1<!DOCTYPE html> 2<html lang="en"> 3<head> 4<title>How to Read the tz Database</title> 5<meta charset="UTF-8"> 6<meta name="viewport" content="width=device-width, initial-scale=1"> 7<style> 8pre {margin-left: 1.3rem; overflow: auto;} 9td {text-align: center;} 10table {border: 1px outset;} 11th, td {border: 1px inset;} 12table.rule {border: none; margin: auto;} 13td.footnote {text-align: left;} 14ul {padding-left: 1.3rem;} 15</style> 16</head> 17<body> 18<main> 19<h1>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 20Database</a> Source Files</h1> 21<h2>by Bill Seymour</h2> 22<p>This guide uses the <code>America/Chicago</code> and 23<code>Pacific/Honolulu</code> zones as examples of how to infer 24times of day from the <a href="tz-link.html">tz database</a> 25source files. It might be helpful, but not absolutely necessary, 26for the reader to have already downloaded the 27latest release of the database and become familiar with the basic layout 28of the data files. The format is explained in the “man page” 29for the zic compiler, <code>zic.8.txt</code>, in 30the <code>code</code> subdirectory. 31Although this guide covers many of the common cases, it is not a 32complete summary of what zic accepts; the man page is the 33authoritative reference.</p> 34 35<p>We’ll begin by talking about the rules for changing between standard 36and daylight saving time since we’ll need that information when we talk 37about the zones.</p> 38 39<p>First, let’s consider the special daylight saving time rules 40for Chicago (from the <code>northamerica</code> file in 41the <code>data</code> subdirectory):</p> 42 43<pre>#Rule NAME FROM TO - IN ON AT SAVE LETTER 44Rule Chicago 1920 only - Jun 13 2:00 1:00 D 45Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 46Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 47Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 48Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 49Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 50</pre> 51<table> 52<tr> 53 <th colspan="6">Reformatted a Bit</th> 54</tr> 55<tr> 56 <th>From</th> 57 <th>To</th> 58 <th colspan="2">On</th> 59 <th>At</th> 60 <th>Action</th> 61</tr> 62<tr> 63 <td colspan="2">1920 only</td> 64 <td colspan="2">June 13<small><sup>th</sup></small></td> 65 <td rowspan="6">02:00 local</td> 66 <td>go to daylight saving time</td> 67</tr> 68<tr> 69 <td>1920</td> 70 <td>1921</td> 71 <td rowspan="5">last Sunday</td> 72 <td>in October</td> 73 <td>return to standard time</td> 74</tr> 75<tr> 76 <td colspan="2">1921 only</td> 77 <td>in March</td> 78 <td rowspan="2">go to daylight saving time</td> 79</tr> 80<tr> 81 <td rowspan="2">1922</td> 82 <td>1966</td> 83 <td>in April</td> 84</tr> 85<tr> 86 <td>1954</td> 87 <td>in September</td> 88 <td rowspan="2">return to standard time</td> 89</tr> 90<tr> 91 <td>1955</td> 92 <td>1966</td> 93 <td>in October</td> 94</tr> 95</table> 96 97<p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the 98first and last calendar years defining a contiguous range over which a specific 99Rule line is to apply. The keyword <code>only</code> can be used in the 100<code>TO</code> field to repeat the value of the <code>FROM</code> field in the 101event that a rule should only apply to a single year. Often, the keyword 102<code>max</code> is used to extend a rule’s application into the 103indefinite future; it is a platform-agnostic stand-in for the largest 104representable year. 105 106<p>The next column, <code>-</code>, is reserved; for compatibility with earlier 107releases, it always contains a hyphen, which acts as a kind of null value. 108Prior to the 2020b release, it was called the <code>TYPE</code> field, though 109it had not been used in the main data since the 2000e release. 