tz-how-to.html revision 1.1
1 <!DOCTYPE html 2 PUBLIC "-//W3C//DTD HTML 4.01//EN" 3 "http://www.w3.org/TR/html4/strict.dtd"> 4 <html> 5 <head><title>How to Read the tz Database</title></head> 6 <body> 7 <h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 8 Database</a> Source Files</h2> 9 <h3>by Bill Seymour</h3> 10 <p>This page uses the <code>America/Chicago</code> and 11 <code>Pacific/Honolulu</code> zones as examples of how to infer 12 times of day from the <a href="tz-link.htm">tz database</a> 13 source files. It might be helpful, but not absolutely necessary, 14 for the reader to have already downloaded the 15 <a href="http://www.iana.org/time-zones/repository/tzdata-latest.tar.gz">latest 16 release of the database</a> and become familiar with the basic layout 17 of the data files. The format is explained in the “man 18 page” for the zic compiler, <code>zic.8.txt</code>, in 19 the <code>code</code> subdirectory.</p> 20 21 <p>We’ll begin by talking about the rules for changing between standard 22 and daylight saving time since we’ll need that information when we talk 23 about the zones.</p> 24 25 <p>First, let’s consider the special daylight saving time rules 26 for Chicago (from the <code>northamerica</code> file in 27 the <code>data</code> subdirectory):</p> 28 29 <table border="1"> 30 <tr> 31 <th colspan="6">From the Source File</th> 32 </tr> 33 <tr> 34 <td colspan="6" align="center"><table><tr><td> 35 <pre> 36 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 37 Rule Chicago 1920 only - Jun 13 2:00 1:00 D 38 Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 39 Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 40 Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 41 Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 42 Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 43 </pre> 44 </td></tr></table></td> 45 </tr> 46 <tr> 47 <th colspan="6">Reformatted a Bit</th> 48 </tr> 49 <tr> 50 <th>From</th> 51 <th>To</th> 52 <th colspan="2">On</th> 53 <th>At</th> 54 <th>Action</th> 55 </tr> 56 <tr align="center"> 57 <td colspan="2">1920 only</td> 58 <td colspan="2">June 13<small><sup>th</sup></small></td> 59 <td rowspan="6">02:00 local</td> 60 <td>go to daylight saving time</td> 61 </tr> 62 <tr align="center"> 63 <td>1920</td> 64 <td>1921</td> 65 <td rowspan="5">last Sunday</td> 66 <td>in October</td> 67 <td>return to standard time</td> 68 </tr> 69 <tr align="center"> 70 <td colspan="2">1921 only</td> 71 <td>in March</td> 72 <td rowspan="2">go to daylight saving time</td> 73 </tr> 74 <tr align="center"> 75 <td rowspan="2">1922</td> 76 <td>1966</td> 77 <td>in April</td> 78 </tr> 79 <tr align="center"> 80 <td>1954</td> 81 <td>in September</td> 82 <td rowspan="2">return to standard time</td> 83 </tr> 84 <tr align="center"> 85 <td>1955</td> 86 <td>1966</td> 87 <td>in October</td> 88 </tr> 89 </table> 90 91 <p>We’ll basically just ignore the <code>TYPE</code> column. 92 In the 2007j release, the most recent as of this writing, the 93 <code>TYPE</code> column never contains anything but a hyphen, 94 a kind of null value. (From the description in <code>zic.8.txt</code>, 95 this appears to be a mechanism for removing years from a set 96 in some localizable way. It’s used in the file, <code>pacificnew</code>, 97 to determine whether a given year will have a US presidential election; 98 but everything related to that use is commented out.) 99 100 <p>The <code>SAVE</code> column contains the wall clock offset from 101 local standard time. 102 This is usually either zero for standard time or one hour for daylight 103 saving time; but there’s no reason, in principle, why it can’t 104 take on other values. 105 106 <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 107 column can contain a variable 108 part of the usual abbreviation of the time zone’s name, or it can just 109 be a hyphen if there’s no variable part. For example, the abbreviation 110 used in the central time zone will be either “CST” or 111 “CDT”. The variable part is ‘S’ or ‘D’; 112 and, sure enough, that’s just what we find in 113 the <code>LETTER</code> column 114 in the <code>Chicago</code> rules. More about this when we talk about 115 “Zone” lines. 