tz-how-to.html revision 1.1
1<!DOCTYPE html 2 PUBLIC "-//W3C//DTD HTML 4.01//EN" 3 "http://www.w3.org/TR/html4/strict.dtd"> 4<html> 5<head><title>How to Read the tz Database</title></head> 6<body> 7<h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 8Database</a> Source Files</h2> 9<h3>by Bill Seymour</h3> 10<p>This page uses the <code>America/Chicago</code> and 11<code>Pacific/Honolulu</code> zones as examples of how to infer 12times of day from the <a href="tz-link.htm">tz database</a> 13source files. It might be helpful, but not absolutely necessary, 14for the reader to have already downloaded the 15<a href="http://www.iana.org/time-zones/repository/tzdata-latest.tar.gz">latest 16release of the database</a> and become familiar with the basic layout 17of the data files. The format is explained in the “man 18page” for the zic compiler, <code>zic.8.txt</code>, in 19the <code>code</code> subdirectory.</p> 20 21<p>We’ll begin by talking about the rules for changing between standard 22and daylight saving time since we’ll need that information when we talk 23about the zones.</p> 24 25<p>First, let’s consider the special daylight saving time rules 26for Chicago (from the <code>northamerica</code> file in 27the <code>data</code> subdirectory):</p> 28 29<table border="1"> 30<tr> 31 <th colspan="6">From the Source File</th> 32</tr> 33<tr> 34 <td colspan="6" align="center"><table><tr><td> 35<pre> 36#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 37Rule Chicago 1920 only - Jun 13 2:00 1:00 D 38Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 39Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 40Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 41Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 42Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 43</pre> 44 </td></tr></table></td> 45</tr> 46<tr> 47 <th colspan="6">Reformatted a Bit</th> 48</tr> 49<tr> 50 <th>From</th> 51 <th>To</th> 52 <th colspan="2">On</th> 53 <th>At</th> 54 <th>Action</th> 55</tr> 56<tr align="center"> 57 <td colspan="2">1920 only</td> 58 <td colspan="2">June 13<small><sup>th</sup></small></td> 59 <td rowspan="6">02:00 local</td> 60 <td>go to daylight saving time</td> 61</tr> 62<tr align="center"> 63 <td>1920</td> 64 <td>1921</td> 65 <td rowspan="5">last Sunday</td> 66 <td>in October</td> 67 <td>return to standard time</td> 68</tr> 69<tr align="center"> 70 <td colspan="2">1921 only</td> 71 <td>in March</td> 72 <td rowspan="2">go to daylight saving time</td> 73</tr> 74<tr align="center"> 75 <td rowspan="2">1922</td> 76 <td>1966</td> 77 <td>in April</td> 78</tr> 79<tr align="center"> 80 <td>1954</td> 81 <td>in September</td> 82 <td rowspan="2">return to standard time</td> 83</tr> 84<tr align="center"> 85 <td>1955</td> 86 <td>1966</td> 87 <td>in October</td> 88</tr> 89</table> 90 91<p>We’ll basically just ignore the <code>TYPE</code> column. 92In the 2007j release, the most recent as of this writing, the 93<code>TYPE</code> column never contains anything but a hyphen, 94a kind of null value. (From the description in <code>zic.8.txt</code>, 95this appears to be a mechanism for removing years from a set 96in some localizable way. It’s used in the file, <code>pacificnew</code>, 97to determine whether a given year will have a US presidential election; 98but everything related to that use is commented out.) 99 100<p>The <code>SAVE</code> column contains the wall clock offset from 101local standard time. 102This is usually either zero for standard time or one hour for daylight 103saving time; but there’s no reason, in principle, why it can’t 104take on other values. 105 106<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 107column can contain a variable 108part of the usual abbreviation of the time zone’s name, or it can just 109be a hyphen if there’s no variable part. For example, the abbreviation 110used in the central time zone will be either “CST” or 111“CDT”. The variable part is ‘S’ or ‘D’; 112and, sure enough, that’s just what we find in 113the <code>LETTER</code> column 114in the <code>Chicago</code> rules. More about this when we talk about 115“Zone” lines. 