110An obsolescent supplementary file used the 111field as a proof-of-concept to allow <code>zic</code> to apply a given Rule 112line only to certain “types” of years within the specified range as 113dictated by the output of a separate script, such as: only years which would 114have a US presidential election, or only years which wouldn’t. 115 116<p>The <code>SAVE</code> column contains the local (wall clock) offset from 117local standard time. 118This is usually either zero for standard time or one hour for daylight 119saving time; but there’s no reason, in principle, why it can’t 120take on other values. 121 122<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 123column can contain a variable 124part of the usual abbreviation of the time zone’s name, or it can just 125be a hyphen if there’s no variable part. For example, the abbreviation 126used in the central time zone will be either “CST” or “CDT”. 127The variable part is ‘S’ or ‘D’; 128and, sure enough, that’s just what we find in 129the <code>LETTER</code> column 130in the <code>Chicago</code> rules. More about this when we talk about 131“Zone” lines. 132 133<p>One important thing to notice is that “Rule” lines 134want at once to be both <i>transitions</i> and <i>steady states</i>: 135<ul> 136<li>On the one hand, they represent transitions between standard and 137daylight saving time; and any number of Rule lines can be in effect 138during a given period (which will always be a non-empty set of 139contiguous calendar years).</li> 140<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 141columns contain state that exists between transitions. More about this 142when we talk about the US rules.</li> 143</ul> 144 145<p>In the example above, the transition to daylight saving time 146happened on the 13<small><sup>th</sup></small> of June in 1920, and on 147the last Sunday in March in 1921; but the return to standard time 148happened on the last Sunday in October in both of those 149years. Similarly, the rule for changing to daylight saving time was 150the same from 1922 to 1966; but the rule for returning to standard 151time changed in 1955. Got it?</p> 152 153<p>OK, now for the somewhat more interesting “US” rules:</p> 154 155<pre>#Rule NAME FROM TO - IN ON AT SAVE LETTER/S 156Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 157Rule US 1918 1919 - Oct lastSun 2:00 0 S 158Rule US 1942 only - Feb 9 2:00 1:00 W # War 159Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 160Rule US 1945 only - Sep 30 2:00 0 S 161Rule US 1967 2006 - Oct lastSun 2:00 0 S 162Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 163Rule US 1974 only - Jan 6 2:00 1:00 D 164Rule US 1975 only - Feb 23 2:00 1:00 D 165Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 166Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 167Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 168Rule US 2007 max - Nov Sun>=1 2:00 0 S 169</pre> 170<table> 171<tr> 172 <th colspan="6">Reformatted a Bit</th> 173</tr> 174<tr> 175 <th>From</th> 176 <th>To</th> 177 <th colspan="2">On</th> 178 <th>At</th> 179 <th>Action</th> 180</tr> 181<tr> 182 <td rowspan="2">1918</td> 183 <td rowspan="2">1919</td> 184 <td rowspan="2">last Sunday</td> 185 <td>in March</td> 186 <td rowspan="3">02:00 local</td> 187 <td>go to daylight saving time</td> 188</tr> 189<tr> 190 <td>in October</td> 191 <td>return to standard time</td> 192</tr> 193<tr> 194 <td colspan="2">1942 only</td> 195 <td colspan="2">February 9<small><sup>th</sup></small></td> 196 <td>go to “war time”</td> 197</tr> 198<tr> 199 <td colspan="2" rowspan="2">1945 only</td> 200 <td colspan="2">August 14<small><sup>th</sup></small></td> 201 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 202 <td> 203 rename “war time” to “peace<br>time;” 204 clocks don’t change 205 </td> 206</tr> 207<tr> 208 <td colspan="2">September 30<small><sup>th</sup></small></td> 209 <td rowspan="9">02:00 local</td> 210 <td rowspan="2">return to standard time</td> 211</tr> 212<tr> 213 <td rowspan="2">1967</td> 