tz-how-to.html revision 1.2
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 latest release of the database and become familiar with the basic layout 16 of the data files. The format is explained in the “man 17 page” for the zic compiler, <code>zic.8.txt</code>, in 18 the <code>code</code> subdirectory.</p> 19 20 <p>We’ll begin by talking about the rules for changing between standard 21 and daylight saving time since we’ll need that information when we talk 22 about the zones.</p> 23 24 <p>First, let’s consider the special daylight saving time rules 25 for Chicago (from the <code>northamerica</code> file in 26 the <code>data</code> subdirectory):</p> 27 28 <table border="1"> 29 <tr> 30 <th colspan="6">From the Source File</th> 31 </tr> 32 <tr> 33 <td colspan="6" align="center"><table><tr><td> 34 <pre> 35 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 36 Rule Chicago 1920 only - Jun 13 2:00 1:00 D 37 Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 38 Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 39 Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 40 Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 41 Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 42 </pre> 43 </td></tr></table></td> 44 </tr> 45 <tr> 46 <th colspan="6">Reformatted a Bit</th> 47 </tr> 48 <tr> 49 <th>From</th> 50 <th>To</th> 51 <th colspan="2">On</th> 52 <th>At</th> 53 <th>Action</th> 54 </tr> 55 <tr align="center"> 56 <td colspan="2">1920 only</td> 57 <td colspan="2">June 13<small><sup>th</sup></small></td> 58 <td rowspan="6">02:00 local</td> 59 <td>go to daylight saving time</td> 60 </tr> 61 <tr align="center"> 62 <td>1920</td> 63 <td>1921</td> 64 <td rowspan="5">last Sunday</td> 65 <td>in October</td> 66 <td>return to standard time</td> 67 </tr> 68 <tr align="center"> 69 <td colspan="2">1921 only</td> 70 <td>in March</td> 71 <td rowspan="2">go to daylight saving time</td> 72 </tr> 73 <tr align="center"> 74 <td rowspan="2">1922</td> 75 <td>1966</td> 76 <td>in April</td> 77 </tr> 78 <tr align="center"> 79 <td>1954</td> 80 <td>in September</td> 81 <td rowspan="2">return to standard time</td> 82 </tr> 83 <tr align="center"> 84 <td>1955</td> 85 <td>1966</td> 86 <td>in October</td> 87 </tr> 88 </table> 89 90 <p>We’ll basically just ignore the <code>TYPE</code> column. 91 In the 2007j release, the most recent as of this writing, the 92 <code>TYPE</code> column never contains anything but a hyphen, 93 a kind of null value. (From the description in <code>zic.8.txt</code>, 94 this appears to be a mechanism for removing years from a set 95 in some localizable way. It’s used in the file, <code>pacificnew</code>, 96 to determine whether a given year will have a US presidential election; 97 but everything related to that use is commented out.) 98 99 <p>The <code>SAVE</code> column contains the wall clock offset from 100 local standard time. 101 This is usually either zero for standard time or one hour for daylight 102 saving time; but there’s no reason, in principle, why it can’t 103 take on other values. 104 105 <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 106 column can contain a variable 107 part of the usual abbreviation of the time zone’s name, or it can just 108 be a hyphen if there’s no variable part. For example, the abbreviation 109 used in the central time zone will be either “CST” or 110 “CDT”. The variable part is ‘S’ or ‘D’; 111 and, sure enough, that’s just what we find in 112 the <code>LETTER</code> column 113 in the <code>Chicago</code> rules. More about this when we talk about 114 “Zone” lines. 115 116 <p>One important thing to notice is that “Rule” lines 117 want at once to be both <i>transitions</i> and <i>steady states</i>: 118 <ul> 119 <li>On the one hand, they represent transitions between standard and 120 daylight saving time; and any number of Rule lines can be in effect 121 during a given period (which will always be a non-empty set of 122 contiguous calendar years).