116 117 <p>One important thing to notice is that “Rule” lines 118 want at once to be both <i>transitions</i> and <i>steady states</i>: 119 <ul> 120 <li>On the one hand, they represent transitions between standard and 121 daylight saving time; and any number of Rule lines can be in effect 122 during a given period (which will always be a non-empty set of 123 contiguous calendar years).</li> 124 <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 125 columns contain state that exists between transitions. More about this 126 when we talk about the US rules.</li> 127 </ul> 128 129 <p>In the example above, the transition to daylight saving time 130 happened on the 13<small><sup>th</sup></small> of June in 1920, and on 131 the last Sunday in March in 1921; but the return to standard time 132 happened on the last Sunday in October in both of those 133 years. Similarly, the rule for changing to daylight saving time was 134 the same from 1922 to 1966; but the rule for returning to standard 135 time changed in 1955. Got it?</p> 136 137 <p>OK, now for the somewhat more interesting “US” rules:</p> 138 139 <table border="1"> 140 <tr> 141 <th colspan="6">From the Source File</th> 142 </tr> 143 <tr> 144 <td colspan="6" align="center"><table><tr><td> 145 <pre> 146 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 147 Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 148 Rule US 1918 1919 - Oct lastSun 2:00 0 S 149 Rule US 1942 only - Feb 9 2:00 1:00 W # War 150 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 151 Rule US 1945 only - Sep 30 2:00 0 S 152 Rule US 1967 2006 - Oct lastSun 2:00 0 S 153 Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 154 Rule US 1974 only - Jan 6 2:00 1:00 D 155 Rule US 1975 only - Feb 23 2:00 1:00 D 156 Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 157 Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 158 Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 159 Rule US 2007 max - Nov Sun>=1 2:00 0 S 160 </pre> 161 </td></tr></table></td> 162 </tr> 163 <tr> 164 <th colspan="6">Reformatted a Bit</th> 165 </tr> 166 <tr> 167 <th>From</th> 168 <th>To</th> 169 <th colspan="2">On</th> 170 <th>At</th> 171 <th>Action</th> 172 </tr> 173 <tr align="center"> 174 <td rowspan="2">1918</td> 175 <td rowspan="2">1919</td> 176 <td rowspan="2">last Sunday</td> 177 <td>in March</td> 178 <td rowspan="3">02:00 local</td> 179 <td>go to daylight saving time</td> 180 </tr> 181 <tr align="center"> 182 <td>in October</td> 183 <td>return to standard time</td> 184 </tr> 185 <tr align="center"> 186 <td colspan="2">1942 only</td> 187 <td colspan="2">February 9<small><sup>th</sup></small></td> 188 <td>go to “war time”</td> 189 </tr> 190 <tr align="center"> 191 <td colspan="2" rowspan="2">1945 only</td> 192 <td colspan="2">August 14<small><sup>th</sup></small></td> 193 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 194 <td> 195 rename “war time” to “peace<br>time;” 196 clocks don’t change 197 </td> 198 </tr> 199 <tr align="center"> 200 <td colspan="2">September 30<small><sup>th</sup></small></td> 201 <td rowspan="9">02:00 local</td> 202 <td rowspan="2">return to standard time</td> 203 </tr> 204 <tr align="center"> 205 <td rowspan="2">1967</td> 206 <td>2006</td> 207 <td rowspan="2">last Sunday</td> 208 <td>in October</td> 209 </tr> 210 <tr align="center"> 211 <td>1973</td> 212 <td>in April</td> 213 <td rowspan="6">go to daylight saving time</td> 214 </tr> 215 <tr align="center"> 216 <td colspan="2">1974 only</td> 217 <td colspan="2">January 6<small><sup>th</sup></small></td> 218 </tr> 219 <tr align="center"> 220 <td colspan="2">1975 only</td> 221 <td colspan="2">February 23<small><sup>rd</sup></small></td> 222 </tr> 223 <tr align="center"> 224 <td>1976</td> 225 <td>1986</td> 226 <td>last Sunday</td> 227 <td rowspan="2">in April</td> 228 </tr> 229 <tr align="center"> 230 <td>1987</td> 231 <td>2006</td> 232 <td>first Sunday</td> 233 </tr> 234 <tr align="center"> 235 <td rowspan="2">2007</td> 236 <td rowspan="2">present</td> 237 <td colspan="2">second Sunday in March</td> 238 </tr> 239 <tr align="center"> 240 <td colspan="2">first Sunday in November</td> 241 <td>return to standard time</td> 242 </tr> 243 </table> 244 245 <p>There are two interesting things to note here.