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 guide 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. 29 Although this guide covers many of the common cases, it is not a 30 complete summary of what zic accepts; the man page is the 31 authoritative reference.</p> 32 33 <p>We’ll begin by talking about the rules for changing between standard 34 and daylight saving time since we’ll need that information when we talk 35 about the zones.</p> 36 37 <p>First, let’s consider the special daylight saving time rules 38 for Chicago (from the <code>northamerica</code> file in 39 the <code>data</code> subdirectory):</p> 40 41 <table> 42 <tr> 43 <th colspan="6">From the Source File</th> 44 </tr> 45 <tr> 46 <td colspan="6"> 47 <table class="rule"> 48 <tr><td style="border:none;text-align:left"> 49 <pre class="td"> 50 #Rule NAME FROM TO - IN ON AT SAVE LETTER 51 Rule Chicago 1920 only - Jun 13 2:00 1:00 D 52 Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 53 Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 54 Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 55 Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 56 Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 57 </pre> 58 </td></tr></table></td> 59 </tr> 60 <tr> 61 <th colspan="6">Reformatted a Bit</th> 62 </tr> 63 <tr> 64 <th>From</th> 65 <th>To</th> 66 <th colspan="2">On</th> 67 <th>At</th> 68 <th>Action</th> 69 </tr> 70 <tr> 71 <td colspan="2">1920 only</td> 72 <td colspan="2">June 13<small><sup>th</sup></small></td> 73 <td rowspan="6">02:00 local</td> 74 <td>go to daylight saving time</td> 75 </tr> 76 <tr> 77 <td>1920</td> 78 <td>1921</td> 79 <td rowspan="5">last Sunday</td> 80 <td>in October</td> 81 <td>return to standard time</td> 82 </tr> 83 <tr> 84 <td colspan="2">1921 only</td> 85 <td>in March</td> 86 <td rowspan="2">go to daylight saving time</td> 87 </tr> 88 <tr> 89 <td rowspan="2">1922</td> 90 <td>1966</td> 91 <td>in April</td> 92 </tr> 93 <tr> 94 <td>1954</td> 95 <td>in September</td> 96 <td rowspan="2">return to standard time</td> 97 </tr> 98 <tr> 99 <td>1955</td> 100 <td>1966</td> 101 <td>in October</td> 102 </tr> 103 </table> 104 105 <p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the 106 first and last calendar years defining a contiguous range over which a specific 107 Rule line is to apply. The keyword <code>only</code> can be used in the 108 <code>TO</code> field to repeat the value of the <code>FROM</code> field in the 109 event that a rule should only apply to a single year. Often, the keyword 110 <code>max</code> is used to extend a rule’s application into the 111 indefinite future; it is a platform-agnostic stand-in for the largest 112 representable year. 113 114 <p>The next column, <code>-</code>, is reserved; for compatibility with earlier 115 releases, it always contains a hyphen, which acts as a kind of null value. 116 Prior to the 2020b release, it was called the <code>TYPE</code> field, though 117 it had not been used in the main data since the 2000e release. 118 An obsolescent supplementary file used the 119 field as a proof-of-concept to allow <code>zic</code> to apply a given Rule 120 line only to certain “types” of years within the specified range as 121 dictated by the output of a separate script, such as: only years which would 122 have a US presidential election, or only years which wouldn’t. 123 124 <p>The <code>SAVE</code> column contains the local (wall clock) offset from 125 local standard time. 126 This is usually either zero for standard time or one hour for daylight 127 saving time; but there’s no reason, in principle, why it can’t 128 take on other values. 129 130 <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 131 column can contain a variable 132 part of the usual abbreviation of the time zone’s name, or it can just 133 be a hyphen if there’s no variable part. For example, the abbreviation 134 used in the central time zone will be either “CST” or 135 “CDT”. The variable part is ‘S’ or ‘D’; 136 and, sure enough, that’s just what we find in 137 the <code>LETTER</code> column 138 in the <code>Chicago</code> rules. More about this when we talk about 139 “Zone” lines. 