</li> 123 <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 124 columns contain state that exists between transitions. More about this 125 when we talk about the US rules.</li> 126 </ul> 127 128 <p>In the example above, the transition to daylight saving time 129 happened on the 13<small><sup>th</sup></small> of June in 1920, and on 130 the last Sunday in March in 1921; but the return to standard time 131 happened on the last Sunday in October in both of those 132 years. Similarly, the rule for changing to daylight saving time was 133 the same from 1922 to 1966; but the rule for returning to standard 134 time changed in 1955. Got it?</p> 135 136 <p>OK, now for the somewhat more interesting “US” rules:</p> 137 138 <table border="1"> 139 <tr> 140 <th colspan="6">From the Source File</th> 141 </tr> 142 <tr> 143 <td colspan="6" align="center"><table><tr><td> 144 <pre> 145 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 146 Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 147 Rule US 1918 1919 - Oct lastSun 2:00 0 S 148 Rule US 1942 only - Feb 9 2:00 1:00 W # War 149 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 150 Rule US 1945 only - Sep 30 2:00 0 S 151 Rule US 1967 2006 - Oct lastSun 2:00 0 S 152 Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 153 Rule US 1974 only - Jan 6 2:00 1:00 D 154 Rule US 1975 only - Feb 23 2:00 1:00 D 155 Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 156 Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 157 Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 158 Rule US 2007 max - Nov Sun>=1 2:00 0 S 159 </pre> 160 </td></tr></table></td> 161 </tr> 162 <tr> 163 <th colspan="6">Reformatted a Bit</th> 164 </tr> 165 <tr> 166 <th>From</th> 167 <th>To</th> 168 <th colspan="2">On</th> 169 <th>At</th> 170 <th>Action</th> 171 </tr> 172 <tr align="center"> 173 <td rowspan="2">1918</td> 174 <td rowspan="2">1919</td> 175 <td rowspan="2">last Sunday</td> 176 <td>in March</td> 177 <td rowspan="3">02:00 local</td> 178 <td>go to daylight saving time</td> 179 </tr> 180 <tr align="center"> 181 <td>in October</td> 182 <td>return to standard time</td> 183 </tr> 184 <tr align="center"> 185 <td colspan="2">1942 only</td> 186 <td colspan="2">February 9<small><sup>th</sup></small></td> 187 <td>go to “war time”</td> 188 </tr> 189 <tr align="center"> 190 <td colspan="2" rowspan="2">1945 only</td> 191 <td colspan="2">August 14<small><sup>th</sup></small></td> 192 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 193 <td> 194 rename “war time” to “peace<br>time;” 195 clocks don’t change 196 </td> 197 </tr> 198 <tr align="center"> 199 <td colspan="2">September 30<small><sup>th</sup></small></td> 200 <td rowspan="9">02:00 local</td> 201 <td rowspan="2">return to standard time</td> 202 </tr> 203 <tr align="center"> 204 <td rowspan="2">1967</td> 205 <td>2006</td> 206 <td rowspan="2">last Sunday</td> 207 <td>in October</td> 208 </tr> 209 <tr align="center"> 210 <td>1973</td> 211 <td>in April</td> 212 <td rowspan="6">go to daylight saving time</td> 213 </tr> 214 <tr align="center"> 215 <td colspan="2">1974 only</td> 216 <td colspan="2">January 6<small><sup>th</sup></small></td> 217 </tr> 218 <tr align="center"> 219 <td colspan="2">1975 only</td> 220 <td colspan="2">February 23<small><sup>rd</sup></small></td> 221 </tr> 222 <tr align="center"> 223 <td>1976</td> 224 <td>1986</td> 225 <td>last Sunday</td> 226 <td rowspan="2">in April</td> 227 </tr> 228 <tr align="center"> 229 <td>1987</td> 230 <td>2006</td> 231 <td>first Sunday</td> 232 </tr> 233 <tr align="center"> 234 <td rowspan="2">2007</td> 235 <td rowspan="2">present</td> 236 <td colspan="2">second Sunday in March</td> 237 </tr> 238 <tr align="center"> 239 <td colspan="2">first Sunday in November</td> 240 <td>return to standard time</td> 241 </tr> 242 </table> 243 244 <p>There are two interesting things to note here.