</p> 246 247 <p>First, the time that something happens (in the <code>AT</code> 248 column) is not necessarily the local wall clock time. The time can be 249 suffixed with ‘s’ (for “standard”) to mean 250 local standard time (different from wall clock time when observing 251 daylight saving time); or it can be suffixed with ‘g’, 252 ‘u’, or ‘z’, all three of which mean the 253 standard time at the 254 <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridan</a>. 255 ‘g’ stands for “<a 256 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 257 ‘u’ stands for “<a 258 href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 259 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 260 (whichever was official at the time); ‘z’ stands for the 261 <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 262 Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 263 The time can also be suffixed with ‘w’ meaning “wall 264 clock time;” but it usually isn’t because that’s the 265 default.</p> 266 267 <p>Second, the day in the <code>ON</code> column, in addition to 268 “<code>lastSun</code>” or a particular day of the month, 269 can have the form, “<code>Sun>=</code><i>x</i>” or 270 “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 271 of the month. For example, “<code>Sun>=8</code>” means 272 “the first Sunday on or after the eighth of the month,” in 273 other words, the second Sunday of the month. Furthermore, although 274 there are no examples above, the weekday needn’t be 275 “<code>Sun</code>” in either form, but can be the usual 276 three-character English abbreviation for any day of the week.</p> 277 278 <p>And the US rules give us more examples of a couple of things 279 already mentioned:</p> 280 281 <ul> 282 <li>The rules for changing to and from daylight saving time are 283 actually <i>different sets</i> of rules; and the two sets can change 284 independently. Consider, for example, that the rule for the return to 285 standard time stayed the same from 1967 to 2006; but the rule for the 286 transition to daylight saving time changed several times in the same 287 period. There can also be periods, 1946 to 1966 for example, when no 288 rule from this group is in effect, and so either no transition 289 happened in those years, or some other rule is in effect (perhaps a 290 state or other more local rule).</li> 291 292 <li>The <code>SAVE</code> and <code>LETTER</code> columns 293 contain <i>steady state</i>, not transitions. Consider, for example, 294 the transition from “war time” to “peace time” 295 that happened on August 14, 1945. The “1:00” in 296 the <code>SAVE</code> column is <i>not</i> an instruction to advance 297 the clock an hour. It means that clocks should <i>be</i> one hour 298 ahead of standard time, which they already are because of the previous 299 rule, so there should be no change.</li> 300 301 </ul> 302 303 <p>OK, now let’s look at a Zone record:</p> 304 305 <table border="1"> 306 <tr> 307 <th colspan="5">From the Source File</th> 308 </tr> 309 <tr> 310 <td colspan="6" align="center"><table><tr><td> 311 <pre> 312 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 313 Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 314 -6:00 US C%sT 1920 315 -6:00 Chicago C%sT 1936 Mar 1 2:00 316 -5:00 - EST 1936 Nov 15 2:00 317 -6:00 Chicago C%sT 1942 318 -6:00 US C%sT 1946 319 -6:00 Chicago C%sT 1967 320 -6:00 US C%sT 321 </pre> 322 </td></tr></table></td> 323 </tr> 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 align="center"> 