214 <td>2006</td> 215 <td rowspan="2">last Sunday</td> 216 <td>in October</td> 217</tr> 218<tr> 219 <td>1973</td> 220 <td>in April</td> 221 <td rowspan="6">go to daylight saving time</td> 222</tr> 223<tr> 224 <td colspan="2">1974 only</td> 225 <td colspan="2">January 6<small><sup>th</sup></small></td> 226</tr> 227<tr> 228 <td colspan="2">1975 only</td> 229 <td colspan="2">February 23<small><sup>rd</sup></small></td> 230</tr> 231<tr> 232 <td>1976</td> 233 <td>1986</td> 234 <td>last Sunday</td> 235 <td rowspan="2">in April</td> 236</tr> 237<tr> 238 <td>1987</td> 239 <td>2006</td> 240 <td>first Sunday</td> 241</tr> 242<tr> 243 <td rowspan="2">2007</td> 244 <td rowspan="2">present</td> 245 <td colspan="2">second Sunday in March</td> 246</tr> 247<tr> 248 <td colspan="2">first Sunday in November</td> 249 <td>return to standard time</td> 250</tr> 251</table> 252 253<p>There are two interesting things to note here.</p> 254 255<p>First, the time that something happens (in the <code>AT</code> 256column) is not necessarily the local (wall clock) time. The time can be 257suffixed with ‘s’ (for “standard”) to mean 258local standard time, different from local (wall clock) time when observing 259daylight saving time; or it can be suffixed with ‘g’, 260‘u’, or ‘z’, all three of which mean the 261standard time at the 262<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 263‘g’ stands for “<a 264href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 265‘u’ stands for “<a 266href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 267href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 268(whichever was official at the time); ‘z’ stands for the 269<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 270Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 271The time can also be suffixed with ‘w’ meaning local (wall 272clock) time; but it usually isn’t because that’s the 273default.</p> 274 275<p>Second, the day in the <code>ON</code> column, in addition to 276“<code>lastSun</code>” or a particular day of the month, 277can have the form, “<code>Sun>=</code><i>x</i>” or 278“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 279of the month. For example, “<code>Sun>=8</code>” means 280“the first Sunday on or after the eighth of the month,” in 281other words, the second Sunday of the month. Furthermore, although 282there are no examples above, the weekday needn’t be 283“<code>Sun</code>” in either form, but can be the usual 284three-character English abbreviation for any day of the week.</p> 285 286<p>And the US rules give us more examples of a couple of things 287already mentioned:</p> 288 289<ul> 290<li>The rules for changing to and from daylight saving time are 291actually <i>different sets</i> of rules; and the two sets can change 292independently. Consider, for example, that the rule for the return to 293standard time stayed the same from 1967 to 2006; but the rule for the 294transition to daylight saving time changed several times in the same 295period. There can also be periods, 1946 to 1966 for example, when no 296rule from this group is in effect, and so either no transition 297happened in those years, or some other rule is in effect (perhaps a 298state or other more local rule).</li> 299 300<li>The <code>SAVE</code> and <code>LETTER</code> columns 301contain <i>steady state</i>, not transitions. Consider, for example, 302the transition from “war time” to “peace time” 303that happened on August 14, 1945. The “1:00” in 304the <code>SAVE</code> column is <i>not</i> an instruction to advance 305the clock an hour. It means that clocks should <i>be</i> one hour 306ahead of standard time, which they already are because of the previous 307rule, so there should be no change.