaning one hour) means that we have set our clocks ahead by that
447 amount.</li>
448
449 <li>Some alphabetic string means that we <i>might have</i> set our
450 clocks ahead; and we need to check the rule the name of which is the
451 given alphabetic string.</li>
452 </ul>
453
454 <p>An example of a specific amount of time is:</p>
455 <pre>
456 #Zone NAME STDOFF RULES FORMAT [UNTIL]
457 Zone Pacific/Honolulu ... 1933 Apr 30 2:00
458 -10:30 1:00 HDT 1933 May 21 12:00
459 ...
460 </pre>
461
462 <p>Hawaii tried daylight saving time for three weeks in 1933 and
463 decided they didnt like it. <code>8-) </code>Note that
464 the <code>STDOFF</code> column always contains the standard time
465 offset, so the local (wall clock) time during this period was GMT
466 10:30 + 1:00 = GMT 9:30.</p>
467
468 <p>The <code>FORMAT</code> column specifies the usual abbreviation of
469 the time zone name. It should have one of four forms:</p>
470 <ul>
471
472 <li>a time zone abbreviation that is a string of three or more
473 characters that are either ASCII alphanumerics,
474 <code>+</code>, or <code>-</code></li>
475
476 <li>the string %z, in which case the
477 <code>%z</code> will be replaced by a numeric time zone
478 abbreviation</li>
479
480 <li>a pair of time zone abbreviations separated by a slash
481 (<code>/</code>), in which case the first string is the
482 abbreviation for the standard time name and the second string is the
483 abbreviation for the daylight saving time name</li>
484
485 <li>a string containing <code>%s</code>, in which case
486 the <code>%s</code> will be replaced by the text in the
487 appropriate Rules <code>LETTER</code> column, and the resulting
488 string should be a time zone abbreviation</li>
489 </ul>
490
491 <p>The last two make sense only if theres a named rule in effect.</p>
492
493 <p>An example of a slash is:</p>
494 <pre>
495 #Zone NAME STDOFF RULES FORMAT [UNTIL]
496 Zone Europe/London ... 1996
497 0:00 EU GMT/BST
498 </pre>
499
500 <p>The current time in the UK is called either Greenwich mean time or
501 British summer time.</p>
502
503 <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what
504 happens when switching to a named rule. To what values should
505 the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p>
506
507 <ul>
508 <li>If at least one transition has happened, use
509 the <code>SAVE</code> and <code>LETTER</code> data from the most
510 recent.</li>
511
512 <li>If switching to a named rule before any transition has happened,
513 assume standard time (<code>SAVE</code> zero), and use
514 the <code>LETTER</code> data from the earliest transition with
515 a <code>SAVE</code> of zero.
516
517 </ul>
518
519 <p>And three last things about the <code>FORMAT</code> column:</p>
520 <ul>
521
522 <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz
523 database</a> gives abbreviations for time zones
524 in popular English-language usage. For
525 example, the last line in
526 <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives
527 HST for Hawaii standard time even though the
528 <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a>
529 name for that time zone is HawaiiAleutian standard time.
530 This author has read that there are also some places in Australia where
531 popular time zone names differ from the legal ones.
532
533 <li>No attempt is made to <a
534 href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a>
535 the abbreviations. They are intended to be the values returned through the
536 <code>"%Z"</code> format specifier to
537 <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>s
538 <a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/strftime.html"><code>strftime</code></a>
539 function in the
540 <a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">C locale</a>.
541
542 <li>If there is no generally accepted abbreviation for a time zone,
543 a numeric offset is used instead, e.g., <code>+07</code> for 7 hours
544 ahead of Greenwich. By convention, <code>-00</code> is used in a
545 zone while uninhabited, where the offset is zero but in some sense
546 the true offset is undefined.