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 align="center"> 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 align="center"> 354 <td>Chicago rules</td> 355 <td>1936-03-01</td> 356 <td rowspan="2">02:00:00</td> 357 </tr> 358 <tr align="center"> 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 align="center"> 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 align="center"> 372 <td>US rules</td> 373 <td>CST, CWT or CPT</td> 374 <td>1946-01-01</td> 375 </tr> 376 <tr align="center"> 377 <td>Chicago rules</td> 378 <td rowspan="2">CST or CDT</td> 379 <td>1967-01-01</td> 380 </tr> 381 <tr align="center"> 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 line’s <code>[UNTIL]</code> 399 column up to the date and time in the current 400 line’s <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 we’re 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 there’s 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 GMTOFF 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. Isn’t 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 doesn’t 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 GMTOFF RULES FORMAT [UNTIL] 457 Zone Pacific/Honolulu ... 1933 Apr 30 2:00 458 -10:30 1:00 HDT 1933 May 21 2:00 459 ... 460 </pre> 461 462 <p>Hawaii tried daylight saving time for three weeks in 1933 and 463 decided they didn’t like it. <code>8-) </code>Note that 464 the <code>GMTOFF</code> column always contains the standard time 465 offset, so the 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 can have one of three forms:</p> 470 <ul> 471 472 <li>a string of three or more characters that are either ASCII alphanumerics, 473 “<code>+</code>”, or “<code>-</code>”, 474 in which case that’s the abbreviation</li> 475 476 <li>a pair of strings separated by a slash 477 (‘<code>/</code>’), in which case the first string is the 478 abbreviation for the standard time name and the second string is the 479 abbreviation for the daylight saving time name</li> 480 481 <li>a string containing “<code>%s</code>,” in which case 482 the “<code>%s</code>” will be replaced by the text in the 483 appropriate Rule’s <code>LETTER</code> column</li> 484 </ul> 485 486 <p>The last two make sense only if there’s a named rule in effect.</p> 487 488 <p>An example of a slash is:</p> 489 <pre> 490 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 491 Zone Europe/London ... 1996 492 0:00 EU GMT/BST 493 </pre> 494 495 <p>The current time in the UK is called either Greenwich mean time or 496 British summer time.</p> 497 498 <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 499 happens when switching to a named rule. To what values should 500 the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 501 502 <ul> 503 <li>If at least one transition has happened, use 504 the <code>SAVE</code> and <code>LETTER</code> data from the most 505 recent.</li> 506 507 <li>If switching to a named rule before any transition has happened, 508 assume standard time (<code>SAVE</code> zero), and use 509 the <code>LETTER</code> data from the earliest transition with 510 a <code>SAVE</code> of zero. 511 512 </ul> 513 514 <p>And three last things about the <code>FORMAT</code> column:</p> 515 <ul> 516 517 <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 518 database</a> gives abbreviations for time zone names in <i>popular 519 usage</i>, which is not necessarily “correct” by law. For 520 example, the last line in 521 <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 522 “HST” for “Hawaii standard time” even though the 523 <a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a> 524 name for that time zone is “Hawaii-Aleutian standard time.” 525 This author has read that there are also some places in Australia where 526 popular time zone names differ from the legal ones. 527 528 <li>No attempt is made to <a 529 href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 530 the abbreviations. They are intended to be the values returned through the 531 <code>"%Z"</code> format specifier to 532 <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 533 <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 534 function in the 535 <a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 536 537 <li>If there is no generally-accepted abbreviation for a time zone, 538 a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 539 ahead of Greenwich. By convention, <code>-00</code> is used in a 540 zone while uninhabited, where the offset is zero but in some sense 541 the true offset is undefined. 