tz-how-to.html revision 1.3
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 </head> 9 <body> 10 <h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 11 Database</a> Source Files</h2> 12 <h3>by Bill Seymour</h3> 13 <p>This page uses the <code>America/Chicago</code> and 14 <code>Pacific/Honolulu</code> zones as examples of how to infer 15 times of day from the <a href="tz-link.htm">tz database</a> 16 source files. It might be helpful, but not absolutely necessary, 17 for the reader to have already downloaded the 18 latest release of the database and become familiar with the basic layout 19 of the data files. The format is explained in the “man 20 page” for the zic compiler, <code>zic.8.txt</code>, in 21 the <code>code</code> subdirectory.</p> 22 23 <p>We’ll begin by talking about the rules for changing between standard 24 and daylight saving time since we’ll need that information when we talk 25 about the zones.</p> 26 27 <p>First, let’s consider the special daylight saving time rules 28 for Chicago (from the <code>northamerica</code> file in 29 the <code>data</code> subdirectory):</p> 30 31 <table border="1"> 32 <tr> 33 <th colspan="6">From the Source File</th> 34 </tr> 35 <tr> 36 <td colspan="6" align="center"><table><tr><td> 37 <pre> 38 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 39 Rule Chicago 1920 only - Jun 13 2:00 1:00 D 40 Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 41 Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 42 Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 43 Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 44 Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 45 </pre> 46 </td></tr></table></td> 47 </tr> 48 <tr> 49 <th colspan="6">Reformatted a Bit</th> 50 </tr> 51 <tr> 52 <th>From</th> 53 <th>To</th> 54 <th colspan="2">On</th> 55 <th>At</th> 56 <th>Action</th> 57 </tr> 58 <tr align="center"> 59 <td colspan="2">1920 only</td> 60 <td colspan="2">June 13<small><sup>th</sup></small></td> 61 <td rowspan="6">02:00 local</td> 62 <td>go to daylight saving time</td> 63 </tr> 64 <tr align="center"> 65 <td>1920</td> 66 <td>1921</td> 67 <td rowspan="5">last Sunday</td> 68 <td>in October</td> 69 <td>return to standard time</td> 70 </tr> 71 <tr align="center"> 72 <td colspan="2">1921 only</td> 73 <td>in March</td> 74 <td rowspan="2">go to daylight saving time</td> 75 </tr> 76 <tr align="center"> 77 <td rowspan="2">1922</td> 78 <td>1966</td> 79 <td>in April</td> 80 </tr> 81 <tr align="center"> 82 <td>1954</td> 83 <td>in September</td> 84 <td rowspan="2">return to standard time</td> 85 </tr> 86 <tr align="center"> 87 <td>1955</td> 88 <td>1966</td> 89 <td>in October</td> 90 </tr> 91 </table> 92 93 <p>We’ll basically just ignore the <code>TYPE</code> column. 94 In the 2007j release, the most recent as of this writing, the 95 <code>TYPE</code> column never contains anything but a hyphen, 96 a kind of null value. (From the description in <code>zic.8.txt</code>, 97 this appears to be a mechanism for removing years from a set 98 in some localizable way. It’s used in the file, <code>pacificnew</code>, 99 to determine whether a given year will have a US presidential election; 100 but everything related to that use is commented out.) 101 102 <p>The <code>SAVE</code> column contains the wall clock offset from 103 local standard time. 104 This is usually either zero for standard time or one hour for daylight 105 saving time; but there’s no reason, in principle, why it can’t 106 take on other values. 107 108 <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 109 column can contain a variable 110 part of the usual abbreviation of the time zone’s name, or it can just 111 be a hyphen if there’s no variable part. For example, the abbreviation 112 used in the central time zone will be either “CST” or 113 “CDT”. The variable part is ‘S’ or ‘D’; 114 and, sure enough, that’s just what we find in 115 the <code>LETTER</code> column 116 in the <code>Chicago</code> rules. More about this when we talk about 117 “Zone” lines. 