tz-how-to.html revision 1.5
1<!DOCTYPE html 2 PUBLIC "-//W3C//DTD HTML 4.01//EN" 3 "http://www.w3.org/TR/html4/strict.dtd"> 4<html> 5<head> 6<title>How to Read the tz Database</title> 7<meta http-equiv="Content-type" content='text/html; charset="UTF-8"'> 8<style type="text/css"> 9pre {margin-left: 2em; white-space: pre-wrap;} 10</style> 11</head> 12<body> 13<h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 14Database</a> Source Files</h2> 15<h3>by Bill Seymour</h3> 16<p>This page uses the <code>America/Chicago</code> and 17<code>Pacific/Honolulu</code> zones as examples of how to infer 18times of day from the <a href="tz-link.html">tz database</a> 19source files. It might be helpful, but not absolutely necessary, 20for the reader to have already downloaded the 21latest release of the database and become familiar with the basic layout 22of the data files. The format is explained in the “man 23page” for the zic compiler, <code>zic.8.txt</code>, in 24the <code>code</code> subdirectory.</p> 25 26<p>We’ll begin by talking about the rules for changing between standard 27and daylight saving time since we’ll need that information when we talk 28about the zones.</p> 29 30<p>First, let’s consider the special daylight saving time rules 31for Chicago (from the <code>northamerica</code> file in 32the <code>data</code> subdirectory):</p> 33 34<table border="1"> 35<tr> 36 <th colspan="6">From the Source File</th> 37</tr> 38<tr> 39 <td colspan="6" align="center"><table><tr><td> 40<pre> 41#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 42Rule Chicago 1920 only - Jun 13 2:00 1:00 D 43Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 44Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 45Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 46Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 47Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 48</pre> 49 </td></tr></table></td> 50</tr> 51<tr> 52 <th colspan="6">Reformatted a Bit</th> 53</tr> 54<tr> 55 <th>From</th> 56 <th>To</th> 57 <th colspan="2">On</th> 58 <th>At</th> 59 <th>Action</th> 60</tr> 61<tr align="center"> 62 <td colspan="2">1920 only</td> 63 <td colspan="2">June 13<small><sup>th</sup></small></td> 64 <td rowspan="6">02:00 local</td> 65 <td>go to daylight saving time</td> 66</tr> 67<tr align="center"> 68 <td>1920</td> 69 <td>1921</td> 70 <td rowspan="5">last Sunday</td> 71 <td>in October</td> 72 <td>return to standard time</td> 73</tr> 74<tr align="center"> 75 <td colspan="2">1921 only</td> 76 <td>in March</td> 77 <td rowspan="2">go to daylight saving time</td> 78</tr> 79<tr align="center"> 80 <td rowspan="2">1922</td> 81 <td>1966</td> 82 <td>in April</td> 83</tr> 84<tr align="center"> 85 <td>1954</td> 86 <td>in September</td> 87 <td rowspan="2">return to standard time</td> 88</tr> 89<tr align="center"> 90 <td>1955</td> 91 <td>1966</td> 92 <td>in October</td> 93</tr> 94</table> 95 96<p>We’ll basically just ignore the <code>TYPE</code> column. 97In the 2007j release, the most recent as of this writing, the 98<code>TYPE</code> column never contains anything but a hyphen, 99a kind of null value. (From the description in <code>zic.8.txt</code>, 100this appears to be a mechanism for removing years from a set 101in some localizable way. It’s used in the file, <code>pacificnew</code>, 102to determine whether a given year will have a US presidential election; 103but everything related to that use is commented out.) 104 105<p>The <code>SAVE</code> column contains the wall clock offset from 106local standard time. 107This is usually either zero for standard time or one hour for daylight 108saving time; but there’s no reason, in principle, why it can’t 109take on other values. 