140 141 <p>One important thing to notice is that “Rule” lines 142 want at once to be both <i>transitions</i> and <i>steady states</i>: 143 <ul> 144 <li>On the one hand, they represent transitions between standard and 145 daylight saving time; and any number of Rule lines can be in effect 146 during a given period (which will always be a non-empty set of 147 contiguous calendar years).</li> 148 <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 149 columns contain state that exists between transitions. More about this 150 when we talk about the US rules.</li> 151 </ul> 152 153 <p>In the example above, the transition to daylight saving time 154 happened on the 13<small><sup>th</sup></small> of June in 1920, and on 155 the last Sunday in March in 1921; but the return to standard time 156 happened on the last Sunday in October in both of those 157 years. Similarly, the rule for changing to daylight saving time was 158 the same from 1922 to 1966; but the rule for returning to standard 159 time changed in 1955. Got it?</p> 160 161 <p>OK, now for the somewhat more interesting “US” rules:</p> 162 163 <table> 164 <tr> 165 <th colspan="6">From the Source File</th> 166 </tr> 167 <tr> 168 <td colspan="6"> 169 <table class="rule"> 170 <tr><td style="border:none;text-align:left"> 171 <pre class="td"> 172 #Rule NAME FROM TO - IN ON AT SAVE LETTER/S 173 Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 174 Rule US 1918 1919 - Oct lastSun 2:00 0 S 175 Rule US 1942 only - Feb 9 2:00 1:00 W # War 176 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 177 Rule US 1945 only - Sep 30 2:00 0 S 178 Rule US 1967 2006 - Oct lastSun 2:00 0 S 179 Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 180 Rule US 1974 only - Jan 6 2:00 1:00 D 181 Rule US 1975 only - Feb 23 2:00 1:00 D 182 Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 183 Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 184 Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 185 Rule US 2007 max - Nov Sun>=1 2:00 0 S 186 </pre> 187 </td></tr></table></td> 188 </tr> 189 <tr> 190 <th colspan="6">Reformatted a Bit</th> 191 </tr> 192 <tr> 193 <th>From</th> 194 <th>To</th> 195 <th colspan="2">On</th> 196 <th>At</th> 197 <th>Action</th> 198 </tr> 199 <tr> 200 <td rowspan="2">1918</td> 201 <td rowspan="2">1919</td> 202 <td rowspan="2">last Sunday</td> 203 <td>in March</td> 204 <td rowspan="3">02:00 local</td> 205 <td>go to daylight saving time</td> 206 </tr> 207 <tr> 208 <td>in October</td> 209 <td>return to standard time</td> 210 </tr> 211 <tr> 212 <td colspan="2">1942 only</td> 213 <td colspan="2">February 9<small><sup>th</sup></small></td> 214 <td>go to “war time”</td> 215 </tr> 216 <tr> 217 <td colspan="2" rowspan="2">1945 only</td> 218 <td colspan="2">August 14<small><sup>th</sup></small></td> 219 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 220 <td> 221 rename “war time” to “peace<br>time;” 222 clocks don’t change 223 </td> 224 </tr> 225 <tr> 226 <td colspan="2">September 30<small><sup>th</sup></small></td> 227 <td rowspan="9">02:00 local</td> 228 <td rowspan="2">return to standard time</td> 229 </tr> 230 <tr> 231 <td rowspan="2">1967</td> 232 <td>2006</td> 233 <td rowspan="2">last Sunday</td> 234 <td>in October</td> 235 </tr> 236 <tr> 237 <td>1973</td> 238 <td>in April</td> 239 <td rowspan="6">go to daylight saving time</td> 240 </tr> 241 <tr> 242 <td colspan="2">1974 only</td> 243 <td colspan="2">January 6<small><sup>th</sup></small></td> 244 </tr> 245 <tr> 246 <td colspan="2">1975 only</td> 247 <td colspan="2">February 23<small><sup>rd</sup></small></td> 248 </tr> 249 <tr> 250 <td>1976</td> 251 <td>1986</td> 252 <td>last Sunday</td> 253 <td rowspan="2">in April</td> 254 </tr> 255 <tr> 256 <td>1987</td> 257 <td>2006</td> 258 <td>first Sunday</td> 259 </tr> 260 <tr> 261 <td rowspan="2">2007</td> 262 <td rowspan="2">present</td> 263 <td colspan="2">second Sunday in March</td> 264 </tr> 265 <tr> 266 <td colspan="2">first Sunday in November</td> 267 <td>return to standard time</td> 268 </tr> 269 </table> 270 271 <p>There are two interesting things to note here.