118 119 <p>One important thing to notice is that “Rule” lines 120 want at once to be both <i>transitions</i> and <i>steady states</i>: 121 <ul> 122 <li>On the one hand, they represent transitions between standard and 123 daylight saving time; and any number of Rule lines can be in effect 124 during a given period (which will always be a non-empty set of 125 contiguous calendar years).</li> 126 <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 127 columns contain state that exists between transitions. More about this 128 when we talk about the US rules.</li> 129 </ul> 130 131 <p>In the example above, the transition to daylight saving time 132 happened on the 13<small><sup>th</sup></small> of June in 1920, and on 133 the last Sunday in March in 1921; but the return to standard time 134 happened on the last Sunday in October in both of those 135 years. Similarly, the rule for changing to daylight saving time was 136 the same from 1922 to 1966; but the rule for returning to standard 137 time changed in 1955. Got it?</p> 138 139 <p>OK, now for the somewhat more interesting “US” rules:</p> 140 141 <table border="1"> 142 <tr> 143 <th colspan="6">From the Source File</th> 144 </tr> 145 <tr> 146 <td colspan="6" align="center"><table><tr><td> 147 <pre> 148 #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 149 Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 150 Rule US 1918 1919 - Oct lastSun 2:00 0 S 151 Rule US 1942 only - Feb 9 2:00 1:00 W # War 152 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 153 Rule US 1945 only - Sep 30 2:00 0 S 154 Rule US 1967 2006 - Oct lastSun 2:00 0 S 155 Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 156 Rule US 1974 only - Jan 6 2:00 1:00 D 157 Rule US 1975 only - Feb 23 2:00 1:00 D 158 Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 159 Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 160 Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 161 Rule US 2007 max - Nov Sun>=1 2:00 0 S 162 </pre> 163 </td></tr></table></td> 164 </tr> 165 <tr> 166 <th colspan="6">Reformatted a Bit</th> 167 </tr> 168 <tr> 169 <th>From</th> 170 <th>To</th> 171 <th colspan="2">On</th> 172 <th>At</th> 173 <th>Action</th> 174 </tr> 175 <tr align="center"> 176 <td rowspan="2">1918</td> 177 <td rowspan="2">1919</td> 178 <td rowspan="2">last Sunday</td> 179 <td>in March</td> 180 <td rowspan="3">02:00 local</td> 181 <td>go to daylight saving time</td> 182 </tr> 183 <tr align="center"> 184 <td>in October</td> 185 <td>return to standard time</td> 186 </tr> 187 <tr align="center"> 188 <td colspan="2">1942 only</td> 189 <td colspan="2">February 9<small><sup>th</sup></small></td> 190 <td>go to “war time”</td> 191 </tr> 192 <tr align="center"> 193 <td colspan="2" rowspan="2">1945 only</td> 194 <td colspan="2">August 14<small><sup>th</sup></small></td> 195 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 196 <td> 197 rename “war time” to “peace<br>time;” 198 clocks don’t change 199 </td> 200 </tr> 201 <tr align="center"> 202 <td colspan="2">September 30<small><sup>th</sup></small></td> 203 <td rowspan="9">02:00 local</td> 204 <td rowspan="2">return to standard time</td> 205 </tr> 206 <tr align="center"> 207 <td rowspan="2">1967</td> 208 <td>2006</td> 209 <td rowspan="2">last Sunday</td> 210 <td>in October</td> 211 </tr> 212 <tr align="center"> 213 <td>1973</td> 214 <td>in April</td> 215 <td rowspan="6">go to daylight saving time</td> 216 </tr> 217 <tr align="center"> 218 <td colspan="2">1974 only</td> 219 <td colspan="2">January 6<small><sup>th</sup></small></td> 220 </tr> 221 <tr align="center"> 222 <td colspan="2">1975 only</td> 223 <td colspan="2">February 23<small><sup>rd</sup></small></td> 224 </tr> 225 <tr align="center"> 226 <td>1976</td> 227 <td>1986</td> 228 <td>last Sunday</td> 229 <td rowspan="2">in April</td> 230 </tr> 231 <tr align="center"> 232 <td>1987</td> 233 <td>2006</td> 234 <td>first Sunday</td> 235 </tr> 236 <tr align="center"> 237 <td rowspan="2">2007</td> 238 <td rowspan="2">present</td> 239 <td colspan="2">second Sunday in March</td> 240 </tr> 241 <tr align="center"> 242 <td colspan="2">first Sunday in November</td> 243 <td>return to standard time</td> 244 </tr> 245 </table> 246 247 <p>There are two interesting things to note here.