110 111<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 112column can contain a variable 113part of the usual abbreviation of the time zone’s name, or it can just 114be a hyphen if there’s no variable part. For example, the abbreviation 115used in the central time zone will be either “CST” or 116“CDT”. The variable part is ‘S’ or ‘D’; 117and, sure enough, that’s just what we find in 118the <code>LETTER</code> column 119in the <code>Chicago</code> rules. More about this when we talk about 120“Zone” lines. 121 122<p>One important thing to notice is that “Rule” lines 123want at once to be both <i>transitions</i> and <i>steady states</i>: 124<ul> 125<li>On the one hand, they represent transitions between standard and 126daylight saving time; and any number of Rule lines can be in effect 127during a given period (which will always be a non-empty set of 128contiguous calendar years).</li> 129<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 130columns contain state that exists between transitions. More about this 131when we talk about the US rules.</li> 132</ul> 133 134<p>In the example above, the transition to daylight saving time 135happened on the 13<small><sup>th</sup></small> of June in 1920, and on 136the last Sunday in March in 1921; but the return to standard time 137happened on the last Sunday in October in both of those 138years. Similarly, the rule for changing to daylight saving time was 139the same from 1922 to 1966; but the rule for returning to standard 140time changed in 1955. Got it?</p> 141 142<p>OK, now for the somewhat more interesting “US” rules:</p> 143 144<table border="1"> 145<tr> 146 <th colspan="6">From the Source File</th> 147</tr> 148<tr> 149 <td colspan="6" align="center"><table><tr><td> 150<pre> 151#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 152Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 153Rule US 1918 1919 - Oct lastSun 2:00 0 S 154Rule US 1942 only - Feb 9 2:00 1:00 W # War 155Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 156Rule US 1945 only - Sep 30 2:00 0 S 157Rule US 1967 2006 - Oct lastSun 2:00 0 S 158Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 159Rule US 1974 only - Jan 6 2:00 1:00 D 160Rule US 1975 only - Feb 23 2:00 1:00 D 161Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 162Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 163Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 164Rule US 2007 max - Nov Sun>=1 2:00 0 S 165</pre> 166 </td></tr></table></td> 167</tr> 168<tr> 169 <th colspan="6">Reformatted a Bit</th> 170</tr> 171<tr> 172 <th>From</th> 173 <th>To</th> 174 <th colspan="2">On</th> 175 <th>At</th> 176 <th>Action</th> 177</tr> 178<tr align="center"> 179 <td rowspan="2">1918</td> 180 <td rowspan="2">1919</td> 181 <td rowspan="2">last Sunday</td> 182 <td>in March</td> 183 <td rowspan="3">02:00 local</td> 184 <td>go to daylight saving time</td> 185</tr> 186<tr align="center"> 187 <td>in October</td> 188 <td>return to standard time</td> 189</tr> 190<tr align="center"> 191 <td colspan="2">1942 only</td> 192 <td colspan="2">February 9<small><sup>th</sup></small></td> 193 <td>go to “war time”</td> 194</tr> 195<tr align="center"> 196 <td colspan="2" rowspan="2">1945 only</td> 197 <td colspan="2">August 14<small><sup>th</sup></small></td> 198 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 199 <td> 200 rename “war time” to “peace<br>time;” 201 clocks don’t change 202 </td> 203</tr> 204<tr align="center"> 205 <td colspan="2">September 30<small><sup>th</sup></small></td> 206 <td rowspan="9">02:00 local</td> 207 <td rowspan="2">return to standard time</td> 208</tr> 209<tr align="center"> 210 <td rowspan="2">1967</td> 211 <td>2006</td> 212 <td rowspan="2">last Sunday</td> 213 <td>in October</td> 214</tr> 215<tr align="center"> 216 <td>1973</td> 217 <td>in April</td> 218 <td rowspan="6">go to daylight saving time</td> 219</tr> 220<tr align="center"> 221 <td colspan="2">1974 only</td> 222 <td colspan="2">January 6<small><sup>th</sup></small></td> 223</tr> 224<tr align="center"> 225 <td colspan="2">1975 only</td> 226 <td