116 117<p>One important thing to notice is that “Rule” lines 118want at once to be both <i>transitions</i> and <i>steady states</i>: 119<ul> 120<li>On the one hand, they represent transitions between standard and 121daylight saving time; and any number of Rule lines can be in effect 122during a given period (which will always be a non-empty set of 123contiguous calendar years).</li> 124<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 125columns contain state that exists between transitions. More about this 126when we talk about the US rules.</li> 127</ul> 128 129<p>In the example above, the transition to daylight saving time 130happened on the 13<small><sup>th</sup></small> of June in 1920, and on 131the last Sunday in March in 1921; but the return to standard time 132happened on the last Sunday in October in both of those 133years. Similarly, the rule for changing to daylight saving time was 134the same from 1922 to 1966; but the rule for returning to standard 135time changed in 1955. Got it?</p> 136 137<p>OK, now for the somewhat more interesting “US” rules:</p> 138 139<table border="1"> 140<tr> 141 <th colspan="6">From the Source File</th> 142</tr> 143<tr> 144 <td colspan="6" align="center"><table><tr><td> 145<pre> 146#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 147Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 148Rule US 1918 1919 - Oct lastSun 2:00 0 S 149Rule US 1942 only - Feb 9 2:00 1:00 W # War 150Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 151Rule US 1945 only - Sep 30 2:00 0 S 152Rule US 1967 2006 - Oct lastSun 2:00 0 S 153Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 154Rule US 1974 only - Jan 6 2:00 1:00 D 155Rule US 1975 only - Feb 23 2:00 1:00 D 156Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 157Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 158Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 159Rule US 2007 max - Nov Sun>=1 2:00 0 S 160</pre> 161 </td></tr></table></td> 162</tr> 163<tr> 164 <th colspan="6">Reformatted a Bit</th> 165</tr> 166<tr> 167 <th>From</th> 168 <th>To</th> 169 <th colspan="2">On</th> 170 <th>At</th> 171 <th>Action</th> 172</tr> 173<tr align="center"> 174 <td rowspan="2">1918</td> 175 <td rowspan="2">1919</td> 176 <td rowspan="2">last Sunday</td> 177 <td>in March</td> 178 <td rowspan="3">02:00 local</td> 179 <td>go to daylight saving time</td> 180</tr> 181<tr align="center"> 182 <td>in October</td> 183 <td>return to standard time</td> 184</tr> 185<tr align="center"> 186 <td colspan="2">1942 only</td> 187 <td colspan="2">February 9<small><sup>th</sup></small></td> 188 <td>go to “war time”</td> 189</tr> 190<tr align="center"> 191 <td colspan="2" rowspan="2">1945 only</td> 192 <td colspan="2">August 14<small><sup>th</sup></small></td> 193 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 194 <td> 195 rename “war time” to “peace<br>time;” 196 clocks don’t change 197 </td> 198</tr> 199<tr align="center"> 200 <td colspan="2">September 30<small><sup>th</sup></small></td> 201 <td rowspan="9">02:00 local</td> 202 <td rowspan="2">return to standard time</td> 203</tr> 204<tr align="center"> 205 <td rowspan="2">1967</td> 206 <td>2006</td> 207 <td rowspan="2">last Sunday</td> 208 <td>in October</td> 209</tr> 210<tr align="center"> 211 <td>1973</td> 212 <td>in April</td> 213 <td rowspan="6">go to daylight saving time</td> 214</tr> 215<tr align="center"> 216 <td colspan="2">1974 only</td> 217 <td colspan="2">January 6<small><sup>th</sup></small></td> 218</tr> 219<tr align="center"> 220 <td colspan="2">1975 only</td> 221 <td colspan="2">February 23<small><sup>rd</sup></small></td> 222</tr> 223<tr align="center"> 224 <td>1976</td> 225 <td>1986</td> 226 <td>last Sunday</td> 227 <td rowspan="2">in April</td> 228</tr> 229<tr align="center"> 230 <td>1987</td> 231 <td>2006</td> 232 <td>first Sunday</td> 233</tr> 234<tr align="center"> 235 <td rowspan="2">2007</td> 236 <td rowspan="2">present</td> 237 <td colspan="2">second Sunday in March</td> 238</tr> 239<tr align="center"> 240 <td colspan="2">first Sunday in November</td> 241 <td>return to standard time</td> 242</tr> 243</table> 244 245<p>There are two interesting things to note here.