tz-how-to.html revision 1.8
1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <title>How to Read the tz Database</title> 5 <meta charset="UTF-8"> 6 <style> 7 pre {margin-left: 2em; white-space: pre-wrap;} 8 pre.td {margin-left: 0;} 9 td {text-align: center;} 10 table {border: 1px outset;} 11 th, td {border: 1px inset;} 12 table.rule {border: none; margin: auto;} 13 td.footnote {text-align: left;} 14 </style> 15 </head> 16 <body> 17 <h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 18 Database</a> Source Files</h2> 19 <h3>by Bill Seymour</h3> 20 <p>This page uses the <code>America/Chicago</code> and 21 <code>Pacific/Honolulu</code> zones as examples of how to infer 22 times of day from the <a href="tz-link.html">tz database</a> 23 source files. It might be helpful, but not absolutely necessary, 24 for the reader to have already downloaded the 25 latest release of the database and become familiar with the basic layout 26 of the data files. The format is explained in the “man 27 page” for the zic compiler, <code>zic.8.txt</code>, in 28 the <code>code</code> subdirectory.</p> 29 30 <p>We’ll begin by talking about the rules for changing between standard 31 and daylight saving time since we’ll need that information when we talk 32 about the zones.</p> 33 34 <p>First, let’s consider the special daylight saving time rules 35 for Chicago (from the <code>northamerica</code> file in 36 the <code>data</code> subdirectory):</p> 37 38 <table> 39 <tr> 40 <th colspan="6">From the Source File</th> 41 </tr> 42 <tr> 43 <td colspan="6"> 44 <table class="rule"> 45 <tr><td style="border:none;text-align:left"> 46 <pre class="td"> 47 #Rule NAME FROM TO - IN ON AT SAVE LETTER 48 Rule Chicago 1920 only - Jun 13 2:00 1:00 D 49 Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 50 Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 51 Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 52 Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 53 Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 54 </pre> 55 </td></tr></table></td> 56 </tr> 57 <tr> 58 <th colspan="6">Reformatted a Bit</th> 59 </tr> 60 <tr> 61 <th>From</th> 62 <th>To</th> 63 <th colspan="2">On</th> 64 <th>At</th> 65 <th>Action</th> 66 </tr> 67 <tr> 68 <td colspan="2">1920 only</td> 69 <td colspan="2">June 13<small><sup>th</sup></small></td> 70 <td rowspan="6">02:00 local</td> 71 <td>go to daylight saving time</td> 72 </tr> 73 <tr> 74 <td>1920</td> 75 <td>1921</td> 76 <td rowspan="5">last Sunday</td> 77 <td>in October</td> 78 <td>return to standard time</td> 79 </tr> 80 <tr> 81 <td colspan="2">1921 only</td> 82 <td>in March</td> 83 <td rowspan="2">go to daylight saving time</td> 84 </tr> 85 <tr> 86 <td rowspan="2">1922</td> 87 <td>1966</td> 88 <td>in April</td> 89 </tr> 90 <tr> 91 <td>1954</td> 92 <td>in September</td> 93 <td rowspan="2">return to standard time</td> 94 </tr> 95 <tr> 96 <td>1955</td> 97 <td>1966</td> 98 <td>in October</td> 99 </tr> 100 </table> 101 102 <p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the 103 first and last calendar years defining a contiguous range over which a specific 104 Rule line is to apply. The keyword <code>only</code> can be used in the 105 <code>TO</code> field to repeat the value of the <code>FROM</code> field in the 106 event that a rule should only apply to a single year. Often, the keyword 107 <code>max</code> is used to extend a rule’s application into the 108 indefinite future; it is a platform-agnostic stand-in for the largest 109 representable year. 110 111 <p>The next column, <code>-</code>, is reserved; for compatibility with earlier 112 releases, it always contains a hyphen, which acts as a kind of null value. 