colspan="2">February 23<small><sup>rd</sup></small></td> 227</tr> 228<tr align="center"> 229 <td>1976</td> 230 <td>1986</td> 231 <td>last Sunday</td> 232 <td rowspan="2">in April</td> 233</tr> 234<tr align="center"> 235 <td>1987</td> 236 <td>2006</td> 237 <td>first Sunday</td> 238</tr> 239<tr align="center"> 240 <td rowspan="2">2007</td> 241 <td rowspan="2">present</td> 242 <td colspan="2">second Sunday in March</td> 243</tr> 244<tr align="center"> 245 <td colspan="2">first Sunday in November</td> 246 <td>return to standard time</td> 247</tr> 248</table> 249 250<p>There are two interesting things to note here.</p> 251 252<p>First, the time that something happens (in the <code>AT</code> 253column) is not necessarily the local wall clock time. The time can be 254suffixed with ‘s’ (for “standard”) to mean 255local standard time (different from wall clock time when observing 256daylight saving time); or it can be suffixed with ‘g’, 257‘u’, or ‘z’, all three of which mean the 258standard time at the 259<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 260‘g’ stands for “<a 261href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 262‘u’ stands for “<a 263href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 264href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 265(whichever was official at the time); ‘z’ stands for the 266<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 267Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 268The time can also be suffixed with ‘w’ meaning “wall 269clock time;” but it usually isn’t because that’s the 270default.</p> 271 272<p>Second, the day in the <code>ON</code> column, in addition to 273“<code>lastSun</code>” or a particular day of the month, 274can have the form, “<code>Sun>=</code><i>x</i>” or 275“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 276of the month. For example, “<code>Sun>=8</code>” means 277“the first Sunday on or after the eighth of the month,” in 278other words, the second Sunday of the month. Furthermore, although 279there are no examples above, the weekday needn’t be 280“<code>Sun</code>” in either form, but can be the usual 281three-character English abbreviation for any day of the week.</p> 282 283<p>And the US rules give us more examples of a couple of things 284already mentioned:</p> 285 286<ul> 287<li>The rules for changing to and from daylight saving time are 288actually <i>different sets</i> of rules; and the two sets can change 289independently. Consider, for example, that the rule for the return to 290standard time stayed the same from 1967 to 2006; but the rule for the 291transition to daylight saving time changed several times in the same 292period. There can also be periods, 1946 to 1966 for example, when no 293rule from this group is in effect, and so either no transition 294happened in those years, or some other rule is in effect (perhaps a 295state or other more local rule).</li> 296 297<li>The <code>SAVE</code> and <code>LETTER</code> columns 298contain <i>steady state</i>, not transitions. Consider, for example, 299the transition from “war time” to “peace time” 300that happened on August 14, 1945. The “1:00” in 301the <code>SAVE</code> column is <i>not</i> an instruction to advance 302the clock an hour. It means that clocks should <i>be</i> one hour 303ahead of standard time, which they already are because of the previous 304rule, so there should be no change.