</p> 272 273 <p>First, the time that something happens (in the <code>AT</code> 274 column) is not necessarily the local (wall clock) time. The time can be 275 suffixed with ‘s’ (for “standard”) to mean 276 local standard time, different from local (wall clock) time when observing 277 daylight saving time; or it can be suffixed with ‘g’, 278 ‘u’, or ‘z’, all three of which mean the 279 standard time at the 280 <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 281 ‘g’ stands for “<a 282 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 283 ‘u’ stands for “<a 284 href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 285 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 286 (whichever was official at the time); ‘z’ stands for the 287 <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 288 Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 289 The time can also be suffixed with ‘w’ meaning local (wall 290 clock) time; but it usually isn’t because that’s the 291 default.</p> 292 293 <p>Second, the day in the <code>ON</code> column, in addition to 294 “<code>lastSun</code>” or a particular day of the month, 295 can have the form, “<code>Sun>=</code><i>x</i>” or 296 “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 297 of the month. For example, “<code>Sun>=8</code>” means 298 “the first Sunday on or after the eighth of the month,” in 299 other words, the second Sunday of the month. Furthermore, although 300 there are no examples above, the weekday needn’t be 301 “<code>Sun</code>” in either form, but can be the usual 302 three-character English abbreviation for any day of the week.</p> 303 304 <p>And the US rules give us more examples of a couple of things 305 already mentioned:</p> 306 307 <ul> 308 <li>The rules for changing to and from daylight saving time are 309 actually <i>different sets</i> of rules; and the two sets can change 310 independently. Consider, for example, that the rule for the return to 311 standard time stayed the same from 1967 to 2006; but the rule for the 312 transition to daylight saving time changed several times in the same 313 period. There can also be periods, 1946 to 1966 for example, when no 314 rule from this group is in effect, and so either no transition 315 happened in those years, or some other rule is in effect (perhaps a 316 state or other more local rule).</li> 317 318 <li>The <code>SAVE</code> and <code>LETTER</code> columns 319 contain <i>steady state</i>, not transitions. Consider, for example, 320 the transition from “war time” to “peace time” 321 that happened on August 14, 1945. The “1:00” in 322 the <code>SAVE</code> column is <i>not</i> an instruction to advance 323 the clock an hour. It means that clocks should <i>be</i> one hour 324 ahead of standard time, which they already are because of the previous 325 rule, so there should be no change.</li> 326 327 </ul> 328 329 <p>OK, now let’s look at a Zone record:</p> 330 331 <table> 332 <tr> 333 <th colspan="5">From the Source File</th> 334 </tr> 335 <tr> 336 <td colspan="5"> 337 <table class="rule"> 338 <tr><td style="border:none;text-align:left"> 339 <pre class="td"> 340 #Zone NAME STDOFF RULES FORMAT [UNTIL] 341 Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 342 -6:00 US C%sT 1920 343 -6:00 Chicago C%sT 1936 Mar 1 2:00 344 -5:00 - EST 1936 Nov 15 2:00 345 -6:00 Chicago C%sT 1942 346 -6:00 US C%sT 1946 347 -6:00 Chicago C%sT 1967 348 -6:00 US C%sT 349 </pre> 350 </td></tr></table></td> 351 </tr> 352 <tr> 353 <th colspan="5">Columns Renamed</th> 354 </tr> 355 <tr> 356 <th rowspan="2">Standard Offset<br> 357 