113 Prior to the 2020b release, it was called the <code>TYPE</code> field, though 114 it was never used in the main data. An obsolescent supplementary file used the 115 field as a proof-of-concept to allow <code>zic</code> to apply a given Rule 116 line only to certain “types” of years within the specified range as 117 dictated by the output of a separate script, such as: only years which would 118 have a US presidential election, or only years which wouldn’t. 119 120 <p>The <code>SAVE</code> column contains the local (wall clock) offset from 121 local standard time. 122 This is usually either zero for standard time or one hour for daylight 123 saving time; but there’s no reason, in principle, why it can’t 124 take on other values. 125 126 <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 127 column can contain a variable 128 part of the usual abbreviation of the time zone’s name, or it can just 129 be a hyphen if there’s no variable part. For example, the abbreviation 130 used in the central time zone will be either “CST” or 131 “CDT”. The variable part is ‘S’ or ‘D’; 132 and, sure enough, that’s just what we find in 133 the <code>LETTER</code> column 134 in the <code>Chicago</code> rules. More about this when we talk about 135 “Zone” lines. 136 137 <p>One important thing to notice is that “Rule” lines 138 want at once to be both <i>transitions</i> and <i>steady states</i>: 139 <ul> 140 <li>On the one hand, they represent transitions between standard and 141 daylight saving time; and any number of Rule lines can be in effect 142 during a given period (which will always be a non-empty set of 143 contiguous calendar years).</li> 144 <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 145 columns contain state that exists between transitions. More about this 146 when we talk about the US rules.</li> 147 </ul> 148 149 <p>In the example above, the transition to daylight saving time 150 happened on the 13<small><sup>th</sup></small> of June in 1920, and on 151 the last Sunday in March in 1921; but the return to standard time 152 happened on the last Sunday in October in both of those 153 years. Similarly, the rule for changing to daylight saving time was 154 the same from 1922 to 1966; but the rule for returning to standard 155 time changed in 1955. Got it?</p> 156 157 <p>OK, now for the somewhat more interesting “US” rules:</p> 158 159 <table> 160 <tr> 161 <th colspan="6">From the Source File</th> 162 </tr> 163 <tr> 164 <td colspan="6"> 165 <table class="rule"> 166 <tr><td style="border:none;text-align:left"> 167 <pre class="td"> 168 #Rule NAME FROM TO - IN ON AT SAVE LETTER/S 169 Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 170 Rule US 1918 1919 - Oct lastSun 2:00 0 S 171 Rule US 1942 only - Feb 9 2:00 1:00 W # War 172 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 173 Rule US 1945 only - Sep 30 2:00 0 S 174 Rule US 1967 2006 - Oct lastSun 2:00 0 S 175 Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 176 Rule US 1974 only - Jan 6 2:00 1:00 D 177 Rule US 1975 only - Feb 23 2:00 1:00 D 178 Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 179 Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 180 Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 181 Rule US 2007 max - Nov Sun>=1 2:00 0 S 182 </pre> 183 </td></tr></table></td> 184 </tr> 185 <tr> 186 <th colspan="6">Reformatted a Bit</th> 187 </tr> 188 <tr> 189 <th>From</th> 190 <th>To</th> 191 <th colspan="2">On</th> 192 <th>At</th> 193 <th>Action</th> 194 </tr> 195 <tr> 196 <td rowspan="2">1918</td> 197 <td rowspan="2">1919</td> 198 <td rowspan="2">last Sunday</td> 199 <td>in March</td> 200 <td rowspan="3">02:00 local</td> 201 <td>go to daylight saving time</td> 202 </tr> 203 <tr> 204 <td>in October</td> 205 <td>return to standard time</td> 206 </tr> 207 <tr> 208 <td colspan="2">1942 only</td> 209 <td colspan="2">February 9<small><sup>th</sup></small></td> 210 <td>go to “war time”</td> 211 </tr> 212 <tr> 213 <td colspan="2" rowspan="2">1945 only</td> 214 <td colspan="2">August 14<small><sup>th</sup></small></td> 215 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 216 <td> 217 rename “war time” to “peace<br>time;” 218 clocks don’t change 219 </td> 220 </tr> 221 <tr> 222 <td colspan="2">September 30<small><sup>th</sup></small></td> 223 <td rowspan="9">02:00 local</td> 224 <td rowspan="2">return to standard time</td> 225 </tr> 226 <tr> 227 <td rowspan="2">1967</td> 