</li> 305 306</ul> 307 308<p>OK, now let’s look at a Zone record:</p> 309 310<table border="1"> 311<tr> 312 <th colspan="5">From the Source File</th> 313</tr> 314<tr> 315 <td colspan="6" align="center"><table><tr><td> 316<pre> 317#Zone NAME GMTOFF RULES FORMAT [UNTIL] 318Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 319 -6:00 US C%sT 1920 320 -6:00 Chicago C%sT 1936 Mar 1 2:00 321 -5:00 - EST 1936 Nov 15 2:00 322 -6:00 Chicago C%sT 1942 323 -6:00 US C%sT 1946 324 -6:00 Chicago C%sT 1967 325 -6:00 US C%sT 326</pre> 327 </td></tr></table></td> 328</tr> 329<tr> 330 <th colspan="5">Columns Renamed</th> 331</tr> 332<tr> 333 <th rowspan="2">Standard Offset<br> 334 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 335 Meridian</a></th> 336 <th rowspan="2">Daylight<br>Saving Time</th> 337 <th rowspan="2">Abbreviation(s)</th> 338 <th colspan="2">Ending at Local Time</th> 339</tr> 340<tr> 341 <th>Date</th> 342 <th>Time</th> 343</tr> 344<tr align="center"> 345 <td>−5:50:36</td> 346 <td>not observed</td> 347 <td>LMT</td> 348 <td>1883-11-18</td> 349 <td>12:09:24</td> 350</tr> 351<tr align="center"> 352 <td rowspan="2">−6:00:00</td> 353 <td>US rules</td> 354 <td rowspan="2">CST or CDT</td> 355 <td>1920-01-01</td> 356 <td>00:00:00</td> 357</tr> 358<tr align="center"> 359 <td>Chicago rules</td> 360 <td>1936-03-01</td> 361 <td rowspan="2">02:00:00</td> 362</tr> 363<tr align="center"> 364 <td>−5:00:00</td> 365 <td>not observed</td> 366 <td>EST</td> 367 <td>1936-11-15</td> 368</tr> 369<tr align="center"> 370 <td rowspan="4">−6:00:00</td> 371 <td>Chicago rules</td> 372 <td>CST or CDT</td> 373 <td>1942-01-01</td> 374 <td rowspan="3">00:00:00</td> 375</tr> 376<tr align="center"> 377 <td>US rules</td> 378 <td>CST, CWT or CPT</td> 379 <td>1946-01-01</td> 380</tr> 381<tr align="center"> 382 <td>Chicago rules</td> 383 <td rowspan="2">CST or CDT</td> 384 <td>1967-01-01</td> 385</tr> 386<tr align="center"> 387 <td>US rules</td> 388 <td colspan="2">—</td> 389</tr> 390</table> 391 392<p>There are a couple of interesting differences between Zones and Rules.</p> 393 394<p>First, and somewhat trivially, whereas Rules are considered to 395contain one or more records, a Zone is considered to be a single 396record with zero or more <i>continuation lines</i>. Thus, the keyword, 397“<code>Zone</code>,” and the zone name are not 398repeated. The last line is the one without anything in 399the <code>[UNTIL]</code> column.</p> 400 401<p>Second, and more fundamentally, each line of a Zone represents a 402steady state, not a transition between states. The state exists from 403the date and time in the previous line’s <code>[UNTIL]</code> 404column up to the date and time in the current 405line’s <code>[UNTIL]</code> column. In other words, the date and 406time in the <code>[UNTIL]</code> column is the instant that separates 407this state from the next. Where that would be ambiguous because 408we’re setting our clocks back, the <code>[UNTIL]</code> column 409specifies the first occurrence of the instant. The state specified by 410the last line, the one without anything in the <code>[UNTIL]</code> 411column, continues to the present.</p> 412 413<p>The first line typically specifies the mean solar time observed 414before the introduction of standard time. Since there’s no line before 415that, it has no beginning. <code>8-) </code> For some places near the <a 416href="https://en.wikipedia.org/wiki/International_Date_Line">International 417Date Line</a>, the first <i>two</i> lines will show solar times 418differing by 24 hours; this corresponds to a movement of the Date 419Line. For example:</p> 420 421<pre> 422#Zone NAME GMTOFF RULES FORMAT [UNTIL] 423Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 424 -8:57:41 - LMT ... 425</pre> 426 427<p>When Alaska was purchased from Russia in 1867, the Date Line moved 428from the Alaska/Canada border to the Bering Strait; and the time in 429Alaska was then 24 hours earlier than it had 430been. <code><aside></code>(6 October in the Julian calendar, 431which Russia was still using then for religious reasons, was followed 432by <i>a second instance of the same day with a different name</i>, 18 433October in the Gregorian calendar. Isn’t civil time 434wonderful? <code>8-)</code>)<code></aside></code></p> 435 436<p>The abbreviation, “LMT” stands for “local mean 437time”, which is an invention of 438the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 439database</a> and was probably never actually used during the 440period. Furthermore, the value is almost certainly wrong except in the 441archetypal place after which the zone is named. (The tz database 442usually doesn’t provide a separate Zone record for places where 443nothing significant happened after 1970.)