</p> 246 247<p>First, the time that something happens (in the <code>AT</code> 248column) is not necessarily the local wall clock time. The time can be 249suffixed with ‘s’ (for “standard”) to mean 250local standard time (different from wall clock time when observing 251daylight saving time); or it can be suffixed with ‘g’, 252‘u’, or ‘z’, all three of which mean the 253standard time at the 254<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridan</a>. 255‘g’ stands for “<a 256href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 257‘u’ stands for “<a 258href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 259href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 260(whichever was official at the time); ‘z’ stands for the 261<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 262Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 263The time can also be suffixed with ‘w’ meaning “wall 264clock time;” but it usually isn’t because that’s the 265default.</p> 266 267<p>Second, the day in the <code>ON</code> column, in addition to 268“<code>lastSun</code>” or a particular day of the month, 269can have the form, “<code>Sun>=</code><i>x</i>” or 270“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 271of the month. For example, “<code>Sun>=8</code>” means 272“the first Sunday on or after the eighth of the month,” in 273other words, the second Sunday of the month. Furthermore, although 274there are no examples above, the weekday needn’t be 275“<code>Sun</code>” in either form, but can be the usual 276three-character English abbreviation for any day of the week.</p> 277 278<p>And the US rules give us more examples of a couple of things 279already mentioned:</p> 280 281<ul> 282<li>The rules for changing to and from daylight saving time are 283actually <i>different sets</i> of rules; and the two sets can change 284independently. Consider, for example, that the rule for the return to 285standard time stayed the same from 1967 to 2006; but the rule for the 286transition to daylight saving time changed several times in the same 287period. There can also be periods, 1946 to 1966 for example, when no 288rule from this group is in effect, and so either no transition 289happened in those years, or some other rule is in effect (perhaps a 290state or other more local rule).</li> 291 292<li>The <code>SAVE</code> and <code>LETTER</code> columns 293contain <i>steady state</i>, not transitions. Consider, for example, 294the transition from “war time” to “peace time” 295that happened on August 14, 1945. The “1:00” in 296the <code>SAVE</code> column is <i>not</i> an instruction to advance 297the clock an hour. It means that clocks should <i>be</i> one hour 298ahead of standard time, which they already are because of the previous 299rule, so there should be no change.