228 <td>2006</td> 229 <td rowspan="2">last Sunday</td> 230 <td>in October</td> 231 </tr> 232 <tr> 233 <td>1973</td> 234 <td>in April</td> 235 <td rowspan="6">go to daylight saving time</td> 236 </tr> 237 <tr> 238 <td colspan="2">1974 only</td> 239 <td colspan="2">January 6<small><sup>th</sup></small></td> 240 </tr> 241 <tr> 242 <td colspan="2">1975 only</td> 243 <td colspan="2">February 23<small><sup>rd</sup></small></td> 244 </tr> 245 <tr> 246 <td>1976</td> 247 <td>1986</td> 248 <td>last Sunday</td> 249 <td rowspan="2">in April</td> 250 </tr> 251 <tr> 252 <td>1987</td> 253 <td>2006</td> 254 <td>first Sunday</td> 255 </tr> 256 <tr> 257 <td rowspan="2">2007</td> 258 <td rowspan="2">present</td> 259 <td colspan="2">second Sunday in March</td> 260 </tr> 261 <tr> 262 <td colspan="2">first Sunday in November</td> 263 <td>return to standard time</td> 264 </tr> 265 </table> 266 267 <p>There are two interesting things to note here.</p> 268 269 <p>First, the time that something happens (in the <code>AT</code> 270 column) is not necessarily the local (wall clock) time. The time can be 271 suffixed with ‘s’ (for “standard”) to mean 272 local standard time, different from local (wall clock) time when observing 273 daylight saving time; or it can be suffixed with ‘g’, 274 ‘u’, or ‘z’, all three of which mean the 275 standard time at the 276 <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 277 ‘g’ stands for “<a 278 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 279 ‘u’ stands for “<a 280 href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 281 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 282 (whichever was official at the time); ‘z’ stands for the 283 <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 284 Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 285 The time can also be suffixed with ‘w’ meaning local (wall 286 clock) time; but it usually isn’t because that’s the 287 default.</p> 288 289 <p>Second, the day in the <code>ON</code> column, in addition to 290 “<code>lastSun</code>” or a particular day of the month, 291 can have the form, “<code>Sun>=</code><i>x</i>” or 292 “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 293 of the month. For example, “<code>Sun>=8</code>” means 294 “the first Sunday on or after the eighth of the month,” in 295 other words, the second Sunday of the month. Furthermore, although 296 there are no examples above, the weekday needn’t be 297 “<code>Sun</code>” in either form, but can be the usual 298 three-character English abbreviation for any day of the week.</p> 299 300 <p>And the US rules give us more examples of a couple of things 301 already mentioned:</p> 302 303 <ul> 304 <li>The rules for changing to and from daylight saving time are 305 actually <i>different sets</i> of rules; and the two sets can change 306 independently. Consider, for example, that the rule for the return to 307 standard time stayed the same from 1967 to 2006; but the rule for the 308 transition to daylight saving time changed several times in the same 309 period. There can also be periods, 1946 to 1966 for example, when no 310 rule from this group is in effect, and so either no transition 311 happened in those years, or some other rule is in effect (perhaps a 312 state or other more local rule).</li> 313 314 <li>The <code>SAVE</code> and <code>LETTER</code> columns 315 contain <i>steady state</i>, not transitions. Consider, for example, 316 the transition from “war time” to “peace time” 317 that happened on August 14, 1945. The “1:00” in 318 the <code>SAVE</code> column is <i>not</i> an instruction to advance 319 the clock an hour. It means that clocks should <i>be</i> one hour 320 ahead of standard time, which they already are because of the previous 321 rule, so there should be no change.