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 358 Meridian</a></th> 359 <th rowspan="2">Daylight<br>Saving Time</th> 360 <th rowspan="2">Abbreviation(s)</th> 361 <th colspan="2">Ending at Local Time</th> 362 </tr> 363 <tr> 364 <th>Date</th> 365 <th>Time</th> 366 </tr> 367 <tr> 368 <td>−5:50:36</td> 369 <td>not observed</td> 370 <td>LMT</td> 371 <td>1883-11-18</td> 372 <td>12:09:24</td> 373 </tr> 374 <tr> 375 <td rowspan="2">−6:00:00</td> 376 <td>US rules</td> 377 <td rowspan="2">CST or CDT</td> 378 <td>1920-01-01</td> 379 <td>00:00:00</td> 380 </tr> 381 <tr> 382 <td>Chicago rules</td> 383 <td>1936-03-01</td> 384 <td rowspan="2">02:00:00</td> 385 </tr> 386 <tr> 387 <td>−5:00:00</td> 388 <td>not observed</td> 389 <td>EST</td> 390 <td>1936-11-15</td> 391 </tr> 392 <tr> 393 <td rowspan="4">−6:00:00</td> 394 <td>Chicago rules</td> 395 <td>CST or CDT</td> 396 <td>1942-01-01</td> 397 <td rowspan="3">00:00:00</td> 398 </tr> 399 <tr> 400 <td>US rules</td> 401 <td>CST, CWT or CPT</td> 402 <td>1946-01-01</td> 403 </tr> 404 <tr> 405 <td>Chicago rules</td> 406 <td rowspan="2">CST or CDT</td> 407 <td>1967-01-01</td> 408 </tr> 409 <tr> 410 <td>US rules</td> 411 <td colspan="2">—</td> 412 </tr> 413 </table> 414 415 <p>There are a couple of interesting differences between Zones and Rules.</p> 416 417 <p>First, and somewhat trivially, whereas Rules are considered to 418 contain one or more records, a Zone is considered to be a single 419 record with zero or more <i>continuation lines</i>. Thus, the keyword, 420 “<code>Zone</code>,” and the zone name are not 421 repeated. The last line is the one without anything in 422 the <code>[UNTIL]</code> column.</p> 423 424 <p>Second, and more fundamentally, each line of a Zone represents a 425 steady state, not a transition between states. The state exists from 426 the date and time in the previous line’s <code>[UNTIL]</code> 427 column up to the date and time in the current 428 line’s <code>[UNTIL]</code> column. In other words, the date and 429 time in the <code>[UNTIL]</code> column is the instant that separates 430 this state from the next. Where that would be ambiguous because 431 we’re setting our clocks back, the <code>[UNTIL]</code> column 432 specifies the first occurrence of the instant. The state specified by 433 the last line, the one without anything in the <code>[UNTIL]</code> 434 column, continues to the present.</p> 435 436 <p>The first line typically specifies the mean solar time observed 437 before the introduction of standard time. Since there’s no line before 438 that, it has no beginning. <code>8-) </code> For some places near the <a 439 href="https://en.wikipedia.org/wiki/International_Date_Line">International 440 Date Line</a>, the first <i>two</i> lines will show solar times 441 differing by 24 hours; this corresponds to a movement of the Date 442 Line. For example:</p> 443 444 <pre> 445 #Zone NAME STDOFF RULES FORMAT [UNTIL] 446 Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 447 -8:57:41 - LMT ... 448 </pre> 449 450 <p>When Alaska was purchased from Russia in 1867, the Date Line moved 451 from the Alaska/Canada border to the Bering Strait; and the time in 452 Alaska was then 24 hours earlier than it had 453 been. <code><aside></code>(6 October in the Julian calendar, 454 which Russia was still using then for religious reasons, was followed 455 by <i>a second instance of the same day with a different name</i>, 18 456 October in the Gregorian calendar. Isn’t civil time 457 wonderful? <code>8-)</code>)<code></aside></code></p> 458 459 <p>The abbreviation, “LMT” stands for “local mean 460 time”, which is an invention of 461 the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 462 database</a> and was probably never actually used during the 463 period. Furthermore, the value is almost certainly wrong except in the 464 archetypal place after which the zone is named. (The tz database 465 usually doesn’t provide a separate Zone record for places where 466 nothing significant happened after 1970.)