</p> 248 249 <p>First, the time that something happens (in the <code>AT</code> 250 column) is not necessarily the local wall clock time. The time can be 251 suffixed with ‘s’ (for “standard”) to mean 252 local standard time (different from wall clock time when observing 253 daylight saving time); or it can be suffixed with ‘g’, 254 ‘u’, or ‘z’, all three of which mean the 255 standard time at the 256 <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 257 ‘g’ stands for “<a 258 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 259 ‘u’ stands for “<a 260 href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 261 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 262 (whichever was official at the time); ‘z’ stands for the 263 <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 264 Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 265 The time can also be suffixed with ‘w’ meaning “wall 266 clock time;” but it usually isn’t because that’s the 267 default.</p> 268 269 <p>Second, the day in the <code>ON</code> column, in addition to 270 “<code>lastSun</code>” or a particular day of the month, 271 can have the form, “<code>Sun>=</code><i>x</i>” or 272 “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 273 of the month. For example, “<code>Sun>=8</code>” means 274 “the first Sunday on or after the eighth of the month,” in 275 other words, the second Sunday of the month. Furthermore, although 276 there are no examples above, the weekday needn’t be 277 “<code>Sun</code>” in either form, but can be the usual 278 three-character English abbreviation for any day of the week.</p> 279 280 <p>And the US rules give us more examples of a couple of things 281 already mentioned:</p> 282 283 <ul> 284 <li>The rules for changing to and from daylight saving time are 285 actually <i>different sets</i> of rules; and the two sets can change 286 independently. Consider, for example, that the rule for the return to 287 standard time stayed the same from 1967 to 2006; but the rule for the 288 transition to daylight saving time changed several times in the same 289 period. There can also be periods, 1946 to 1966 for example, when no 290 rule from this group is in effect, and so either no transition 291 happened in those years, or some other rule is in effect (perhaps a 292 state or other more local rule).</li> 293 294 <li>The <code>SAVE</code> and <code>LETTER</code> columns 295 contain <i>steady state</i>, not transitions. Consider, for example, 296 the transition from “war time” to “peace time” 297 that happened on August 14, 1945. The “1:00” in 298 the <code>SAVE</code> column is <i>not</i> an instruction to advance 299 the clock an hour. It means that clocks should <i>be</i> one hour 300 ahead of standard time, which they already are because of the previous 301 rule, so there should be no change.