</li> 322 323 </ul> 324 325 <p>OK, now let’s look at a Zone record:</p> 326 327 <table> 328 <tr> 329 <th colspan="5">From the Source File</th> 330 </tr> 331 <tr> 332 <td colspan="5"> 333 <table class="rule"> 334 <tr><td style="border:none;text-align:left"> 335 <pre class="td"> 336 #Zone NAME STDOFF RULES FORMAT [UNTIL] 337 Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 338 -6:00 US C%sT 1920 339 -6:00 Chicago C%sT 1936 Mar 1 2:00 340 -5:00 - EST 1936 Nov 15 2:00 341 -6:00 Chicago C%sT 1942 342 -6:00 US C%sT 1946 343 -6:00 Chicago C%sT 1967 344 -6:00 US C%sT 345 </pre> 346 </td></tr></table></td> 347 </tr> 348 <tr> 349 <th colspan="5">Columns Renamed</th> 350 </tr> 351 <tr> 352 <th rowspan="2">Standard Offset<br> 353 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 354 Meridian</a></th> 355 <th rowspan="2">Daylight<br>Saving Time</th> 356 <th rowspan="2">Abbreviation(s)</th> 357 <th colspan="2">Ending at Local Time</th> 358 </tr> 359 <tr> 360 <th>Date</th> 361 <th>Time</th> 362 </tr> 363 <tr> 364 <td>−5:50:36</td> 365 <td>not observed</td> 366 <td>LMT</td> 367 <td>1883-11-18</td> 368 <td>12:09:24</td> 369 </tr> 370 <tr> 371 <td rowspan="2">−6:00:00</td> 372 <td>US rules</td> 373 <td rowspan="2">CST or CDT</td> 374 <td>1920-01-01</td> 375 <td>00:00:00</td> 376 </tr> 377 <tr> 378 <td>Chicago rules</td> 379 <td>1936-03-01</td> 380 <td rowspan="2">02:00:00</td> 381 </tr> 382 <tr> 383 <td>−5:00:00</td> 384 <td>not observed</td> 385 <td>EST</td> 386 <td>1936-11-15</td> 387 </tr> 388 <tr> 389 <td rowspan="4">−6:00:00</td> 390 <td>Chicago rules</td> 391 <td>CST or CDT</td> 392 <td>1942-01-01</td> 393 <td rowspan="3">00:00:00</td> 394 </tr> 395 <tr> 396 <td>US rules</td> 397 <td>CST, CWT or CPT</td> 398 <td>1946-01-01</td> 399 </tr> 400 <tr> 401 <td>Chicago rules</td> 402 <td rowspan="2">CST or CDT</td> 403 <td>1967-01-01</td> 404 </tr> 405 <tr> 406 <td>US rules</td> 407 <td colspan="2">—</td> 408 </tr> 409 </table> 410 411 <p>There are a couple of interesting differences between Zones and Rules.</p> 412 413 <p>First, and somewhat trivially, whereas Rules are considered to 414 contain one or more records, a Zone is considered to be a single 415 record with zero or more <i>continuation lines</i>. Thus, the keyword, 416 “<code>Zone</code>,” and the zone name are not 417 repeated. The last line is the one without anything in 418 the <code>[UNTIL]</code> column.</p> 419 420 <p>Second, and more fundamentally, each line of a Zone represents a 421 steady state, not a transition between states. The state exists from 422 the date and time in the previous line’s <code>[UNTIL]</code> 423 column up to the date and time in the current 424 line’s <code>[UNTIL]</code> column. In other words, the date and 425 time in the <code>[UNTIL]</code> column is the instant that separates 426 this state from the next. Where that would be ambiguous because 427 we’re setting our clocks back, the <code>[UNTIL]</code> column 428 specifies the first occurrence of the instant. The state specified by 429 the last line, the one without anything in the <code>[UNTIL]</code> 430 column, continues to the present.</p> 431 432 <p>The first line typically specifies the mean solar time observed 433 before the introduction of standard time. Since there’s no line before 434 that, it has no beginning. <code>8-) </code> For some places near the <a 435 href="https://en.wikipedia.org/wiki/International_Date_Line">International 436 Date Line</a>, the first <i>two</i> lines will show solar times 437 differing by 24 hours; this corresponds to a movement of the Date 438 Line. For example:</p> 439 440 <pre> 441 #Zone NAME STDOFF RULES FORMAT [UNTIL] 442 Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 443 -8:57:41 - LMT ... 444 </pre> 445 446 <p>When Alaska was purchased from Russia in 1867, the Date Line moved 447 from the Alaska/Canada border to the Bering Strait; and the time in 448 Alaska was then 24 hours earlier than it had 449 been. <code><aside></code>(6 October in the Julian calendar, 450 which Russia was still using then for religious reasons, was followed 451 by <i>a second instance of the same day with a different name</i>, 18 452 October in the Gregorian calendar. Isn’t civil time 453 wonderful? <code>8-)</code>)<code></aside></code></p> 454 455 <p>The abbreviation, “LMT” stands for “local mean 456 time”, which is an invention of 457 the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 458 database</a> and was probably never actually used during the 459 period. Furthermore, the value is almost certainly wrong except in the 460 archetypal place after which the zone is named. (The tz database 461 usually doesn’t provide a separate Zone record for places where 462 nothing significant happened after 1970.)