</p> 444 445<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 446<ul> 447<li>A hyphen, a kind of null value, means that we have not set our 448clocks ahead of standard time.</li> 449 450<li>An amount of time (usually but not necessarily “1:00” 451meaning one hour) means that we have set our clocks ahead by that 452amount.</li> 453 454<li>Some alphabetic string means that we <i>might have</i> set our 455clocks ahead; and we need to check the rule the name of which is the 456given alphabetic string.</li> 457</ul> 458 459<p>An example of a specific amount of time is:</p> 460<pre> 461#Zone NAME GMTOFF RULES FORMAT [UNTIL] 462Zone Pacific/Honolulu ... 1933 Apr 30 2:00 463 -10:30 1:00 HDT 1933 May 21 2:00 464 ... 465</pre> 466 467<p>Hawaii tried daylight saving time for three weeks in 1933 and 468decided they didn’t like it. <code>8-) </code>Note that 469the <code>GMTOFF</code> column always contains the standard time 470offset, so the wall clock time during this period was GMT − 47110:30 + 1:00 = GMT − 9:30.</p> 472 473<p>The <code>FORMAT</code> column specifies the usual abbreviation of 474the time zone name. It can have one of three forms:</p> 475<ul> 476 477<li>a string of three or more characters that are either ASCII alphanumerics, 478“<code>+</code>”, or “<code>-</code>”, 479in which case that’s the abbreviation</li> 480 481<li>a pair of strings separated by a slash 482(‘<code>/</code>’), in which case the first string is the 483abbreviation for the standard time name and the second string is the 484abbreviation for the daylight saving time name</li> 485 486<li>a string containing “<code>%s</code>,” in which case 487the “<code>%s</code>” will be replaced by the text in the 488appropriate Rule’s <code>LETTER</code> column</li> 489</ul> 490 491<p>The last two make sense only if there’s a named rule in effect.</p> 492 493<p>An example of a slash is:</p> 494<pre> 495#Zone NAME GMTOFF RULES FORMAT [UNTIL] 496Zone Europe/London ... 1996 497 0:00 EU GMT/BST 498</pre> 499 500<p>The current time in the UK is called either Greenwich mean time or 501British summer time.</p> 502 503<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 504happens when switching to a named rule. To what values should 505the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 506 507<ul> 508<li>If at least one transition has happened, use 509the <code>SAVE</code> and <code>LETTER</code> data from the most 510recent.</li> 511 512<li>If switching to a named rule before any transition has happened, 513assume standard time (<code>SAVE</code> zero), and use 514the <code>LETTER</code> data from the earliest transition with 515a <code>SAVE</code> of zero. 516 517</ul> 518 519<p>And three last things about the <code>FORMAT</code> column:</p> 520<ul> 521 522<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 523database</a> gives abbreviations for time zones in <i>popular 524usage</i>, which is not necessarily “correct” by law. For 525example, the last line in 526<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 527“HST” for “Hawaii standard time” even though the 528<a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 529name for that time zone is “Hawaii-Aleutian standard time.” 530This author has read that there are also some places in Australia where 531popular time zone names differ from the legal ones. 532 533<li>No attempt is made to <a 534href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 535the abbreviations. They are intended to be the values returned through the 536<code>"%Z"</code> format specifier to 537<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 538<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 539function in the 540<a href="http://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 541 542<li>If there is no generally-accepted abbreviation for a time zone, 543a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 544ahead of Greenwich. By convention, <code>-00</code> is used in a 545zone while uninhabited, where the offset is zero but in some sense 546the true offset is undefined. 