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> 7pre {margin-left: 2em; white-space: pre-wrap;} 8pre.td {margin-left: 0;} 9td {text-align: center;} 10table {border: 1px outset;} 11th, td {border: 1px inset;} 12table.rule {border: none; margin: auto;} 13td.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 18Database</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 22times of day from the <a href="tz-link.html">tz database</a> 23source files. It might be helpful, but not absolutely necessary, 24for the reader to have already downloaded the 25latest release of the database and become familiar with the basic layout 26of the data files. The format is explained in the “man 27page” for the zic compiler, <code>zic.8.txt</code>, in 28the <code>code</code> subdirectory. 29Although this guide covers many of the common cases, it is not a 30complete summary of what zic accepts; the man page is the 31authoritative reference.</p> 32 33<p>We’ll begin by talking about the rules for changing between standard 34and daylight saving time since we’ll need that information when we talk 35about the zones.</p> 36 37<p>First, let’s consider the special daylight saving time rules 38for Chicago (from the <code>northamerica</code> file in 39the <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 51Rule Chicago 1920 only - Jun 13 2:00 1:00 D 52Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 53Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 54Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 55Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 56Rule 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 106first and last calendar years defining a contiguous range over which a specific 107Rule 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 109event 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 111indefinite future; it is a platform-agnostic stand-in for the largest 112representable year. 113 114<p>The next column, <code>-</code>, is reserved; for compatibility with earlier 115releases, it always contains a hyphen, which acts as a kind of null value. 116Prior to the 2020b release, it was called the <code>TYPE</code> field, though 117it had not been used in the main data since the 2000e release. 118An obsolescent supplementary file used the 119field as a proof-of-concept to allow <code>zic</code> to apply a given Rule 120line only to certain “types” of years within the specified range as 121dictated by the output of a separate script, such as: only years which would 122have 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 125local standard time. 126This is usually either zero for standard time or one hour for daylight 127saving time; but there’s no reason, in principle, why it can’t 128take on other values. 129 130<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 131column can contain a variable 132part of the usual abbreviation of the time zone’s name, or it can just 133be a hyphen if there’s no variable part. For example, the abbreviation 134used in the central time zone will be either “CST” or 135“CDT”. The variable part is ‘S’ or ‘D’; 136and, sure enough, that’s just what we find in 137the <code>LETTER</code> column 138in 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 142want 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 145daylight saving time; and any number of Rule lines can be in effect 146during a given period (which will always be a non-empty set of 147contiguous calendar years).</li> 148<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 149columns contain state that exists between transitions. More about this 150when we talk about the US rules.</li> 151</ul> 152 153<p>In the example above, the transition to daylight saving time 154happened on the 13<small><sup>th</sup></small> of June in 1920, and on 155the last Sunday in March in 1921; but the return to standard time 156happened on the last Sunday in October in both of those 157years. Similarly, the rule for changing to daylight saving time was 158the same from 1922 to 1966; but the rule for returning to standard 159time 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 173Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 174Rule US 1918 1919 - Oct lastSun 2:00 0 S 175Rule US 1942 only - Feb 9 2:00 1:00 W # War 176Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 177Rule US 1945 only - Sep 30 2:00 0 S 178Rule US 1967 2006 - Oct lastSun 2:00 0 S 179Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 180Rule US 1974 only - Jan 6 2:00 1:00 D 181Rule US 1975 only - Feb 23 2:00 1:00 D 182Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 183Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 184Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 185Rule 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> 274column) is not necessarily the local (wall clock) time. The time can be 275suffixed with ‘s’ (for “standard”) to mean 276local standard time, different from local (wall clock) time when observing 277daylight saving time; or it can be suffixed with ‘g’, 278‘u’, or ‘z’, all three of which mean the 279standard time at the 280<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 281‘g’ stands for “<a 282href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 283‘u’ stands for “<a 284href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 285href="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> 288Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 289The time can also be suffixed with ‘w’ meaning local (wall 290clock) time; but it usually isn’t because that’s the 291default.</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, 295can have the form, “<code>Sun>=</code><i>x</i>” or 296“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 297of the month. For example, “<code>Sun>=8</code>” means 298“the first Sunday on or after the eighth of the month,” in 299other words, the second Sunday of the month. Furthermore, although 300there are no examples above, the weekday needn’t be 301“<code>Sun</code>” in either form, but can be the usual 302three-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 305already mentioned:</p> 306 307<ul> 308<li>The rules for changing to and from daylight saving time are 309actually <i>different sets</i> of rules; and the two sets can change 310independently. Consider, for example, that the rule for the return to 311standard time stayed the same from 1967 to 2006; but the rule for the 312transition to daylight saving time changed several times in the same 313period. There can also be periods, 1946 to 1966 for example, when no 314rule from this group is in effect, and so either no transition 315happened in those years, or some other rule is in effect (perhaps a 316state or other more local rule).