</li> 300 301</ul> 302 303<p>OK, now let’s look at a Zone record:</p> 304 305<table border="1"> 306<tr> 307 <th colspan="5">From the Source File</th> 308</tr> 309<tr> 310 <td colspan="6" align="center"><table><tr><td> 311<pre> 312#Zone NAME GMTOFF RULES FORMAT [UNTIL] 313Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 314 -6:00 US C%sT 1920 315 -6:00 Chicago C%sT 1936 Mar 1 2:00 316 -5:00 - EST 1936 Nov 15 2:00 317 -6:00 Chicago C%sT 1942 318 -6:00 US C%sT 1946 319 -6:00 Chicago C%sT 1967 320 -6:00 US C%sT 321</pre> 322 </td></tr></table></td> 323</tr> 324<tr> 325 <th colspan="5">Columns Renamed</th> 326</tr> 327<tr> 328 <th rowspan="2">Standard Offset<br> 329 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 330 Meridian</a></th> 331 <th rowspan="2">Daylight<br>Saving Time</th> 332 <th rowspan="2">Abbreviation(s)</th> 333 <th colspan="2">Ending at Local Time</th> 334</tr> 335<tr> 336 <th>Date</th> 337 <th>Time</th> 338</tr> 339<tr align="center"> 340 <td>−5:50:36</td> 341 <td>not observed</td> 342 <td>LMT</td> 343 <td>1883-11-18</td> 344 <td>12:09:24</td> 345</tr> 346<tr align="center"> 347 <td rowspan="2">−6:00:00</td> 348 <td>US rules</td> 349 <td rowspan="2">CST or CDT</td> 350 <td>1920-01-01</td> 351 <td>00:00:00</td> 352</tr> 353<tr align="center"> 354 <td>Chicago rules</td> 355 <td>1936-03-01</td> 356 <td rowspan="2">02:00:00</td> 357</tr> 358<tr align="center"> 359 <td>−5:00:00</td> 360 <td>not observed</td> 361 <td>EST</td> 362 <td>1936-11-15</td> 363</tr> 364<tr align="center"> 365 <td rowspan="4">−6:00:00</td> 366 <td>Chicago rules</td> 367 <td>CST or CDT</td> 368 <td>1942-01-01</td> 369 <td rowspan="3">00:00:00</td> 370</tr> 371<tr align="center"> 372 <td>US rules</td> 373 <td>CST, CWT or CPT</td> 374 <td>1946-01-01</td> 375</tr> 376<tr align="center"> 377 <td>Chicago rules</td> 378 <td rowspan="2">CST or CDT</td> 379 <td>1967-01-01</td> 380</tr> 381<tr align="center"> 382 <td>US rules</td> 383 <td colspan="2">—</td> 384</tr> 385</table> 386 387<p>There are a couple of interesting differences between Zones and Rules.</p> 388 389<p>First, and somewhat trivially, whereas Rules are considered to 390contain one or more records, a Zone is considered to be a single 391record with zero or more <i>continuation lines</i>. Thus, the keyword, 392“<code>Zone</code>,” and the zone name are not 393repeated. The last line is the one without anything in 394the <code>[UNTIL]</code> column.</p> 395 396<p>Second, and more fundamentally, each line of a Zone represents a 397steady state, not a transition between states. The state exists from 398the date and time in the previous line’s <code>[UNTIL]</code> 399column up to the date and time in the current 400line’s <code>[UNTIL]</code> column. In other words, the date and 401time in the <code>[UNTIL]</code> column is the instant that separates 402this state from the next. Where that would be ambiguous because 403we’re setting our clocks back, the <code>[UNTIL]</code> column 404specifies the first occurrence of the instant. The state specified by 405the last line, the one without anything in the <code>[UNTIL]</code> 406column, continues to the present.</p> 407 408<p>The first line typically specifies the mean solar time observed 409before the introduction of standard time. Since there’s no line before 410that, it has no beginning. <code>8-) </code> For some places near the <a 411href="https://en.wikipedia.org/wiki/International_Date_Line">International 412Date Line</a>, the first <i>two</i> lines will show solar times 413differing by 24 hours; this corresponds to a movement of the Date 414Line. For example:</p> 415 416<pre> 417#Zone NAME GMTOFF RULES FORMAT [UNTIL] 418Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 419 -8:57:41 - LMT ... 420</pre> 421 422<p>When Alaska was purchased from Russia in 1867, the Date Line moved 423from the Alaska/Canada border to the Bering Strait; and the time in 424Alaska was then 24 hours earlier than it had 425been. <code><aside></code>(6 October in the Julian calendar, 426which Russia was still using then for religious reasons, was followed 427by <i>a second instance of the same day with a different name</i>, 18 428October in the Gregorian calendar. Isn’t civil time 429wonderful? <code>8-)</code>)<code></aside></code></p> 430 431<p>The abbreviation, “LMT” stands for “local mean 432time”, which is an invention of 433the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 434database</a> and was probably never actually used during the 435period. Furthermore, the value is almost certainly wrong except in the 436archetypal place after which the zone is named. (The tz database 437usually doesn’t provide a separate Zone record for places where 438nothing significant happened after 1970.)