</li> 308 309</ul> 310 311<p>OK, now let’s look at a Zone record:</p> 312 313<pre> 314#Zone NAME STDOFF RULES FORMAT [UNTIL] 315Zone 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<table> 325<tr> 326 <th colspan="5">Columns Renamed</th> 327</tr> 328<tr> 329 <th rowspan="2">Standard Offset<br> 330 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 331 Meridian</a></th> 332 <th rowspan="2">Daylight<br>Saving Time</th> 333 <th rowspan="2">Abbreviation(s)</th> 334 <th colspan="2">Ending at Local Time</th> 335</tr> 336<tr> 337 <th>Date</th> 338 <th>Time</th> 339</tr> 340<tr> 341 <td>−5:50:36</td> 342 <td>not observed</td> 343 <td>LMT</td> 344 <td>1883-11-18</td> 345 <td>12:09:24</td> 346</tr> 347<tr> 348 <td rowspan="2">−6:00:00</td> 349 <td>US rules</td> 350 <td rowspan="2">CST or CDT</td> 351 <td>1920-01-01</td> 352 <td>00:00:00</td> 353</tr> 354<tr> 355 <td>Chicago rules</td> 356 <td>1936-03-01</td> 357 <td rowspan="2">02:00:00</td> 358</tr> 359<tr> 360 <td>−5:00:00</td> 361 <td>not observed</td> 362 <td>EST</td> 363 <td>1936-11-15</td> 364</tr> 365<tr> 366 <td rowspan="4">−6:00:00</td> 367 <td>Chicago rules</td> 368 <td>CST or CDT</td> 369 <td>1942-01-01</td> 370 <td rowspan="3">00:00:00</td> 371</tr> 372<tr> 373 <td>US rules</td> 374 <td>CST, CWT or CPT</td> 375 <td>1946-01-01</td> 376</tr> 377<tr> 378 <td>Chicago rules</td> 379 <td rowspan="2">CST or CDT</td> 380 <td>1967-01-01</td> 381</tr> 382<tr> 383 <td>US rules</td> 384 <td colspan="2">–</td> 385</tr> 386</table> 387 388<p>There are a couple of interesting differences between Zones and Rules.</p> 389 390<p>First, and somewhat trivially, whereas Rules are considered to 391contain one or more records, a Zone is considered to be a single 392record with zero or more <i>continuation lines</i>. Thus, the keyword, 393“<code>Zone</code>,” and the zone name are not 394repeated. The last line is the one without anything in 395the <code>[UNTIL]</code> column.</p> 396 397<p>Second, and more fundamentally, each line of a Zone represents a 398steady state, not a transition between states. The state exists from 399the date and time in the previous line’s <code>[UNTIL]</code> 400column up to the date and time in the current 401line’s <code>[UNTIL]</code> column. In other words, the date and 402time in the <code>[UNTIL]</code> column is the instant that separates 403this state from the next. Where that would be ambiguous because 404we’re setting our clocks back, the <code>[UNTIL]</code> column 405specifies the first occurrence of the instant. The state specified by 406the last line, the one without anything in the <code>[UNTIL]</code> 407column, continues to the present.</p> 408 409<p>The first line typically specifies the mean solar time observed 410before the introduction of standard time. Since there’s no line before 411that, it has no beginning. <code>8-) </code> For some places near the <a 412href="https://en.wikipedia.org/wiki/International_Date_Line">International 413Date Line</a>, the first <i>two</i> lines will show solar times 414differing by 24 hours; this corresponds to a movement of the Date 415Line. For example:</p> 416 417<pre> 418#Zone NAME STDOFF RULES FORMAT [UNTIL] 419Zone America/Juneau 15:02:19 - LMT 1867 Oct 19 0:31:13u 420 -8:57:41 - LMT ... 421</pre> 422 423<p>When Alaska was purchased from Russia in 1867, the Date Line moved 424from the Alaska/Canada border to the Bering Strait; and the time in 425Alaska was then 24 hours earlier than it had 426been. <code><aside></code>(This also means that, although the sun had set 427on Friday 6 October and risen on Saturday 7 October in the Julian calendar, 428which Russia was still using then for religious reasons, upon the transfer 429Saturday afternoon became Friday afternoon, and then the sun set and rose once 430more on <i>second instances of the same days with different names</i>, 431Friday 18 and Saturday 19 October in the Gregorian calendar. Isn’t civil time 432wonderful? <code>8-)</code>)<code></aside></code></p> 433 434<p>The abbreviation, “LMT” stands for “local mean 435time”, which is an invention of 436the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 437database</a> and was probably never actually used during the 438period. Furthermore, the value is almost certainly wrong except in the 439archetypal place after which the zone is named. (The tz database 440usually doesn’t provide a separate Zone record for places where 441nothing significant happened after 1970.)