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 <main> 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