</li> 302 303 </ul> 304 305 <p>OK, now let’s look at a Zone record:</p> 306 307 <table border="1"> 308 <tr> 309 <th colspan="5">From the Source File</th> 310 </tr> 311 <tr> 312 <td colspan="6" align="center"><table><tr><td> 313 <pre> 314 #Zone NAME GMTOFF 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 </td></tr></table></td> 325 </tr> 326 <tr> 327 <th colspan="5">Columns Renamed</th> 328 </tr> 329 <tr> 330 <th rowspan="2">Standard Offset<br> 331 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 332 Meridian</a></th> 333 <th rowspan="2">Daylight<br>Saving Time</th> 334 <th rowspan="2">Abbreviation(s)</th> 335 <th colspan="2">Ending at Local Time</th> 336 </tr> 337 <tr> 338 <th>Date</th> 339 <th>Time</th> 340 </tr> 341 <tr align="center"> 342 <td>−5:50:36</td> 343 <td>not observed</td> 344 <td>LMT</td> 345 <td>1883-11-18</td> 346 <td>12:09:24</td> 347 </tr> 348 <tr align="center"> 349 <td rowspan="2">−6:00:00</td> 350 <td>US rules</td> 351 <td rowspan="2">CST or CDT</td> 352 <td>1920-01-01</td> 353 <td>00:00:00</td> 354 </tr> 355 <tr align="center"> 356 <td>Chicago rules</td> 357 <td>1936-03-01</td> 358 <td rowspan="2">02:00:00</td> 359 </tr> 360 <tr align="center"> 361 <td>−5:00:00</td> 362 <td>not observed</td> 363 <td>EST</td> 364 <td>1936-11-15</td> 365 </tr> 366 <tr align="center"> 367 <td rowspan="4">−6:00:00</td> 368 <td>Chicago rules</td> 369 <td>CST or CDT</td> 370 <td>1942-01-01</td> 371 <td rowspan="3">00:00:00</td> 372 </tr> 373 <tr align="center"> 374 <td>US rules</td> 375 <td>CST, CWT or CPT</td> 376 <td>1946-01-01</td> 377 </tr> 378 <tr align="center"> 379 <td>Chicago rules</td> 380 <td rowspan="2">CST or CDT</td> 381 <td>1967-01-01</td> 382 </tr> 383 <tr align="center"> 384 <td>US rules</td> 385 <td colspan="2">—</td> 386 </tr> 387 </table> 388 389 <p>There are a couple of interesting differences between Zones and Rules.</p> 390 391 <p>First, and somewhat trivially, whereas Rules are considered to 392 contain one or more records, a Zone is considered to be a single 393 record with zero or more <i>continuation lines</i>. Thus, the keyword, 394 “<code>Zone</code>,” and the zone name are not 395 repeated. The last line is the one without anything in 396 the <code>[UNTIL]</code> column.</p> 397 398 <p>Second, and more fundamentally, each line of a Zone represents a 399 steady state, not a transition between states. The state exists from 400 the date and time in the previous line’s <code>[UNTIL]</code> 401 column up to the date and time in the current 402 line’s <code>[UNTIL]</code> column. In other words, the date and 403 time in the <code>[UNTIL]</code> column is the instant that separates 404 this state from the next. Where that would be ambiguous because 405 we’re setting our clocks back, the <code>[UNTIL]</code> column 406 specifies the first occurrence of the instant. The state specified by 407 the last line, the one without anything in the <code>[UNTIL]</code> 408 column, continues to the present.</p> 409 410 <p>The first line typically specifies the mean solar time observed 411 before the introduction of standard time. Since there’s no line before 412 that, it has no beginning. <code>8-) </code> For some places near the <a 413 href="https://en.wikipedia.org/wiki/International_Date_Line">International 414 Date Line</a>, the first <i>two</i> lines will show solar times 415 differing by 24 hours; this corresponds to a movement of the Date 416 Line. For example:</p> 417 418 <pre> 419 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 420 Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 421 -8:57:41 - LMT ... 422 </pre> 423 424 <p>When Alaska was purchased from Russia in 1867, the Date Line moved 425 from the Alaska/Canada border to the Bering Strait; and the time in 426 Alaska was then 24 hours earlier than it had 427 been. <code><aside></code>(6 October in the Julian calendar, 428 which Russia was still using then for religious reasons, was followed 429 by <i>a second instance of the same day with a different name</i>, 18 430 October in the Gregorian calendar. Isn’t civil time 431 wonderful? <code>8-)</code>)<code></aside></code></p> 432 433 <p>The abbreviation, “LMT” stands for “local mean 434 time”, which is an invention of 435 the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 436 database</a> and was probably never actually used during the 437 period. Furthermore, the value is almost certainly wrong except in the 438 archetypal place after which the zone is named. (The tz database 439 usually doesn’t provide a separate Zone record for places where 440 nothing significant happened after 1970.)