</li> 317 318<li>The <code>SAVE</code> and <code>LETTER</code> columns 319contain <i>steady state</i>, not transitions. Consider, for example, 320the transition from “war time” to “peace time” 321that happened on August 14, 1945. The “1:00” in 322the <code>SAVE</code> column is <i>not</i> an instruction to advance 323the clock an hour. It means that clocks should <i>be</i> one hour 324ahead of standard time, which they already are because of the previous 325rule, 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] 341Zone 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 418contain one or more records, a Zone is considered to be a single 419record with zero or more <i>continuation lines</i>. Thus, the keyword, 420“<code>Zone</code>,” and the zone name are not 421repeated. The last line is the one without anything in 422the <code>[UNTIL]</code> column.</p> 423 424<p>Second, and more fundamentally, each line of a Zone represents a 425steady state, not a transition between states. The state exists from 426the date and time in the previous line’s <code>[UNTIL]</code> 427column up to the date and time in the current 428line’s <code>[UNTIL]</code> column. In other words, the date and 429time in the <code>[UNTIL]</code> column is the instant that separates 430this state from the next. Where that would be ambiguous because 431we’re setting our clocks back, the <code>[UNTIL]</code> column 432specifies the first occurrence of the instant. The state specified by 433the last line, the one without anything in the <code>[UNTIL]</code> 434column, continues to the present.</p> 435 436<p>The first line typically specifies the mean solar time observed 437before the introduction of standard time. Since there’s no line before 438that, it has no beginning. <code>8-) </code> For some places near the <a 439href="https://en.wikipedia.org/wiki/International_Date_Line">International 440Date Line</a>, the first <i>two</i> lines will show solar times 441differing by 24 hours; this corresponds to a movement of the Date 442Line. For example:</p> 443 444<pre> 445#Zone NAME STDOFF RULES FORMAT [UNTIL] 446Zone 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 451from the Alaska/Canada border to the Bering Strait; and the time in 452Alaska was then 24 hours earlier than it had 453been. <code><aside></code>(6 October in the Julian calendar, 454which Russia was still using then for religious reasons, was followed 455by <i>a second instance of the same day with a different name</i>, 18 456October in the Gregorian calendar. Isn’t civil time 457wonderful? <code>8-)</code>)<code></aside></code></p> 458 459<p>The abbreviation, “LMT” stands for “local mean 460time”, which is an invention of 461the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 462database</a> and was probably never actually used during the 463period. Furthermore, the value is almost certainly wrong except in the 464archetypal place after which the zone is named. (The tz database 465usually doesn’t provide a separate Zone record for places where 466nothing 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 471clocks ahead of standard time.</li> 472 473<li>An amount of time (usually but not necessarily “1:00” 474meaning one hour) means that we have set our clocks ahead by that 475amount.</li> 476 477<li>Some alphabetic string means that we <i>might have</i> set our 478clocks ahead; and we need to check the rule the name of which is the 479given 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] 485Zone 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 491decided they didn’t like it. <code>8-) </code>Note that 492the <code>STDOFF</code> column always contains the standard time 493offset, so the local (wall clock) time during this period was GMT − 49410:30 + 1:00 = GMT − 9:30.</p> 495 496<p>The <code>FORMAT</code> column specifies the usual abbreviation of 497the 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 501characters 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 506abbreviation</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 510abbreviation for the standard time name and the second string is the 511abbreviation for the daylight saving time name</li> 512 513<li>a string containing “<code>%s</code>”, in which case 514the “<code>%s</code>” will be replaced by the text in the 515appropriate Rule’s <code>LETTER</code> column, and the resulting 516string 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] 524Zone 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 529British summer time.</p> 530 531<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 532happens when switching to a named rule. To what values should 533the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 534 535<ul> 536<li>If at least one transition has happened, use 537the <code>SAVE</code> and <code>LETTER</code> data from the most 538recent.</li> 539 540<li>If switching to a named rule before any transition has happened, 541assume standard time (<code>SAVE</code> zero), and use 542the <code>LETTER</code> data from the earliest transition with 543a <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 551database</a> gives abbreviations for time zones in <i>popular 552usage</i>, which is not necessarily “correct” by law. For 553example, 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> 557name for that time zone is “Hawaii-Aleutian standard time.” 558This author has read that there are also some places in Australia where 559popular time zone names differ from the legal ones. 560 561<li>No attempt is made to <a 562href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 563the 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> 567function 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, 571a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 572ahead of Greenwich. By convention, <code>-00</code> is used in a 573zone while uninhabited, where the offset is zero but in some sense 574the 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 589Rule US 1918 1919 - Oct lastSun 2:00 0 S 590Rule US 1942 only - Feb 9 2:00 1:00 W # War 591Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 592Rule 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] 605Zone 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> 712This web page is in the public domain, so clarified as of 7132015-10-20 by Bill Seymour. 714<br> 715All suggestions and corrections will be welcome; all flames will be amusing. 716Mail to was at pobox dot com. 717</address> 718</body> 719</html> 720