</p> 463 464 <p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 465 <ul> 466 <li>A hyphen, a kind of null value, means that we have not set our 467 clocks ahead of standard time.</li> 468 469 <li>An amount of time (usually but not necessarily “1:00” 470 meaning one hour) means that we have set our clocks ahead by that 471 amount.</li> 472 473 <li>Some alphabetic string means that we <i>might have</i> set our 474 clocks ahead; and we need to check the rule the name of which is the 475 given alphabetic string.</li> 476 </ul> 477 478 <p>An example of a specific amount of time is:</p> 479 <pre> 480 #Zone NAME STDOFF RULES FORMAT [UNTIL] 481 Zone Pacific/Honolulu ... 1933 Apr 30 2:00 482 -10:30 1:00 HDT 1933 May 21 12:00 483 ... 484 </pre> 485 486 <p>Hawaii tried daylight saving time for three weeks in 1933 and 487 decided they didn’t like it. <code>8-) </code>Note that 488 the <code>STDOFF</code> column always contains the standard time 489 offset, so the local (wall clock) time during this period was GMT − 490 10:30 + 1:00 = GMT − 9:30.</p> 491 492 <p>The <code>FORMAT</code> column specifies the usual abbreviation of 493 the time zone name. It can have one of three forms:</p> 494 <ul> 495 496 <li>a string of three or more characters that are either ASCII alphanumerics, 497 “<code>+</code>”, or “<code>-</code>”, 498 in which case that’s the abbreviation</li> 499 500 <li>a pair of strings separated by a slash 501 (‘<code>/</code>’), in which case the first string is the 502 abbreviation for the standard time name and the second string is the 503 abbreviation for the daylight saving time name</li> 504 505 <li>a string containing “<code>%s</code>,” in which case 506 the “<code>%s</code>” will be replaced by the text in the 507 appropriate Rule’s <code>LETTER</code> column</li> 508 </ul> 509 510 <p>The last two make sense only if there’s a named rule in effect.</p> 511 512 <p>An example of a slash is:</p> 513 <pre> 514 #Zone NAME STDOFF RULES FORMAT [UNTIL] 515 Zone Europe/London ... 1996 516 0:00 EU GMT/BST 517 </pre> 518 519 <p>The current time in the UK is called either Greenwich mean time or 520 British summer time.</p> 521 522 <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 523 happens when switching to a named rule. To what values should 524 the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 525 526 <ul> 527 <li>If at least one transition has happened, use 528 the <code>SAVE</code> and <code>LETTER</code> data from the most 529 recent.</li> 530 531 <li>If switching to a named rule before any transition has happened, 532 assume standard time (<code>SAVE</code> zero), and use 533 the <code>LETTER</code> data from the earliest transition with 534 a <code>SAVE</code> of zero. 535 536 </ul> 537 538 <p>And three last things about the <code>FORMAT</code> column:</p> 539 <ul> 540 541 <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 542 database</a> gives abbreviations for time zones in <i>popular 543 usage</i>, which is not necessarily “correct” by law. For 544 example, the last line in 545 <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 546 “HST” for “Hawaii standard time” even though the 547 <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 548 name for that time zone is “Hawaii-Aleutian standard time.” 549 This author has read that there are also some places in Australia where 550 popular time zone names differ from the legal ones. 551 552 <li>No attempt is made to <a 553 href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 554 the abbreviations. They are intended to be the values returned through the 555 <code>"%Z"</code> format specifier to 556 <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 557 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 558 function in the 559 <a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 560 561 <li>If there is no generally-accepted abbreviation for a time zone, 562 a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 563 ahead of Greenwich. By convention, <code>-00</code> is used in a 564 zone while uninhabited, where the offset is zero but in some sense 565 the true offset is undefined. 