547</ul> 548 549<p>As a final example, here’s the complete history for Hawaii:</p> 550 551<table border="1"> 552<tr> 553 <th colspan="6">Relevant Excerpts from the US Rules</th> 554</tr> 555<tr> 556 <td colspan="6" align="center"><table><tr><td> 557<pre> 558#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 559Rule US 1918 1919 - Oct lastSun 2:00 0 S 560Rule US 1942 only - Feb 9 2:00 1:00 W # War 561Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 562Rule US 1945 only - Sep 30 2:00 0 S 563</pre> 564 </td></tr></table></td> 565</tr> 566<tr> 567 <th colspan="6">The Zone Record</th> 568</tr> 569<tr> 570 <td colspan="6" align="center"><table><tr><td> 571<pre> 572#Zone NAME GMTOFF RULES FORMAT [UNTIL] 573Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 574 -10:30 - HST 1933 Apr 30 2:00 575 -10:30 1:00 HDT 1933 May 21 2:00 576 -10:30 US H%sT 1947 Jun 8 2:00 577 -10:00 - HST 578</pre> 579 </td></tr></table></td> 580</tr> 581<tr> 582 <th colspan="6">What We Infer</th> 583</tr> 584<tr> 585 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 586 <th rowspan="2">Adjust<br>Clocks</th> 587 <th colspan="2">Time Zone</th> 588 <th colspan="2">Ending at Local Time</th> 589</tr> 590<tr> 591 <th>Abbrv.</th> 592 <th>Name</th> 593 <th>Date</th> 594 <th>Time</th> 595</tr> 596<tr align="center"> 597 <td>−10:31:26</td> 598 <td>—</td> 599 <td>LMT</td> 600 <td>local mean time</td> 601 <td>1900-01-01</td> 602 <td>12:00</td> 603</tr> 604<tr align="center"> 605 <td>−10:30</td> 606 <td>+0:01:26</td> 607 <td>HST</td> 608 <td>Hawaii standard time</td> 609 <td>1933-04-30</td> 610 <td rowspan="3">02:00</td> 611</tr> 612<tr align="center"> 613 <td>−9:30</td> 614 <td>+1:00</td> 615 <td>HDT</td> 616 <td>Hawaii daylight time</td> 617 <td>1933-05-21</td> 618</tr> 619<tr align="center"> 620 <td>−10:30¹</td> 621 <td>−1:00¹</td> 622 <td>HST¹</td> 623 <td>Hawaii standard time</td> 624 <td>1942-02-09</td> 625</tr> 626<tr align="center"> 627 <td rowspan="2">−9:30</td> 628 <td>+1:00</td> 629 <td>HWT</td> 630 <td>Hawaii war time</td> 631 <td>1945-08-14</td> 632 <td>13:30²</td> 633</tr> 634<tr align="center"> 635 <td>0</td> 636 <td>HPT</td> 637 <td>Hawaii peace time</td> 638 <td>1945-09-30</td> 639 <td rowspan="2">02:00</td> 640</tr> 641<tr align="center"> 642 <td>−10:30</td> 643 <td>−1:00</td> 644 <td rowspan="2">HST</td> 645 <td rowspan="2">Hawaii standard time</td> 646 <td>1947-06-08</td> 647</tr> 648<tr align="center"> 649 <td>−10:00³</td> 650 <td>+0:30³</td> 651 <td colspan="2">—</td> 652</tr> 653<tr> 654 <td colspan="6"> 655 ¹Switching to US rules…most recent transition (in 1919) was to standard time 656 </td> 657</tr> 658<tr> 659 <td colspan="6"> 660 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 661 + (−9:30) = 13:30 local 662 </td> 663</tr> 664<tr> 665 <td colspan="6"> 666 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 667 the civil time in Hawaii has been 668 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 669 − 10:00 year-round. 670 </td> 671</tr> 672</table> 673 674<p>There will be a short quiz later. <code>8-)</code></p> 675 676<hr> 677<address> 678This web page is in the public domain, so clarified as of 6792015-10-20 by Bill Seymour. 680<br> 681All suggestions and corrections will be welcome; all flames will be amusing. 682Mail to was at pobox dot com. 683</address> 684</body> 685</html> 686