</p> 441 442 <p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 443 <ul> 444 <li>A hyphen, a kind of null value, means that we have not set our 445 clocks ahead of standard time.</li> 446 447 <li>An amount of time (usually but not necessarily “1:00” 448 meaning one hour) means that we have set our clocks ahead by that 449 amount.</li> 450 451 <li>Some alphabetic string means that we <i>might have</i> set our 452 clocks ahead; and we need to check the rule the name of which is the 453 given alphabetic string.</li> 454 </ul> 455 456 <p>An example of a specific amount of time is:</p> 457 <pre> 458 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 459 Zone Pacific/Honolulu ... 1933 Apr 30 2:00 460 -10:30 1:00 HDT 1933 May 21 2:00 461 ... 462 </pre> 463 464 <p>Hawaii tried daylight saving time for three weeks in 1933 and 465 decided they didn’t like it. <code>8-) </code>Note that 466 the <code>GMTOFF</code> column always contains the standard time 467 offset, so the wall clock time during this period was GMT − 468 10:30 + 1:00 = GMT − 9:30.</p> 469 470 <p>The <code>FORMAT</code> column specifies the usual abbreviation of 471 the time zone name. It can have one of three forms:</p> 472 <ul> 473 474 <li>a string of three or more characters that are either ASCII alphanumerics, 475 “<code>+</code>”, or “<code>-</code>”, 476 in which case that’s the abbreviation</li> 477 478 <li>a pair of strings separated by a slash 479 (‘<code>/</code>’), in which case the first string is the 480 abbreviation for the standard time name and the second string is the 481 abbreviation for the daylight saving time name</li> 482 483 <li>a string containing “<code>%s</code>,” in which case 484 the “<code>%s</code>” will be replaced by the text in the 485 appropriate Rule’s <code>LETTER</code> column</li> 486 </ul> 487 488 <p>The last two make sense only if there’s a named rule in effect.</p> 489 490 <p>An example of a slash is:</p> 491 <pre> 492 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 493 Zone Europe/London ... 1996 494 0:00 EU GMT/BST 495 </pre> 496 497 <p>The current time in the UK is called either Greenwich mean time or 498 British summer time.</p> 499 500 <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 501 happens when switching to a named rule. To what values should 502 the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 503 504 <ul> 505 <li>If at least one transition has happened, use 506 the <code>SAVE</code> and <code>LETTER</code> data from the most 507 recent.</li> 508 509 <li>If switching to a named rule before any transition has happened, 510 assume standard time (<code>SAVE</code> zero), and use 511 the <code>LETTER</code> data from the earliest transition with 512 a <code>SAVE</code> of zero. 513 514 </ul> 515 516 <p>And three last things about the <code>FORMAT</code> column:</p> 517 <ul> 518 519 <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 520 database</a> gives abbreviations for time zone names in <i>popular 521 usage</i>, which is not necessarily “correct” by law. For 522 example, the last line in 523 <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 524 “HST” for “Hawaii standard time” even though the 525 <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 526 name for that time zone is “Hawaii-Aleutian standard time.” 527 This author has read that there are also some places in Australia where 528 popular time zone names differ from the legal ones. 529 530 <li>No attempt is made to <a 531 href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 532 the abbreviations. They are intended to be the values returned through the 533 <code>"%Z"</code> format specifier to 534 <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 535 <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 536 function in the 537 <a href="http://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 538 539 <li>If there is no generally-accepted abbreviation for a time zone, 540 a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 541 ahead of Greenwich. By convention, <code>-00</code> is used in a 542 zone while uninhabited, where the offset is zero but in some sense 543 the true offset is undefined. 