</p> 245 246 <p>First, the time that something happens (in the <code>AT</code> 247 column) is not necessarily the local wall clock time. The time can be 248 suffixed with ‘s’ (for “standard”) to mean 249 local standard time (different from wall clock time when observing 250 daylight saving time); or it can be suffixed with ‘g’, 251 ‘u’, or ‘z’, all three of which mean the 252 standard time at the 253 <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 254 ‘g’ stands for “<a 255 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 256 ‘u’ stands for “<a 257 href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 258 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 259 (whichever was official at the time); ‘z’ stands for the 260 <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 261 Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 262 The time can also be suffixed with ‘w’ meaning “wall 263 clock time;” but it usually isn’t because that’s the 264 default.</p> 265 266 <p>Second, the day in the <code>ON</code> column, in addition to 267 “<code>lastSun</code>” or a particular day of the month, 268 can have the form, “<code>Sun>=</code><i>x</i>” or 269 “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 270 of the month. For example, “<code>Sun>=8</code>” means 271 “the first Sunday on or after the eighth of the month,” in 272 other words, the second Sunday of the month. Furthermore, although 273 there are no examples above, the weekday needn’t be 274 “<code>Sun</code>” in either form, but can be the usual 275 three-character English abbreviation for any day of the week.</p> 276 277 <p>And the US rules give us more examples of a couple of things 278 already mentioned:</p> 279 280 <ul> 281 <li>The rules for changing to and from daylight saving time are 282 actually <i>different sets</i> of rules; and the two sets can change 283 independently. Consider, for example, that the rule for the return to 284 standard time stayed the same from 1967 to 2006; but the rule for the 285 transition to daylight saving time changed several times in the same 286 period. There can also be periods, 1946 to 1966 for example, when no 287 rule from this group is in effect, and so either no transition 288 happened in those years, or some other rule is in effect (perhaps a 289 state or other more local rule).</li> 290 291 <li>The <code>SAVE</code> and <code>LETTER</code> columns 292 contain <i>steady state</i>, not transitions. Consider, for example, 293 the transition from “war time” to “peace time” 294 that happened on August 14, 1945. The “1:00” in 295 the <code>SAVE</code> column is <i>not</i> an instruction to advance 296 the clock an hour. It means that clocks should <i>be</i> one hour 297 ahead of standard time, which they already are because of the previous 298 rule, so there should be no change.</li> 299 300 </ul> 301 302 <p>OK, now let’s look at a Zone record:</p> 303 304 <table border="1"> 305 <tr> 306 <th colspan="5">From the Source File</th> 307 </tr> 308 <tr> 309 <td colspan="6" align="center"><table><tr><td> 310 <pre> 311 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 312 Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 313 -6:00 US C%sT 1920 314 -6:00 Chicago C%sT 1936 Mar 1 2:00 315 -5:00 - EST 1936 Nov 15 2:00 316 -6:00 Chicago C%sT 1942 317 -6:00 US C%sT 1946 318 -6:00 Chicago C%sT 1967 319 -6:00 US C%sT 320 </pre> 321 </td></tr></table></td> 322 </tr> 323 <tr> 324 <th colspan="5">Columns Renamed</th> 325 </tr> 326 <tr> 327 <th rowspan="2">Standard Offset<br> 328 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 329 Meridian</a></th> 330 <th rowspan="2">Daylight<br>Saving Time</th> 331 <th