542 </ul> 543 544 <p>As a final example, here’s the complete history for Hawaii:</p> 545 546 <table border="1"> 547 <tr> 548 <th colspan="6">Relevant Excerpts from the US Rules</th> 549 </tr> 550 <tr> 551 <td colspan="6" align="center"><table><tr><td> 552 <pre> 553 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 554 Rule US 1918 1919 - Oct lastSun 2:00 0 S 555 Rule US 1942 only - Feb 9 2:00 1:00 W # War 556 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 557 Rule US 1945 only - Sep 30 2:00 0 S 558 </pre> 559 </td></tr></table></td> 560 </tr> 561 <tr> 562 <th colspan="6">The Zone Record</th> 563 </tr> 564 <tr> 565 <td colspan="6" align="center"><table><tr><td> 566 <pre> 567 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 568 Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 569 -10:30 - HST 1933 Apr 30 2:00 570 -10:30 1:00 HDT 1933 May 21 2:00 571 -10:30 US H%sT 1947 Jun 8 2:00 572 -10:00 - HST 573 </pre> 574 </td></tr></table></td> 575 </tr> 576 <tr> 577 <th colspan="6">What We Infer</th> 578 </tr> 579 <tr> 580 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 581 <th rowspan="2">Adjust<br>Clocks</th> 582 <th colspan="2">Time Zone</th> 583 <th colspan="2">Ending at Local Time</th> 584 </tr> 585 <tr> 586 <th>Abbrv.</th> 587 <th>Name</th> 588 <th>Date</th> 589 <th>Time</th> 590 </tr> 591 <tr align="center"> 592 <td>−10:31:26</td> 593 <td>—</td> 594 <td>LMT</td> 595 <td>local mean time</td> 596 <td>1900-01-01</td> 597 <td>12:00</td> 598 </tr> 599 <tr align="center"> 600 <td>−10:30</td> 601 <td>+0:01:26</td> 602 <td>HST</td> 603 <td>Hawaii standard time</td> 604 <td>1933-04-30</td> 605 <td rowspan="3">02:00</td> 606 </tr> 607 <tr align="center"> 608 <td>−9:30</td> 609 <td>+1:00</td> 610 <td>HDT</td> 611 <td>Hawaii daylight time</td> 612 <td>1933-05-21</td> 613 </tr> 614 <tr align="center"> 615 <td>−10:30¹</td> 616 <td>−1:00¹</td> 617 <td>HST¹</td> 618 <td>Hawaii standard time</td> 619 <td>1942-02-09</td> 620 </tr> 621 <tr align="center"> 622 <td rowspan="2">−9:30</td> 623 <td>+1:00</td> 624 <td>HWT</td> 625 <td>Hawaii war time</td> 626 <td>1945-08-14</td> 627 <td>13:30²</td> 628 </tr> 629 <tr align="center"> 630 <td>0</td> 631 <td>HPT</td> 632 <td>Hawaii peace time</td> 633 <td>1945-09-30</td> 634 <td rowspan="2">02:00</td> 635 </tr> 636 <tr align="center"> 637 <td>−10:30</td> 638 <td>−1:00</td> 639 <td rowspan="2">HST</td> 640 <td rowspan="2">Hawaii standard time</td> 641 <td>1947-06-08</td> 642 </tr> 643 <tr align="center"> 644 <td>−10:00³</td> 645 <td>+0:30³</td> 646 <td colspan="2">—</td> 647 </tr> 648 <tr> 649 <td colspan="6"> 650 ¹Switching to US rules…most recent transition (in 1919) was to standard time 651 </td> 652 </tr> 653 <tr> 654 <td colspan="6"> 655 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 656 + (−9:30) = 13:30 local 657 </td> 658 </tr> 659 <tr> 660 <td colspan="6"> 661 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 662 the civil time in Hawaii has been 663 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 664 − 10:00 year-round. 665 </td> 666 </tr> 667 </table> 668 669 <p>There will be a short quiz later. <code>8-)</code></p> 670 671 <hr> 672 <address> 673 This web page is in the public domain, so clarified as of 674 2015-10-20 by Bill Seymour. 675 <br> 676 All suggestions and corrections will be welcome; all flames will be amusing. 677 Mail to was at pobox dot com. 678 </address> 679 </body> 680 </html> 681
Indexes created Mon Sep 29 13:09:55 GMT 2025