tz-how-to.html revision 1.6
1 <!DOCTYPE html 2 PUBLIC "-//W3C//DTD HTML 4.01//EN" 3 "http://www.w3.org/TR/html4/strict.dtd"> 4 <html> 5 <head> 6 <title>How to Read the tz Database</title> 7 <meta http-equiv="Content-type" content='text/html; charset="UTF-8"'> 8 <style type="text/css"> 9 pre {margin-left: 2em; white-space: pre-wrap;} 10 </style> 11 </head> 12 <body> 13 <h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 14 Database</a> Source Files</h2> 15 <h3>by Bill Seymour</h3> 16 <p>This page uses the <code>America/Chicago</code> and 17 <code>Pacific/Honolulu</code> zones as examples of how to infer 18 times of day from the <a href="tz-link.html">tz database</a> 19 source files. It might be helpful, but not absolutely necessary, 20 for the reader to have already downloaded the 21 latest release of the database and become familiar with the basic layout 22 of the data files. The format is explained in the “man 23 page” for the zic compiler, <code>zic.8.txt</code>, in 24 the <code>code</code> subdirectory.</p> 25 26 <p>We’ll begin by talking about the rules for changing between standard 27 and daylight saving time since we’ll need that information when we talk 28 about the zones.</p> 29 30 <p>First, let’s consider the special daylight saving time rules 31 for Chicago (from the <code>northamerica</code> file in 32 the <code>data</code> subdirectory):</p> 33 34 <table border="1"> 35 <tr> 36 <th colspan="6">From the Source File</th> 37 </tr> 38 <tr> 39 <td colspan="6" align="center"><table><tr><td> 40 <pre> 41 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 42 Rule Chicago 1920 only - Jun 13 2:00 1:00 D 43 Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 44 Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 45 Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 46 Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 47 Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 48 </pre> 49 </td></tr></table></td> 50 </tr> 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 align="center"> 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 align="center"> 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 align="center"> 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 align="center"> 80 <td rowspan="2">1922</td> 81 <td>1966</td> 82 <td>in April</td> 83 </tr> 84 <tr align="center"> 85 <td>1954</td> 86 <td>in September</td> 87 <td rowspan="2">return to standard time</td> 88 </tr> 89 <tr align="center"> 90 <td>1955</td> 91 <td>1966</td> 92 <td>in October</td> 93 </tr> 94 </table> 95 96 <p>We’ll basically just ignore the <code>TYPE</code> column. 97 In the 2007j release, the most recent as of this writing, the 98 <code>TYPE</code> column never contains anything but a hyphen, 99 a kind of null value. (From the description in <code>zic.8.txt</code>, 100 this appears to be a mechanism for removing years from a set 101 in some localizable way. It’s used in the file, <code>pacificnew</code>, 102 to determine whether a given year will have a US presidential election; 103 but everything related to that use is commented out.) 104 105 <p>The <code>SAVE</code> column contains the wall clock offset from 106 local standard time. 107 This is usually either zero for standard time or one hour for daylight 108 saving time; but there’s no reason, in principle, why it can’t 109 take on other values. 110 111 <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 112 column can contain a variable 113 part of the usual abbreviation of the time zone’s name, or it can just 114 be a hyphen if there’s no variable part. For example, the abbreviation 115 used in the central time zone will be either “CST” or 116 “CDT”. The variable part is ‘S’ or ‘D’; 117 and, sure enough, that’s just what we find in 118 the <code>LETTER</code> column 119 in the <code>Chicago</code> rules. More about this when we talk about 120 “Zone” lines. 121 122 <p>One important thing to notice is that “Rule” lines 123 want at once to be both <i>transitions</i> and <i>steady states</i>: 124 <ul> 125 <li>On the one hand, they represent transitions between standard and 126 daylight saving time; and any number of Rule lines can be in effect 127 during a given period (which will always be a non-empty set of 128 contiguous calendar years).