544 </ul> 545 546 <p>As a final example, here’s the complete history for Hawaii:</p> 547 548 <table border="1"> 549 <tr> 550 <th colspan="6">Relevant Excerpts from the US Rules</th> 551 </tr> 552 <tr> 553 <td colspan="6" align="center"><table><tr><td> 554 <pre> 555 #Rule NAME FROM TO TYPE 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 30 2:00 0 S 560 </pre> 561 </td></tr></table></td> 562 </tr> 563 <tr> 564 <th colspan="6">The Zone Record</th> 565 </tr> 566 <tr> 567 <td colspan="6" align="center"><table><tr><td> 568 <pre> 569 #Zone NAME GMTOFF RULES FORMAT [UNTIL] 570 Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 571 -10:30 - HST 1933 Apr 30 2:00 572 -10:30 1:00 HDT 1933 May 21 2:00 573 -10:30 US H%sT 1947 Jun 8 2:00 574 -10:00 - HST 575 </pre> 576 </td></tr></table></td> 577 </tr> 578 <tr> 579 <th colspan="6">What We Infer</th> 580 </tr> 581 <tr> 582 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 583 <th rowspan="2">Adjust<br>Clocks</th> 584 <th colspan="2">Time Zone</th> 585 <th colspan="2">Ending at Local Time</th> 586 </tr> 587 <tr> 588 <th>Abbrv.</th> 589 <th>Name</th> 590 <th>Date</th> 591 <th>Time</th> 592 </tr> 593 <tr align="center"> 594 <td>−10:31:26</td> 595 <td>—</td> 596 <td>LMT</td> 597 <td>local mean time</td> 598 <td>1900-01-01</td> 599 <td>12:00</td> 600 </tr> 601 <tr align="center"> 602 <td>−10:30</td> 603 <td>+0:01:26</td> 604 <td>HST</td> 605 <td>Hawaii standard time</td> 606 <td>1933-04-30</td> 607 <td rowspan="3">02:00</td> 608 </tr> 609 <tr align="center"> 610 <td>−9:30</td> 611 <td>+1:00</td> 612 <td>HDT</td> 613 <td>Hawaii daylight time</td> 614 <td>1933-05-21</td> 615 </tr> 616 <tr align="center"> 617 <td>−10:30¹</td> 618 <td>−1:00¹</td> 619 <td>HST¹</td> 620 <td>Hawaii standard time</td> 621 <td>1942-02-09</td> 622 </tr> 623 <tr align="center"> 624 <td rowspan="2">−9:30</td> 625 <td>+1:00</td> 626 <td>HWT</td> 627 <td>Hawaii war time</td> 628 <td>1945-08-14</td> 629 <td>13:30²</td> 630 </tr> 631 <tr align="center"> 632 <td>0</td> 633 <td>HPT</td> 634 <td>Hawaii peace time</td> 635 <td>1945-09-30</td> 636 <td rowspan="2">02:00</td> 637 </tr> 638 <tr align="center"> 639 <td>−10:30</td> 640 <td>−1:00</td> 641 <td rowspan="2">HST</td> 642 <td rowspan="2">Hawaii standard time</td> 643 <td>1947-06-08</td> 644 </tr> 645 <tr align="center"> 646 <td>−10:00³</td> 647 <td>+0:30³</td> 648 <td colspan="2">—</td> 649 </tr> 650 <tr> 651 <td colspan="6"> 652 ¹Switching to US rules…most recent transition (in 1919) was to standard time 653 </td> 654 </tr> 655 <tr> 656 <td colspan="6"> 657 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 658 + (−9:30) = 13:30 local 659 </td> 660 </tr> 661 <tr> 662 <td colspan="6"> 663 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 664 the civil time in Hawaii has been 665 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 666 − 10:00 year-round. 667 </td> 668 </tr> 669 </table> 670 671 <p>There will be a short quiz later. <code>8-)</code></p> 672 673 <hr> 674 <address> 675 This web page is in the public domain, so clarified as of 676 2015-10-20 by Bill Seymour. 677 <br> 678 All suggestions and corrections will be welcome; all flames will be amusing. 679 Mail to was at pobox dot com. 680 </address> 681 </body> 682 </html> 683
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