</p> 467 468 <p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 469 <ul> 470 <li>A hyphen, a kind of null value, means that we have not set our 471 clocks ahead of standard time.</li> 472 473 <li>An amount of time (usually but not necessarily “1:00” 474 meaning one hour) means that we have set our clocks ahead by that 475 amount.</li> 476 477 <li>Some alphabetic string means that we <i>might have</i> set our 478 clocks ahead; and we need to check the rule the name of which is the 479 given alphabetic string.</li> 480 </ul> 481 482 <p>An example of a specific amount of time is:</p> 483 <pre> 484 #Zone NAME STDOFF RULES FORMAT [UNTIL] 485 Zone Pacific/Honolulu ... 1933 Apr 30 2:00 486 -10:30 1:00 HDT 1933 May 21 12:00 487 ... 488 </pre> 489 490 <p>Hawaii tried daylight saving time for three weeks in 1933 and 491 decided they didn’t like it. <code>8-) </code>Note that 492 the <code>STDOFF</code> column always contains the standard time 493 offset, so the local (wall clock) time during this period was GMT − 494 10:30 + 1:00 = GMT − 9:30.</p> 495 496 <p>The <code>FORMAT</code> column specifies the usual abbreviation of 497 the time zone name. It should have one of four forms:</p> 498 <ul> 499 500 <li>a time zone abbreviation that is a string of three or more 501 characters that are either ASCII alphanumerics, 502 “<code>+</code>”, or “<code>-</code>”</li> 503 504 <li>the string “%z”, in which case the 505 “<code>%z</code>” will be replaced by a numeric time zone 506 abbreviation</li> 507 508 <li>a pair of time zone abbreviations separated by a slash 509 (‘<code>/</code>’), in which case the first string is the 510 abbreviation for the standard time name and the second string is the 511 abbreviation for the daylight saving time name</li> 512 513 <li>a string containing “<code>%s</code>”, in which case 514 the “<code>%s</code>” will be replaced by the text in the 515 appropriate Rule’s <code>LETTER</code> column, and the resulting 516 string should be a time zone abbreviation</li> 517 </ul> 518 519 <p>The last two make sense only if there’s a named rule in effect.</p> 520 521 <p>An example of a slash is:</p> 522 <pre> 523 #Zone NAME STDOFF RULES FORMAT [UNTIL] 524 Zone Europe/London ... 1996 525 0:00 EU GMT/BST 526 </pre> 527 528 <p>The current time in the UK is called either Greenwich mean time or 529 British summer time.</p> 530 531 <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 532 happens when switching to a named rule. To what values should 533 the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 534 535 <ul> 536 <li>If at least one transition has happened, use 537 the <code>SAVE</code> and <code>LETTER</code> data from the most 538 recent.</li> 539 540 <li>If switching to a named rule before any transition has happened, 541 assume standard time (<code>SAVE</code> zero), and use 542 the <code>LETTER</code> data from the earliest transition with 543 a <code>SAVE</code> of zero. 544 545 </ul> 546 547 <p>And three last things about the <code>FORMAT</code> column:</p> 548 <ul> 549 550 <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 551 database</a> gives abbreviations for time zones 552 in popular English-language usage. For 553 example, the last line in 554 <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 555 “HST” for “Hawaii standard time” even though the 556 <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 557 name for that time zone is “Hawaii-Aleutian standard time.” 558 This author has read that there are also some places in Australia where 559 popular time zone names differ from the legal ones. 560 561 <li>No attempt is made to <a 562 href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 563 the abbreviations. They are intended to be the values returned through the 564 <code>"%Z"</code> format specifier to 565 <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 566 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 567 function in the 568 <a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 569 570 <li>If there is no generally accepted abbreviation for a time zone, 571 a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 572 ahead of Greenwich. By convention, <code>-00</code> is used in a 573 zone while uninhabited, where the offset is zero but in some sense 574 the true offset is undefined. 