547 </ul>
548
549 <p>As a final example, heres the complete history for Hawaii:</p>
550
551 <pre># Relevant Excerpts from the US Rules
552 #Rule NAME FROM TO - IN ON AT SAVE LETTER/S
553 Rule US 1918 1919 - Oct lastSun 2:00 0 S
554 Rule US 1942 only - Feb 9 2:00 1:00 W # War
555 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
556 Rule US 1945 only - Sep lastSun 2:00 0 S
557
558 # The Zone Record
559 #Zone NAME STDOFF RULES FORMAT [UNTIL]
560 Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00
561 -10:30 - HST 1933 Apr 30 2:00
562 -10:30 1:00 HDT 1933 May 21 2:00
563 -10:30 US H%sT 1947 Jun 8 2:00
564 -10:00 - HST
565 </pre>
566 <table>
567 <tr>
568 <th colspan="6">What We Infer</th>
569 </tr>
570 <tr>
571 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th>
572 <th rowspan="2">Adjust<br>Clocks</th>
573 <th colspan="2">Time Zone</th>
574 <th colspan="2">Ending at Local Time</th>
575 </tr>
576 <tr>
577 <th>Abbrv.</th>
578 <th>Name</th>
579 <th>Date</th>
580 <th>Time</th>
581 </tr>
582 <tr>
583 <td>10:31:26</td>
584 <td></td>
585 <td>LMT</td>
586 <td>local mean time</td>
587 <td>1896-01-13</td>
588 <td>12:00</td>
589 </tr>
590 <tr>
591 <td>10:30</td>
592 <td>+0:01:26</td>
593 <td>HST</td>
594 <td>Hawaii standard time</td>
595 <td>1933-04-30</td>
596 <td>02:00</td>
597 </tr>
598 <tr>
599 <td>9:30</td>
600 <td>+1:00</td>
601 <td>HDT</td>
602 <td>Hawaii daylight time</td>
603 <td>1933-05-21</td>
604 <td>12:00</td>
605 </tr>
606 <tr>
607 <td>10:30¹</td>
608 <td>1:00¹</td>
609 <td>HST¹</td>
610 <td>Hawaii standard time</td>
611 <td>1942-02-09</td>
612 <td>02:00</td>
613 </tr>
614 <tr>
615 <td rowspan="2">9:30</td>
616 <td>+1:00</td>
617 <td>HWT</td>
618 <td>Hawaii war time</td>
619 <td>1945-08-14</td>
620 <td>13:30²</td>
621 </tr>
622 <tr>
623 <td>0</td>
624 <td>HPT</td>
625 <td>Hawaii peace time</td>
626 <td>1945-09-30</td>
627 <td rowspan="2">02:00</td>
628 </tr>
629 <tr>
630 <td>10:30</td>
631 <td>1:00</td>
632 <td rowspan="2">HST</td>
633 <td rowspan="2">Hawaii standard time</td>
634 <td>1947-06-08</td>
635 </tr>
636 <tr>
637 <td>10:00³</td>
638 <td>+0:30³</td>
639 <td colspan="2"></td>
640 </tr>
641 <tr>
642 <td colspan="6" class="footnote">
643 ¹Switching to US rules...most recent transition (in 1919) was to standard time
644 </td>
645 </tr>
646 <tr>
647 <td colspan="6" class="footnote">
648 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>
649 + (9:30) = 13:30 local
650 </td>
651 </tr>
652 <tr>
653 <td colspan="6" class="footnote">
654 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947-06-08T12:30Z</a>,
655 the civil time in Hawaii has been
656 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
657 10:00 year-round.
658 </td>
659 </tr>
660 </table>
661
662 <p>There will be a short quiz later. <code>8-)</code></p>
663
664 <hr>
665 <address>
666 This web page is in the public domain, so clarified as of
667 2015-10-20 by Bill Seymour.
668 <br>
669 All suggestions and corrections will be welcome; all flames will be amusing.
670 Mail to was at pobox dot com.
671 </address>
672 </body>
673 </html>
674