rowspan="2">Abbreviation(s)</th> 332 <th colspan="2">Ending at Local Time</th> 333 </tr> 334 <tr> 335 <th>Date</th> 336 <th>Time</th> 337 </tr> 338 <tr align="center"> 339 <td>−5:50:36</td> 340 <td>not observed</td> 341 <td>LMT</td> 342 <td>1883-11-18</td> 343 <td>12:09:24</td> 344 </tr> 345 <tr align="center"> 346 <td rowspan="2">−6:00:00</td> 347 <td>US rules</td> 348 <td rowspan="2">CST or CDT</td> 349 <td>1920-01-01</td> 350 <td>00:00:00</td> 351 </tr> 352 <tr align="center"> 353 <td>Chicago rules</td> 354 <td>1936-03-01</td> 355 <td rowspan="2">02:00:00</td> 356 </tr> 357 <tr align="center"> 358 <td>−5:00:00</td> 359 <td>not observed</td> 360 <td>EST</td> 361 <td>1936-11-15</td> 362 </tr> 363 <tr align="center"> 364 <td rowspan="4">−6:00:00</td> 365 <td>Chicago rules</td> 366 <td>CST or CDT</td> 367 <td>1942-01-01</td> 368 <td rowspan="3">00:00:00</td> 369 </tr> 370 <tr align="center"> 371 <td>US rules</td> 372 <td>CST, CWT or CPT</td> 373 <td>1946-01-01</td> 374 </tr> 375 <tr align="center"> 376 <td>Chicago rules</td> 377 <td rowspan="2">CST or CDT</td> 378 <td>1967-01-01</td> 379 </tr> 380 <tr align="center"> 381 <td>US rules</td> 382 <td colspan="2">—</td> 383 </tr> 384 </table> 385 386 <p>There are a couple of interesting differences between Zones and Rules.</p> 387 388 <p>First, and somewhat trivially, whereas Rules are considered to 389 contain one or more records, a Zone is considered to be a single 390 record with zero or more <i>continuation lines</i>. Thus, the keyword, 391 “<code>Zone</code>,” and the zone name are not 392 repeated. The last line is the one without anything in 393 the <code>[UNTIL]</code> column.</p> 394 395 <p>Second, and more fundamentally, each line of a Zone represents a 396 steady state, not a transition between states. The state exists from 397 the date and time in the previous line’s <code>[UNTIL]</code> 398 column up to the date and time in the current 399 line’s <code>[UNTIL]</code> column. In other words, the date and 400 time in the <code>[UNTIL]</code> column is the instant that separates 401 this state from the next. Where that would be ambiguous because 402 we’re setting our clocks back, the <code>[UNTIL]</code> column 403 specifies the first occurrence of the instant. The state specified by 404 the last line, the one without anything in the <code>[UNTIL]</code> 405 column, continues to the present.</p> 406 407 <p>The first line typically specifies the mean solar time observed 408 before the introduction of standard time. Since there’s no line before 409 that, it has no beginning. <code>8-) </code> For some places near the <a 410 href="https://en.wikipedia.org/wiki/International_Date_Line">International 411 Date Line</a>, the first <i>two</i> lines will show solar times 412 differing by 24 hours; this corresponds to a movement of the Date 413 Line. For example:</p> 414 415 <pre> 416 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 417 Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 418 -8:57:41 - LMT ... 419 </pre> 420 421 <p>When Alaska was purchased from Russia in 1867, the Date Line moved 422 from the Alaska/Canada border to the Bering Strait; and the time in 423 Alaska was then 24 hours earlier than it had 424 been. <code><aside></code>(6 October in the Julian calendar, 425 which Russia was still using then for religious reasons, was followed 426 by <i>a second instance of the same day with a different name</i>, 18 427 October in the Gregorian calendar. Isn’t civil time 428 wonderful? <code>8-)</code>)<code></aside></code></p> 429 430 <p>The abbreviation, “LMT” stands for “local mean 431 time”, which is an invention of 432 the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 433 database</a> and was probably never actually used during the 434 period. Furthermore, the value is almost certainly wrong except in the 435 archetypal place after which the zone is named. (The tz database 436 usually doesn’t provide a separate Zone record for places where 437 nothing significant happened after 1970.)