</p> 439 440<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 441<ul> 442<li>A hyphen, a kind of null value, means that we have not set our 443clocks ahead of standard time.</li> 444 445<li>An amount of time (usually but not necessarily “1:00” 446meaning one hour) means that we have set our clocks ahead by that 447amount.</li> 448 449<li>Some alphabetic string means that we <i>might have</i> set our 450clocks ahead; and we need to check the rule the name of which is the 451given alphabetic string.</li> 452</ul> 453 454<p>An example of a specific amount of time is:</p> 455<pre> 456#Zone NAME GMTOFF RULES FORMAT [UNTIL] 457Zone Pacific/Honolulu ... 1933 Apr 30 2:00 458 -10:30 1:00 HDT 1933 May 21 2:00 459 ... 460</pre> 461 462<p>Hawaii tried daylight saving time for three weeks in 1933 and 463decided they didn’t like it. <code>8-) </code>Note that 464the <code>GMTOFF</code> column always contains the standard time 465offset, so the wall clock time during this period was GMT − 46610:30 + 1:00 = GMT − 9:30.</p> 467 468<p>The <code>FORMAT</code> column specifies the usual abbreviation of 469the time zone name. It can have one of three forms:</p> 470<ul> 471 472<li>a string of three or more characters that are either ASCII alphanumerics, 473“<code>+</code>”, or “<code>-</code>”, 474in which case that’s the abbreviation</li> 475 476<li>a pair of strings separated by a slash 477(‘<code>/</code>’), in which case the first string is the 478abbreviation for the standard time name and the second string is the 479abbreviation for the daylight saving time name</li> 480 481<li>a string containing “<code>%s</code>,” in which case 482the “<code>%s</code>” will be replaced by the text in the 483appropriate Rule’s <code>LETTER</code> column</li> 484</ul> 485 486<p>The last two make sense only if there’s a named rule in effect.</p> 487 488<p>An example of a slash is:</p> 489<pre> 490#Zone NAME GMTOFF RULES FORMAT [UNTIL] 491Zone Europe/London ... 1996 492 0:00 EU GMT/BST 493</pre> 494 495<p>The current time in the UK is called either Greenwich mean time or 496British summer time.</p> 497 498<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 499happens when switching to a named rule. To what values should 500the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 501 502<ul> 503<li>If at least one transition has happened, use 504the <code>SAVE</code> and <code>LETTER</code> data from the most 505recent.</li> 506 507<li>If switching to a named rule before any transition has happened, 508assume standard time (<code>SAVE</code> zero), and use 509the <code>LETTER</code> data from the earliest transition with 510a <code>SAVE</code> of zero. 511 512</ul> 513 514<p>And three last things about the <code>FORMAT</code> column:</p> 515<ul> 516 517<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 518database</a> gives abbreviations for time zone names in <i>popular 519usage</i>, which is not necessarily “correct” by law. For 520example, the last line in 521<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 522“HST” for “Hawaii standard time” even though the 523<a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a> 524name for that time zone is “Hawaii-Aleutian standard time.” 525This author has read that there are also some places in Australia where 526popular time zone names differ from the legal ones. 527 528<li>No attempt is made to <a 529href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 530the abbreviations. They are intended to be the values returned through the 531<code>"%Z"</code> format specifier to 532<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 533<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 534function in the 535<a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 536 537<li>If there is no generally-accepted abbreviation for a time zone, 538a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 539ahead of Greenwich. By convention, <code>-00</code> is used in a 540zone while uninhabited, where the offset is zero but in some sense 541the true offset is undefined. 