</li> 129 <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 130 columns contain state that exists between transitions. More about this 131 when we talk about the US rules.</li> 132 </ul> 133 134 <p>In the example above, the transition to daylight saving time 135 happened on the 13<small><sup>th</sup></small> of June in 1920, and on 136 the last Sunday in March in 1921; but the return to standard time 137 happened on the last Sunday in October in both of those 138 years. Similarly, the rule for changing to daylight saving time was 139 the same from 1922 to 1966; but the rule for returning to standard 140 time changed in 1955. Got it?</p> 141 142 <p>OK, now for the somewhat more interesting “US” rules:</p> 143 144 <table border="1"> 145 <tr> 146 <th colspan="6">From the Source File</th> 147 </tr> 148 <tr> 149 <td colspan="6" align="center"><table><tr><td> 150 <pre> 151 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 152 Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 153 Rule US 1918 1919 - Oct lastSun 2:00 0 S 154 Rule US 1942 only - Feb 9 2:00 1:00 W # War 155 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 156 Rule US 1945 only - Sep 30 2:00 0 S 157 Rule US 1967 2006 - Oct lastSun 2:00 0 S 158 Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 159 Rule US 1974 only - Jan 6 2:00 1:00 D 160 Rule US 1975 only - Feb 23 2:00 1:00 D 161 Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 162 Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 163 Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 164 Rule US 2007 max - Nov Sun>=1 2:00 0 S 165 </pre> 166 </td></tr></table></td> 167 </tr> 168 <tr> 169 <th colspan="6">Reformatted a Bit</th> 170 </tr> 171 <tr> 172 <th>From</th> 173 <th>To</th> 174 <th colspan="2">On</th> 175 <th>At</th> 176 <th>Action</th> 177 </tr> 178 <tr align="center"> 179 <td rowspan="2">1918</td> 180 <td rowspan="2">1919</td> 181 <td rowspan="2">last Sunday</td> 182 <td>in March</td> 183 <td rowspan="3">02:00 local</td> 184 <td>go to daylight saving time</td> 185 </tr> 186 <tr align="center"> 187 <td>in October</td> 188 <td>return to standard time</td> 189 </tr> 190 <tr align="center"> 191 <td colspan="2">1942 only</td> 192 <td colspan="2">February 9<small><sup>th</sup></small></td> 193 <td>go to “war time”</td> 194 </tr> 195 <tr align="center"> 196 <td colspan="2" rowspan="2">1945 only</td> 197 <td colspan="2">August 14<small><sup>th</sup></small></td> 198 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 199 <td> 200 rename “war time” to “peace<br>time;” 201 clocks don’t change 202 </td> 203 </tr> 204 <tr align="center"> 205 <td colspan="2">September 30<small><sup>th</sup></small></td> 206 <td rowspan="9">02:00 local</td> 207 <td rowspan="2">return to standard time</td> 208 </tr> 209 <tr align="center"> 210 <td rowspan="2">1967</td> 211 <td>2006</td> 212 <td rowspan="2">last Sunday</td> 213 <td>in October</td> 214 </tr> 215 <tr align="center"> 216 <td>1973</td> 217 <td>in April</td> 218 <td rowspan="6">go to daylight saving time</td> 219 </tr> 220 <tr align="center"> 221 <td colspan="2">1974 only</td> 222 <td colspan="2">January 6<small><sup>th</sup></small></td> 223 </tr> 224 <tr align="center"> 225 <td colspan="2">1975 only</td> 226 <td colspan="2">February 23<small><sup>rd</sup></small></td> 227 </tr> 228 <tr align="center"> 229 <td>1976</td> 230 <td>1986</td> 231 <td>last Sunday</td> 232 <td rowspan="2">in April</td> 233 </tr> 234 <tr align="center"> 235 <td>1987</td> 236 <td>2006</td> 237 <td>first Sunday</td> 238 </tr> 239 <tr align="center"> 240 <td rowspan="2">2007</td> 241 <td rowspan="2">present</td> 242 <td colspan="2">second Sunday in March</td> 243 </tr> 244 <tr align="center"> 245 <td colspan="2">first Sunday in November</td> 246 <td>return to standard time</td> 247 </tr> 248 </table> 249 250 <p>There are two interesting things to note here.</p> 251 252 <p>First, the time that something happens (in the <code>AT</code> 253 column) is not necessarily the local wall clock time. The time can be 254 suffixed with ‘s’ (for “standard”) to mean 255 local standard time (different from wall clock time when observing 256 daylight saving time); or it can be suffixed with ‘g’, 257 ‘u’, or ‘z’, all three of which mean the 258 standard time at the 259 <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 260 ‘g’ stands for “<a 261 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 262 ‘u’ stands for “<a 263 href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 264 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 265 (whichever was official at the time); ‘z’ stands for the 266 <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 267 Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 268 The time can also be suffixed with ‘w’ meaning “wall 269 clock time;” but it usually isn’t because that’s the 270 default.