</p> 438 439 <p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 440 <ul> 441 <li>A hyphen, a kind of null value, means that we have not set our 442 clocks ahead of standard time.</li> 443 444 <li>An amount of time (usually but not necessarily “1:00” 445 meaning one hour) means that we have set our clocks ahead by that 446 amount.</li> 447 448 <li>Some alphabetic string means that we <i>might have</i> set our 449 clocks ahead; and we need to check the rule the name of which is the 450 given alphabetic string.</li> 451 </ul> 452 453 <p>An example of a specific amount of time is:</p> 454 <pre> 455 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 456 Zone Pacific/Honolulu ... 1933 Apr 30 2:00 457 -10:30 1:00 HDT 1933 May 21 2:00 458 ... 459 </pre> 460 461 <p>Hawaii tried daylight saving time for three weeks in 1933 and 462 decided they didn’t like it. <code>8-) </code>Note that 463 the <code>GMTOFF</code> column always contains the standard time 464 offset, so the wall clock time during this period was GMT − 465 10:30 + 1:00 = GMT − 9:30.</p> 466 467 <p>The <code>FORMAT</code> column specifies the usual abbreviation of 468 the time zone name. It can have one of three forms:</p> 469 <ul> 470 471 <li>a string of three or more characters that are either ASCII alphanumerics, 472 “<code>+</code>”, or “<code>-</code>”, 473 in which case that’s the abbreviation</li> 474 475 <li>a pair of strings separated by a slash 476 (‘<code>/</code>’), in which case the first string is the 477 abbreviation for the standard time name and the second string is the 478 abbreviation for the daylight saving time name</li> 479 480 <li>a string containing “<code>%s</code>,” in which case 481 the “<code>%s</code>” will be replaced by the text in the 482 appropriate Rule’s <code>LETTER</code> column</li> 483 </ul> 484 485 <p>The last two make sense only if there’s a named rule in effect.</p> 486 487 <p>An example of a slash is:</p> 488 <pre> 489 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 490 Zone Europe/London ... 1996 491 0:00 EU GMT/BST 492 </pre> 493 494 <p>The current time in the UK is called either Greenwich mean time or 495 British summer time.</p> 496 497 <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 498 happens when switching to a named rule. To what values should 499 the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 500 501 <ul> 502 <li>If at least one transition has happened, use 503 the <code>SAVE</code> and <code>LETTER</code> data from the most 504 recent.</li> 505 506 <li>If switching to a named rule before any transition has happened, 507 assume standard time (<code>SAVE</code> zero), and use 508 the <code>LETTER</code> data from the earliest transition with 509 a <code>SAVE</code> of zero. 510 511 </ul> 512 513 <p>And three last things about the <code>FORMAT</code> column:</p> 514 <ul> 515 516 <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 517 database</a> gives abbreviations for time zone names in <i>popular 518 usage</i>, which is not necessarily “correct” by law. For 519 example, the last line in 520 <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 521 “HST” for “Hawaii standard time” even though the 522 <a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a> 523 name for that time zone is “Hawaii-Aleutian standard time.” 524 This author has read that there are also some places in Australia where 525 popular time zone names differ from the legal ones. 526 527 <li>No attempt is made to <a 528 href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 529 the abbreviations. They are intended to be the values returned through the 530 <code>"%Z"</code> format specifier to 531 <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 532 <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 533 function in the 534 <a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 535 536 <li>If there is no generally-accepted abbreviation for a time zone, 537 a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 538 ahead of Greenwich. By convention, <code>-00</code> is used in a 539 zone while uninhabited, where the offset is zero but in some sense 540 the true offset is undefined. 