566 </ul> 567 568 <p>As a final example, here’s the complete history for Hawaii:</p> 569 570 <table> 571 <tr> 572 <th colspan="6">Relevant Excerpts from the US Rules</th> 573 </tr> 574 <tr> 575 <td colspan="6"> 576 <table class="rule"> 577 <tr><td style="border:none;text-align:left"> 578 <pre class="td"> 579 #Rule NAME FROM TO - IN ON AT SAVE LETTER/S 580 Rule US 1918 1919 - Oct lastSun 2:00 0 S 581 Rule US 1942 only - Feb 9 2:00 1:00 W # War 582 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 583 Rule US 1945 only - Sep lastSun 2:00 0 S 584 </pre> 585 </td></tr></table></td> 586 </tr> 587 <tr> 588 <th colspan="6">The Zone Record</th> 589 </tr> 590 <tr> 591 <td colspan="6"> 592 <table class="rule"> 593 <tr><td style="border:none;text-align:left"> 594 <pre class="td"> 595 #Zone NAME STDOFF RULES FORMAT [UNTIL] 596 Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 597 -10:30 - HST 1933 Apr 30 2:00 598 -10:30 1:00 HDT 1933 May 21 2:00 599 -10:30 US H%sT 1947 Jun 8 2:00 600 -10:00 - HST 601 </pre> 602 </td></tr></table></td> 603 </tr> 604 <tr> 605 <th colspan="6">What We Infer</th> 606 </tr> 607 <tr> 608 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 609 <th rowspan="2">Adjust<br>Clocks</th> 610 <th colspan="2">Time Zone</th> 611 <th colspan="2">Ending at Local Time</th> 612 </tr> 613 <tr> 614 <th>Abbrv.</th> 615 <th>Name</th> 616 <th>Date</th> 617 <th>Time</th> 618 </tr> 619 <tr> 620 <td>−10:31:26</td> 621 <td>—</td> 622 <td>LMT</td> 623 <td>local mean time</td> 624 <td>1896-01-13</td> 625 <td>12:00</td> 626 </tr> 627 <tr> 628 <td>−10:30</td> 629 <td>+0:01:26</td> 630 <td>HST</td> 631 <td>Hawaii standard time</td> 632 <td>1933-04-30</td> 633 <td>02:00</td> 634 </tr> 635 <tr> 636 <td>−9:30</td> 637 <td>+1:00</td> 638 <td>HDT</td> 639 <td>Hawaii daylight time</td> 640 <td>1933-05-21</td> 641 <td>12:00</td> 642 </tr> 643 <tr> 644 <td>−10:30¹</td> 645 <td>−1:00¹</td> 646 <td>HST¹</td> 647 <td>Hawaii standard time</td> 648 <td>1942-02-09</td> 649 <td>02:00</td> 650 </tr> 651 <tr> 652 <td rowspan="2">−9:30</td> 653 <td>+1:00</td> 654 <td>HWT</td> 655 <td>Hawaii war time</td> 656 <td>1945-08-14</td> 657 <td>13:30²</td> 658 </tr> 659 <tr> 660 <td>0</td> 661 <td>HPT</td> 662 <td>Hawaii peace time</td> 663 <td>1945-09-30</td> 664 <td rowspan="2">02:00</td> 665 </tr> 666 <tr> 667 <td>−10:30</td> 668 <td>−1:00</td> 669 <td rowspan="2">HST</td> 670 <td rowspan="2">Hawaii standard time</td> 671 <td>1947-06-08</td> 672 </tr> 673 <tr> 674 <td>−10:00³</td> 675 <td>+0:30³</td> 676 <td colspan="2">—</td> 677 </tr> 678 <tr> 679 <td colspan="6" class="footnote"> 680 ¹Switching to US rules…most recent transition (in 1919) was to standard time 681 </td> 682 </tr> 683 <tr> 684 <td colspan="6" class="footnote"> 685 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 686 + (−9:30) = 13:30 local 687 </td> 688 </tr> 689 <tr> 690 <td colspan="6" class="footnote"> 691 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 692 the civil time in Hawaii has been 693 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 694 − 10:00 year-round. 695 </td> 696 </tr> 697 </table> 698 699 <p>There will be a short quiz later. <code>8-)</code></p> 700 701 <hr> 702 <address> 703 This web page is in the public domain, so clarified as of 704 2015-10-20 by Bill Seymour. 705 <br> 706 All suggestions and corrections will be welcome; all flames will be amusing. 707 Mail to was at pobox dot com. 708 </address> 709 </body> 710 </html> 711
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