</p> 271 272 <p>Second, the day in the <code>ON</code> column, in addition to 273 “<code>lastSun</code>” or a particular day of the month, 274 can have the form, “<code>Sun>=</code><i>x</i>” or 275 “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 276 of the month. For example, “<code>Sun>=8</code>” means 277 “the first Sunday on or after the eighth of the month,” in 278 other words, the second Sunday of the month. Furthermore, although 279 there are no examples above, the weekday needn’t be 280 “<code>Sun</code>” in either form, but can be the usual 281 three-character English abbreviation for any day of the week.</p> 282 283 <p>And the US rules give us more examples of a couple of things 284 already mentioned:</p> 285 286 <ul> 287 <li>The rules for changing to and from daylight saving time are 288 actually <i>different sets</i> of rules; and the two sets can change 289 independently. Consider, for example, that the rule for the return to 290 standard time stayed the same from 1967 to 2006; but the rule for the 291 transition to daylight saving time changed several times in the same 292 period. There can also be periods, 1946 to 1966 for example, when no 293 rule from this group is in effect, and so either no transition 294 happened in those years, or some other rule is in effect (perhaps a 295 state or other more local rule).</li> 296 297 <li>The <code>SAVE</code> and <code>LETTER</code> columns 298 contain <i>steady state</i>, not transitions. Consider, for example, 299 the transition from “war time” to “peace time” 300 that happened on August 14, 1945. The “1:00” in 301 the <code>SAVE</code> column is <i>not</i> an instruction to advance 302 the clock an hour. It means that clocks should <i>be</i> one hour 303 ahead of standard time, which they already are because of the previous 304 rule, so there should be no change.</li> 305 306 </ul> 307 308 <p>OK, now let’s look at a Zone record:</p> 309 310 <table border="1"> 311 <tr> 312 <th colspan="5">From the Source File</th> 313 </tr> 314 <tr> 315 <td colspan="6" align="center"><table><tr><td> 316 <pre> 317 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 318 Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 319 -6:00 US C%sT 1920 320 -6:00 Chicago C%sT 1936 Mar 1 2:00 321 -5:00 - EST 1936 Nov 15 2:00 322 -6:00 Chicago C%sT 1942 323 -6:00 US C%sT 1946 324 -6:00 Chicago C%sT 1967 325 -6:00 US C%sT 326 </pre> 327 </td></tr></table></td> 328 </tr> 329 <tr> 330 <th colspan="5">Columns Renamed</th> 331 </tr> 332 <tr> 333 <th rowspan="2">Standard Offset<br> 334 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 335 Meridian</a></th> 336 <th rowspan="2">Daylight<br>Saving Time</th> 337 <th rowspan="2">Abbreviation(s)</th> 338 <th colspan="2">Ending at Local Time</th> 339 </tr> 340 <tr> 341 <th>Date</th> 342 <th>Time</th> 343 </tr> 344 <tr align="center"> 345 <td>−5:50:36</td> 346 <td>not observed</td> 347 <td>LMT</td> 348 <td>1883-11-18</td> 349 <td>12:09:24</td> 350 </tr> 351 <tr align="center"> 352 <td rowspan="2">−6:00:00</td> 353 <td>US rules</td> 354 <td rowspan="2">CST or CDT</td> 355 <td>1920-01-01</td> 356 <td>00:00:00</td> 357 </tr> 358 <tr align="center"> 359 <td>Chicago rules</td> 360 <td>1936-03-01</td> 361 <td rowspan="2">02:00:00</td> 362 </tr> 363 <tr align="center"> 364 <td>−5:00:00</td> 365 <td>not observed</td> 366 <td>EST</td> 367 <td>1936-11-15</td> 368 </tr> 369 <tr align="center"> 370 <td rowspan="4">−6:00:00</td> 371 <td>Chicago rules</td> 372 <td>CST or CDT</td> 373 <td>1942-01-01</td> 374 <td rowspan="3">00:00:00</td> 375 </tr> 376 <tr align="center"> 377 <td>US rules</td> 378 <td>CST, CWT or CPT</td> 379 <td>1946-01-01</td> 380 </tr> 381 <tr align="center"> 382 <td>Chicago rules</td> 383 <td rowspan="2">CST or CDT</td> 384 <td>1967-01-01</td> 385 </tr> 386 <tr align="center"> 387 <td>US rules</td> 388 <td colspan="2">—</td> 389 </tr> 390 </table> 391 392 <p>There are a couple of interesting differences between Zones and Rules.