</p> 442 443<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 444<ul> 445<li>A hyphen, a kind of null value, means that we have not set our 446clocks ahead of standard time.</li> 447 448<li>An amount of time (usually but not necessarily “1:00” 449meaning one hour) means that we have set our clocks ahead by that 450amount.</li> 451 452<li>Some alphabetic string means that we <i>might have</i> set our 453clocks ahead; and we need to check the rule the name of which is the 454given alphabetic string.</li> 455</ul> 456 457<p>An example of a specific amount of time is:</p> 458<pre> 459#Zone NAME STDOFF RULES FORMAT [UNTIL] 460Zone Pacific/Honolulu ... 1933 Apr 30 2:00 461 -10:30 1:00 HDT 1933 May 21 12:00 462 ... 463</pre> 464 465<p>Hawaii tried daylight saving time for three weeks in 1933 and 466decided they didn’t like it. <code>8-) </code>Note that 467the <code>STDOFF</code> column always contains the standard time 468offset, so the local (wall clock) time during this period was GMT − 46910:30 + 1:00 = GMT − 9:30.</p> 470 471<p>The <code>FORMAT</code> column specifies the usual abbreviation of 472the time zone name. It should have one of four forms:</p> 473<ul> 474 475<li>a time zone abbreviation that is a string of three or more 476characters that are either ASCII alphanumerics, 477“<code>+</code>”, or “<code>-</code>”</li> 478 479<li>the string “%z”, in which case the 480“<code>%z</code>” will be replaced by a numeric time zone 481abbreviation</li> 482 483<li>a pair of time zone abbreviations separated by a slash 484(‘<code>/</code>’), in which case the first string is the 485abbreviation for the standard time name and the second string is the 486abbreviation for the daylight saving time name</li> 487 488<li>a string containing “<code>%s</code>”, in which case 489the “<code>%s</code>” will be replaced by the text in the 490appropriate Rule’s <code>LETTER</code> column, and the resulting 491string should be a time zone abbreviation</li> 492</ul> 493 494<p>The last two make sense only if there’s a named rule in effect.</p> 495 496<p>An example of a slash is:</p> 497<pre> 498#Zone NAME STDOFF RULES FORMAT [UNTIL] 499Zone Europe/London ... 1996 500 0:00 EU GMT/BST 501</pre> 502 503<p>The current time in the UK is called either Greenwich mean time or 504British summer time.</p> 505 506<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 507happens when switching to a named rule. To what values should 508the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 509 510<ul> 511<li>If at least one transition has happened, use 512the <code>SAVE</code> and <code>LETTER</code> data from the most 513recent.</li> 514 515<li>If switching to a named rule before any transition has happened, 516assume standard time (<code>SAVE</code> zero), and use 517the <code>LETTER</code> data from the earliest transition with 518a <code>SAVE</code> of zero. 519 520</ul> 521 522<p>And three last things about the <code>FORMAT</code> column:</p> 523<ul> 524 525<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 526database</a> gives abbreviations for time zones 527in popular English-language usage. For 528example, the last line in 529<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 530“HST” for “Hawaii standard time” even though the 531<a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 532name for that time zone is “Hawaii–Aleutian standard time”. 