541 </ul> 542 543 <p>As a final example, here’s the complete history for Hawaii:</p> 544 545 <table border="1"> 546 <tr> 547 <th colspan="6">Relevant Excerpts from the US Rules</th> 548 </tr> 549 <tr> 550 <td colspan="6" align="center"><table><tr><td> 551 <pre> 552 #Rule NAME FROM TO TYPE 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 30 2:00 0 S 557 </pre> 558 </td></tr></table></td> 559 </tr> 560 <tr> 561 <th colspan="6">The Zone Record</th> 562 </tr> 563 <tr> 564 <td colspan="6" align="center"><table><tr><td> 565 <pre> 566 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 567 Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 568 -10:30 - HST 1933 Apr 30 2:00 569 -10:30 1:00 HDT 1933 May 21 2:00 570 -10:30 US H%sT 1947 Jun 8 2:00 571 -10:00 - HST 572 </pre> 573 </td></tr></table></td> 574 </tr> 575 <tr> 576 <th colspan="6">What We Infer</th> 577 </tr> 578 <tr> 579 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 580 <th rowspan="2">Adjust<br>Clocks</th> 581 <th colspan="2">Time Zone</th> 582 <th colspan="2">Ending at Local Time</th> 583 </tr> 584 <tr> 585 <th>Abbrv.</th> 586 <th>Name</th> 587 <th>Date</th> 588 <th>Time</th> 589 </tr> 590 <tr align="center"> 591 <td>−10:31:26</td> 592 <td>—</td> 593 <td>LMT</td> 594 <td>local mean time</td> 595 <td>1900-01-01</td> 596 <td>12:00</td> 597 </tr> 598 <tr align="center"> 599 <td>−10:30</td> 600 <td>+0:01:26</td> 601 <td>HST</td> 602 <td>Hawaii standard time</td> 603 <td>1933-04-30</td> 604 <td rowspan="3">02:00</td> 605 </tr> 606 <tr align="center"> 607 <td>−9:30</td> 608 <td>+1:00</td> 609 <td>HDT</td> 610 <td>Hawaii daylight time</td> 611 <td>1933-05-21</td> 612 </tr> 613 <tr align="center"> 614 <td>−10:30¹</td> 615 <td>−1:00¹</td> 616 <td>HST¹</td> 617 <td>Hawaii standard time</td> 618 <td>1942-02-09</td> 619 </tr> 620 <tr align="center"> 621 <td rowspan="2">−9:30</td> 622 <td>+1:00</td> 623 <td>HWT</td> 624 <td>Hawaii war time</td> 625 <td>1945-08-14</td> 626 <td>13:30²</td> 627 </tr> 628 <tr align="center"> 629 <td>0</td> 630 <td>HPT</td> 631 <td>Hawaii peace time</td> 632 <td>1945-09-30</td> 633 <td rowspan="2">02:00</td> 634 </tr> 635 <tr align="center"> 636 <td>−10:30</td> 637 <td>−1:00</td> 638 <td rowspan="2">HST</td> 639 <td rowspan="2">Hawaii standard time</td> 640 <td>1947-06-08</td> 641 </tr> 642 <tr align="center"> 643 <td>−10:00³</td> 644 <td>+0:30³</td> 645 <td colspan="2">—</td> 646 </tr> 647 <tr> 648 <td colspan="6"> 649 ¹Switching to US rules…most recent transition (in 1919) was to standard time 650 </td> 651 </tr> 652 <tr> 653 <td colspan="6"> 654 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 655 + (−9:30) = 13:30 local 656 </td> 657 </tr> 658 <tr> 659 <td colspan="6"> 660 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 661 the civil time in Hawaii has been 662 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 663 − 10:00 year-round. 664 </td> 665 </tr> 666 </table> 667 668 <p>There will be a short quiz later. <code>8-)</code></p> 669 670 <hr> 671 <address> 672 This web page is in the public domain, so clarified as of 673 2015-10-20 by Bill Seymour. 674 <br> 675 All suggestions and corrections will be welcome; all flames will be amusing. 676 Mail to was at pobox dot com. 677 </address> 678 </body> 679 </html> 680
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