542</ul> 543 544<p>As a final example, here’s the complete history for Hawaii:</p> 545 546<table border="1"> 547<tr> 548 <th colspan="6">Relevant Excerpts from the US Rules</th> 549</tr> 550<tr> 551 <td colspan="6" align="center"><table><tr><td> 552<pre> 553#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 554Rule US 1918 1919 - Oct lastSun 2:00 0 S 555Rule US 1942 only - Feb 9 2:00 1:00 W # War 556Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 557Rule US 1945 only - Sep 30 2:00 0 S 558</pre> 559 </td></tr></table></td> 560</tr> 561<tr> 562 <th colspan="6">The Zone Record</th> 563</tr> 564<tr> 565 <td colspan="6" align="center"><table><tr><td> 566<pre> 567#Zone NAME GMTOFF RULES FORMAT [UNTIL] 568Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 569 -10:30 - HST 1933 Apr 30 2:00 570 -10:30 1:00 HDT 1933 May 21 2:00 571 -10:30 US H%sT 1947 Jun 8 2:00 572 -10:00 - HST 573</pre> 574 </td></tr></table></td> 575</tr> 576<tr> 577 <th colspan="6">What We Infer</th> 578</tr> 579<tr> 580 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 581 <th rowspan="2">Adjust<br>Clocks</th> 582 <th colspan="2">Time Zone</th> 583 <th colspan="2">Ending at Local Time</th> 584</tr> 585<tr> 586 <th>Abbrv.</th> 587 <th>Name</th> 588 <th>Date</th> 589 <th>Time</th> 590</tr> 591<tr align="center"> 592 <td>−10:31:26</td> 593 <td>—</td> 594 <td>LMT</td> 595 <td>local mean time</td> 596 <td>1900-01-01</td> 597 <td>12:00</td> 598</tr> 599<tr align="center"> 600 <td>−10:30</td> 601 <td>+0:01:26</td> 602 <td>HST</td> 603 <td>Hawaii standard time</td> 604 <td>1933-04-30</td> 605 <td rowspan="3">02:00</td> 606</tr> 607<tr align="center"> 608 <td>−9:30</td> 609 <td>+1:00</td> 610 <td>HDT</td> 611 <td>Hawaii daylight time</td> 612 <td>1933-05-21</td> 613</tr> 614<tr align="center"> 615 <td>−10:30¹</td> 616 <td>−1:00¹</td> 617 <td>HST¹</td> 618 <td>Hawaii standard time</td> 619 <td>1942-02-09</td> 620</tr> 621<tr align="center"> 622 <td rowspan="2">−9:30</td> 623 <td>+1:00</td> 624 <td>HWT</td> 625 <td>Hawaii war time</td> 626 <td>1945-08-14</td> 627 <td>13:30²</td> 628</tr> 629<tr align="center"> 630 <td>0</td> 631 <td>HPT</td> 632 <td>Hawaii peace time</td> 633 <td>1945-09-30</td> 634 <td rowspan="2">02:00</td> 635</tr> 636<tr align="center"> 637 <td>−10:30</td> 638 <td>−1:00</td> 639 <td rowspan="2">HST</td> 640 <td rowspan="2">Hawaii standard time</td> 641 <td>1947-06-08</td> 642</tr> 643<tr align="center"> 644 <td>−10:00³</td> 645 <td>+0:30³</td> 646 <td colspan="2">—</td> 647</tr> 648<tr> 649 <td colspan="6"> 650 ¹Switching to US rules…most recent transition (in 1919) was to standard time 651 </td> 652</tr> 653<tr> 654 <td colspan="6"> 655 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 656 + (−9:30) = 13:30 local 657 </td> 658</tr> 659<tr> 660 <td colspan="6"> 661 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 662 the civil time in Hawaii has been 663 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 664 − 10:00 year-round. 665 </td> 666</tr> 667</table> 668 669<p>There will be a short quiz later. <code>8-)</code></p> 670 671<hr> 672<address> 673This web page is in the public domain, so clarified as of 6742015-10-20 by Bill Seymour. 675<br> 676All suggestions and corrections will be welcome; all flames will be amusing. 677Mail to was at pobox dot com. 678</address> 679</body> 680</html> 681