</p> 393 394 <p>First, and somewhat trivially, whereas Rules are considered to 395 contain one or more records, a Zone is considered to be a single 396 record with zero or more <i>continuation lines</i>. Thus, the keyword, 397 “<code>Zone</code>,” and the zone name are not 398 repeated. The last line is the one without anything in 399 the <code>[UNTIL]</code> column.</p> 400 401 <p>Second, and more fundamentally, each line of a Zone represents a 402 steady state, not a transition between states. The state exists from 403 the date and time in the previous line’s <code>[UNTIL]</code> 404 column up to the date and time in the current 405 line’s <code>[UNTIL]</code> column. In other words, the date and 406 time in the <code>[UNTIL]</code> column is the instant that separates 407 this state from the next. Where that would be ambiguous because 408 we’re setting our clocks back, the <code>[UNTIL]</code> column 409 specifies the first occurrence of the instant. The state specified by 410 the last line, the one without anything in the <code>[UNTIL]</code> 411 column, continues to the present.</p> 412 413 <p>The first line typically specifies the mean solar time observed 414 before the introduction of standard time. Since there’s no line before 415 that, it has no beginning. <code>8-) </code> For some places near the <a 416 href="https://en.wikipedia.org/wiki/International_Date_Line">International 417 Date Line</a>, the first <i>two</i> lines will show solar times 418 differing by 24 hours; this corresponds to a movement of the Date 419 Line. For example:</p> 420 421 <pre> 422 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 423 Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 424 -8:57:41 - LMT ... 425 </pre> 426 427 <p>When Alaska was purchased from Russia in 1867, the Date Line moved 428 from the Alaska/Canada border to the Bering Strait; and the time in 429 Alaska was then 24 hours earlier than it had 430 been. <code><aside></code>(6 October in the Julian calendar, 431 which Russia was still using then for religious reasons, was followed 432 by <i>a second instance of the same day with a different name</i>, 18 433 October in the Gregorian calendar. Isn’t civil time 434 wonderful? <code>8-)</code>)<code></aside></code></p> 435 436 <p>The abbreviation, “LMT” stands for “local mean 437 time”, which is an invention of 438 the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 439 database</a> and was probably never actually used during the 440 period. Furthermore, the value is almost certainly wrong except in the 441 archetypal place after which the zone is named. (The tz database 442 usually doesn’t provide a separate Zone record for places where 443 nothing significant happened after 1970.)</p> 444 445 <p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 446 <ul> 447 <li>A hyphen, a kind of null value, means that we have not set our 448 clocks ahead of standard time.</li> 449 450 <li>An amount of time (usually but not necessarily “1:00” 451 meaning one hour) means that we have set our clocks ahead by that 452 amount.</li> 453 454 <li>Some alphabetic string means that we <i>might have</i> set our 455 clocks ahead; and we need to check the rule the name of which is the 456 given alphabetic string.</li> 457 </ul> 458 459 <p>An example of a specific amount of time is:</p> 460 <pre> 461 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 462 Zone Pacific/Honolulu ... 1933 Apr 30 2:00 463 -10:30 1:00 HDT 1933 May 21 12:00 464 ... 465 </pre> 466 467 <p>Hawaii tried daylight saving time for three weeks in 1933 and 468 decided they didn’t like it. <code>8-) </code>Note that 469 the <code>GMTOFF</code> column always contains the standard time 470 offset, so the wall clock time during this period was GMT − 471 10:30 + 1:00 = GMT − 9:30.</p> 472 473 <p>The <code>FORMAT</code> column specifies the usual abbreviation of 474 the time zone name. It can have one of three forms:</p> 475 <ul> 476 477 <li>a string of three or more characters that are either ASCII alphanumerics, 478 “<code>+</code>”, or “<code>-</code>”, 479 in which case that’s the abbreviation</li> 480 481 <li>a pair of strings separated by a slash 482 (‘<code>/</code>’), in which case the first string is the 483 abbreviation for the standard time name and the second string is the 484 abbreviation for the daylight saving time name</li> 485 486 <li>a string containing “<code>%s</code>,” in which case 487 the “<code>%s</code>” will be replaced by the text in the 488 appropriate Rule’s <code>LETTER</code> column</li> 489 </ul> 490 491 <p>The last two make sense only if there’s a named rule in effect.