533This author has read that there are also some places in Australia where 534popular time zone names differ from the legal ones. 535 536<li>No attempt is made to <a 537href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 538the abbreviations. They are intended to be the values returned through the 539<code>"%Z"</code> format specifier to 540<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 541<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/strftime.html"><code>strftime</code></a> 542function in the 543<a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 544 545<li>If there is no generally accepted abbreviation for a time zone, 546a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 547ahead of Greenwich. By convention, <code>-00</code> is used in a 548zone while uninhabited, where the offset is zero but in some sense 549the true offset is undefined. 550</ul> 551 552<p>As a final example, here’s the complete history for Hawaii:</p> 553 554<pre># Relevant Excerpts from the US Rules 555#Rule NAME FROM TO - IN ON AT SAVE LETTER/S 556Rule US 1918 1919 - Oct lastSun 2:00 0 S 557Rule US 1942 only - Feb 9 2:00 1:00 W # War 558Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 559Rule US 1945 only - Sep lastSun 2:00 0 S 560 561# The Zone Record 562#Zone NAME STDOFF RULES FORMAT [UNTIL] 563Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 564 -10:30 - HST 1933 Apr 30 2:00 565 -10:30 1:00 HDT 1933 May 21 2:00 566 -10:30 US H%sT 1947 Jun 8 2:00 567 -10:00 - HST 568</pre> 569<table> 570<tr> 571 <th colspan="6">What We Infer</th> 572</tr> 573<tr> 574 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 575 <th rowspan="2">Adjust<br>Clocks</th> 576 <th colspan="2">Time Zone</th> 577 <th colspan="2">Ending at Local Time</th> 578</tr> 579<tr> 580 <th>Abbrv.</th> 581 <th>Name</th> 582 <th>Date</th> 583 <th>Time</th> 584</tr> 585<tr> 586 <td>−10:31:26</td> 587 <td>–</td> 588 <td>LMT</td> 589 <td>local mean time</td> 590 <td>1896-01-13</td> 591 <td>12:00</td> 592</tr> 593<tr> 594 <td>−10:30</td> 595 <td>+0:01:26</td> 596 <td>HST</td> 597 <td>Hawaii standard time</td> 598 <td>1933-04-30</td> 599 <td>02:00</td> 600</tr> 601<tr> 602 <td>−9:30</td> 603 <td>+1:00</td> 604 <td>HDT</td> 605 <td>Hawaii daylight time</td> 606 <td>1933-05-21</td> 607 <td>12:00</td> 608</tr> 609<tr> 610 <td>−10:30¹</td> 611 <td>−1:00¹</td> 612 <td>HST¹</td> 613 <td>Hawaii standard time</td> 614 <td>1942-02-09</td> 615 <td>02:00</td> 616</tr> 617<tr> 618 <td rowspan="2">−9:30</td> 619 <td>+1:00</td> 620 <td>HWT</td> 621 <td>Hawaii war time</td> 622 <td>1945-08-14</td> 623 <td>13:30²</td> 624</tr> 625<tr> 626 <td>0</td> 627 <td>HPT</td> 628 <td>Hawaii peace time</td> 629 <td>1945-09-30</td> 630 <td rowspan="2">02:00</td> 631</tr> 632<tr> 633 <td>−10:30</td> 634 <td>−1:00</td> 635 <td rowspan="2">HST</td> 636 <td rowspan="2">Hawaii standard time</td> 637 <td>1947-06-08</td> 638</tr> 639<tr> 640 <td>−10:00³</td> 641 <td>+0:30³</td> 642 <td colspan="2">–</td> 643</tr> 644<tr> 645 <td colspan="6" class="footnote"> 646 ¹Switching to US rules...most recent transition (in 1919) was to standard time 647 </td> 648</tr> 649<tr> 650 <td colspan="6" class="footnote"> 651 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 652 + (−9:30) = 13:30 local 653 </td> 654</tr> 655<tr> 656 <td colspan="6" class="footnote"> 657 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947-06-08T12:30Z</a>, 658 the civil time in Hawaii has been 659 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 660 −10:00 year-round. 661 </td> 662</tr> 663</table> 664 665<p>There will be a short quiz later. <code>8-)</code></p> 666</main> 667 668<footer> 669<hr> 670This web page is in the public domain, so clarified as of 6712015-10-20 by Bill Seymour. 672<br> 673All suggestions and corrections will be welcome; all flames will be amusing. 674Mail to was at pobox dot com. 675</footer> 676</body> 677</html> 678