1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <title>How to Read the tz Database</title> 5 <meta charset="UTF-8"> 6 <meta name="viewport" content="width=device-width, initial-scale=1"> 7 <style> 8 pre {margin-left: 1.3rem; overflow: auto;} 9 td {text-align: center;} 10 table {border: 1px outset;} 11 th, td {border: 1px inset;} 12 table.rule {border: none; margin: auto;} 13 td.footnote {text-align: left;} 14 ul {padding-left: 1.3rem;} 15 </style> 16 </head> 17 <body> 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
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