575 </ul> 576 577 <p>As a final example, here’s the complete history for Hawaii:</p> 578 579 <table> 580 <tr> 581 <th colspan="6">Relevant Excerpts from the US Rules</th> 582 </tr> 583 <tr> 584 <td colspan="6"> 585 <table class="rule"> 586 <tr><td style="border:none;text-align:left"> 587 <pre class="td"> 588 #Rule NAME FROM TO - IN ON AT SAVE LETTER/S 589 Rule US 1918 1919 - Oct lastSun 2:00 0 S 590 Rule US 1942 only - Feb 9 2:00 1:00 W # War 591 Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 592 Rule US 1945 only - Sep lastSun 2:00 0 S 593 </pre> 594 </td></tr></table></td> 595 </tr> 596 <tr> 597 <th colspan="6">The Zone Record</th> 598 </tr> 599 <tr> 600 <td colspan="6"> 601 <table class="rule"> 602 <tr><td style="border:none;text-align:left"> 603 <pre class="td"> 604 #Zone NAME STDOFF RULES FORMAT [UNTIL] 605 Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 606 -10:30 - HST 1933 Apr 30 2:00 607 -10:30 1:00 HDT 1933 May 21 2:00 608 -10:30 US H%sT 1947 Jun 8 2:00 609 -10:00 - HST 610 </pre> 611 </td></tr></table></td> 612 </tr> 613 <tr> 614 <th colspan="6">What We Infer</th> 615 </tr> 616 <tr> 617 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 618 <th rowspan="2">Adjust<br>Clocks</th> 619 <th colspan="2">Time Zone</th> 620 <th colspan="2">Ending at Local Time</th> 621 </tr> 622 <tr> 623 <th>Abbrv.</th> 624 <th>Name</th> 625 <th>Date</th> 626 <th>Time</th> 627 </tr> 628 <tr> 629 <td>−10:31:26</td> 630 <td>—</td> 631 <td>LMT</td> 632 <td>local mean time</td> 633 <td>1896-01-13</td> 634 <td>12:00</td> 635 </tr> 636 <tr> 637 <td>−10:30</td> 638 <td>+0:01:26</td> 639 <td>HST</td> 640 <td>Hawaii standard time</td> 641 <td>1933-04-30</td> 642 <td>02:00</td> 643 </tr> 644 <tr> 645 <td>−9:30</td> 646 <td>+1:00</td> 647 <td>HDT</td> 648 <td>Hawaii daylight time</td> 649 <td>1933-05-21</td> 650 <td>12:00</td> 651 </tr> 652 <tr> 653 <td>−10:30¹</td> 654 <td>−1:00¹</td> 655 <td>HST¹</td> 656 <td>Hawaii standard time</td> 657 <td>1942-02-09</td> 658 <td>02:00</td> 659 </tr> 660 <tr> 661 <td rowspan="2">−9:30</td> 662 <td>+1:00</td> 663 <td>HWT</td> 664 <td>Hawaii war time</td> 665 <td>1945-08-14</td> 666 <td>13:30²</td> 667 </tr> 668 <tr> 669 <td>0</td> 670 <td>HPT</td> 671 <td>Hawaii peace time</td> 672 <td>1945-09-30</td> 673 <td rowspan="2">02:00</td> 674 </tr> 675 <tr> 676 <td>−10:30</td> 677 <td>−1:00</td> 678 <td rowspan="2">HST</td> 679 <td rowspan="2">Hawaii standard time</td> 680 <td>1947-06-08</td> 681 </tr> 682 <tr> 683 <td>−10:00³</td> 684 <td>+0:30³</td> 685 <td colspan="2">—</td> 686 </tr> 687 <tr> 688 <td colspan="6" class="footnote"> 689 ¹Switching to US rules…most recent transition (in 1919) was to standard time 690 </td> 691 </tr> 692 <tr> 693 <td colspan="6" class="footnote"> 694 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 695 + (−9:30) = 13:30 local 696 </td> 697 </tr> 698 <tr> 699 <td colspan="6" class="footnote"> 700 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 701 the civil time in Hawaii has been 702 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 703 − 10:00 year-round. 704 </td> 705 </tr> 706 </table> 707 708 <p>There will be a short quiz later. <code>8-)</code></p> 709 710 <hr> 711 <address> 712 This web page is in the public domain, so clarified as of 713 2015-10-20 by Bill Seymour. 714 <br> 715 All suggestions and corrections will be welcome; all flames will be amusing. 716 Mail to was at pobox dot com. 717 </address> 718 </body> 719 </html> 720
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