</p> 492 493 <p>An example of a slash is:</p> 494 <pre> 495 #Zone NAME GMTOFF 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 in <i>popular 524 usage</i>, which is not necessarily “correct” by law. 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 “Hawaii-Aleutian 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="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 539 function in the 540 <a href="http://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, here’s the complete history for Hawaii:</p> 550 551 <table border="1"> 552 <tr> 553 <th colspan="6">Relevant Excerpts from the US Rules</th> 554 </tr> 555 <tr> 556 <td colspan="6" align="center"><table><tr><td> 557 <pre> 558 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 559 Rule US 1918 1919 - Oct lastSun 2:00 0 S 560 Rule US 1942 only - Feb 9 2:00 1:00 W # War 561 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 562 Rule US 1945 only - Sep lastSun 2:00 0 S 563 </pre> 564 </td></tr></table></td> 565 </tr> 566 <tr> 567 <th colspan="6">The Zone Record</th> 568 </tr> 569 <tr> 570 <td colspan="6" align="center"><table><tr><td> 571 <pre> 572 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 573 Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 574 -10:30 - HST 1933 Apr 30 2:00 575 -10:30 1:00 HDT 1933 May 21 2:00 576 -10:30 US H%sT 1947 Jun 8 2:00 577 -10:00 - HST 578 </pre> 579 </td></tr></table></td> 580 </tr> 581 <tr> 582 <th colspan="6">What We Infer</th> 583 </tr> 584 <tr> 585 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 586 <th rowspan="2">Adjust<br>Clocks</th> 587 <th colspan="2">Time Zone</th> 588 <th colspan="2">Ending at Local Time</th> 589 </tr> 590 <tr> 591 <th>Abbrv.</th> 592 <th>Name</th> 593 <th>Date</th> 594 <th>Time</th> 595 </tr> 596 <tr align="center"> 597 <td>−10:31:26</td> 598 <td>—</td> 599 <td>LMT</td> 600 <td>local mean time</td> 601 <td>1896-01-13</td> 602 <td>12:00</td> 603 </tr> 604 <tr align="center"> 605 <td>−10:30</td> 606 <td>+0:01:26</td> 607 <td>HST</td> 608 <td>Hawaii standard time</td> 609 <td>1933-04-30</td> 610 <td>02:00</td> 611 </tr> 612 <tr align="center"> 613 <td>−9:30</td> 614 <td>+1:00</td> 615 <td>HDT</td> 616 <td>Hawaii daylight time</td> 617 <td>1933-05-21</td> 618 <td>12:00</td> 619 </tr> 620 <tr align="center"> 621 <td>−10:30¹</td> 622 <td>−1:00¹</td> 623 <td>HST¹</td> 624 <td>Hawaii standard time</td> 625 <td>1942-02-09</td> 626 <td>02:00</td> 627 </tr> 628 <tr align="center"> 629 <td rowspan="2">−9:30</td> 630 <td>+1:00</td> 631 <td>HWT</td> 632 <td>Hawaii war time</td> 633 <td>1945-08-14</td> 634 <td>13:30²</td> 635 </tr> 636 <tr align="center"> 637 <td>0</td> 638 <td>HPT</td> 639 <td>Hawaii peace time</td> 640 <td>1945-09-30</td> 641 <td rowspan="2">02:00</td> 642 </tr> 643 <tr align="center"> 644 <td>−10:30</td> 645 <td>−1:00</td> 646 <td rowspan="2">HST</td> 647 <td rowspan="2">Hawaii standard time</td> 648 <td>1947-06-08</td> 649 </tr> 650 <tr align="center"> 651 <td>−10:00³</td> 652 <td>+0:30³</td> 653 <td colspan="2">—</td> 654 </tr> 655 <tr> 656 <td colspan="6"> 657 ¹Switching to US rules…most recent transition (in 1919) was to standard time 658 </td> 659 </tr> 660 <tr> 661 <td colspan="6"> 662 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 663 + (−9:30) = 13:30 local 664 </td> 665 </tr> 666 <tr> 667 <td colspan="6"> 668 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 669 the civil time in Hawaii has been 670 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 671 − 10:00 year-round. 672 </td> 673 </tr> 674 </table> 675 676 <p>There will be a short quiz later. <code>8-)</code></p> 677 678 <hr> 679 <address> 680 This web page is in the public domain, so clarified as of 681 2015-10-20 by Bill Seymour. 682 <br> 683 All suggestions and corrections will be welcome; all flames will be amusing. 684 Mail to was at pobox dot com. 685 </address> 686 </body> 687 </html> 688
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