tz-how-to.html revision 1.2
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 15latest release of the database and become familiar with the basic layout 16of the data files. The format is explained in the “man 17page” for the zic compiler, <code>zic.8.txt</code>, in 18the <code>code</code> subdirectory.</p> 19 20<p>We’ll begin by talking about the rules for changing between standard 21and daylight saving time since we’ll need that information when we talk 22about the zones.</p> 23 24<p>First, let’s consider the special daylight saving time rules 25for Chicago (from the <code>northamerica</code> file in 26the <code>data</code> subdirectory):</p> 27 28<table border="1"> 29<tr> 30 <th colspan="6">From the Source File</th> 31</tr> 32<tr> 33 <td colspan="6" align="center"><table><tr><td> 34<pre> 35#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 36Rule Chicago 1920 only - Jun 13 2:00 1:00 D 37Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 38Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 39Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 40Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 41Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 42</pre> 43 </td></tr></table></td> 44</tr> 45<tr> 46 <th colspan="6">Reformatted a Bit</th> 47</tr> 48<tr> 49 <th>From</th> 50 <th>To</th> 51 <th colspan="2">On</th> 52 <th>At</th> 53 <th>Action</th> 54</tr> 55<tr align="center"> 56 <td colspan="2">1920 only</td> 57 <td colspan="2">June 13<small><sup>th</sup></small></td> 58 <td rowspan="6">02:00 local</td> 59 <td>go to daylight saving time</td> 60</tr> 61<tr align="center"> 62 <td>1920</td> 63 <td>1921</td> 64 <td rowspan="5">last Sunday</td> 65 <td>in October</td> 66 <td>return to standard time</td> 67</tr> 68<tr align="center"> 69 <td colspan="2">1921 only</td> 70 <td>in March</td> 71 <td rowspan="2">go to daylight saving time</td> 72</tr> 73<tr align="center"> 74 <td rowspan="2">1922</td> 75 <td>1966</td> 76 <td>in April</td> 77</tr> 78<tr align="center"> 79 <td>1954</td> 80 <td>in September</td> 81 <td rowspan="2">return to standard time</td> 82</tr> 83<tr align="center"> 84 <td>1955</td> 85 <td>1966</td> 86 <td>in October</td> 87</tr> 88</table> 89 90<p>We’ll basically just ignore the <code>TYPE</code> column. 91In the 2007j release, the most recent as of this writing, the 92<code>TYPE</code> column never contains anything but a hyphen, 93a kind of null value. (From the description in <code>zic.8.txt</code>, 94this appears to be a mechanism for removing years from a set 95in some localizable way. It’s used in the file, <code>pacificnew</code>, 96to determine whether a given year will have a US presidential election; 97but everything related to that use is commented out.) 98 99<p>The <code>SAVE</code> column contains the wall clock offset from 100local standard time. 101This is usually either zero for standard time or one hour for daylight 102saving time; but there’s no reason, in principle, why it can’t 103take on other values. 104 105<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 106column can contain a variable 107part of the usual abbreviation of the time zone’s name, or it can just 108be a hyphen if there’s no variable part. For example, the abbreviation 109used in the central time zone will be either “CST” or 110“CDT”. The variable part is ‘S’ or ‘D’; 111and, sure enough, that’s just what we find in 112the <code>LETTER</code> column 113in the <code>Chicago</code> rules. More about this when we talk about 114“Zone” lines. 115 116<p>One important thing to notice is that “Rule” lines 117want at once to be both <i>transitions</i> and <i>steady states</i>: 118<ul> 119<li>On the one hand, they represent transitions between standard and 120daylight saving time; and any number of Rule lines can be in effect 121during a given period (which will always be a non-empty set of 122contiguous calendar years).</li> 123<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 124columns contain state that exists between transitions. More about this 125when we talk about the US rules.</li> 126</ul> 127 128<p>In the example above, the transition to daylight saving time 129happened on the 13<small><sup>th</sup></small> of June in 1920, and on 130the last Sunday in March in 1921; but the return to standard time 131happened on the last Sunday in October in both of those 132years. Similarly, the rule for changing to daylight saving time was 133the same from 1922 to 1966; but the rule for returning to standard 134time changed in 1955. Got it?</p> 135 136<p>OK, now for the somewhat more interesting “US” rules:</p> 137 138<table border="1"> 139<tr> 140 <th colspan="6">From the Source File</th> 141</tr> 142<tr> 143 <td colspan="6" align="center"><table><tr><td> 144<pre> 145#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 146Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 147Rule US 1918 1919 - Oct lastSun 2:00 0 S 148Rule US 1942 only - Feb 9 2:00 1:00 W # War 149Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 150Rule US 1945 only - Sep 30 2:00 0 S 151Rule US 1967 2006 - Oct lastSun 2:00 0 S 152Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 153Rule US 1974 only - Jan 6 2:00 1:00 D 154Rule US 1975 only - Feb 23 2:00 1:00 D 155Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 156Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 157Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 158Rule US 2007 max - Nov Sun>=1 2:00 0 S 159</pre> 160 </td></tr></table></td> 161</tr> 162<tr> 163 <th colspan="6">Reformatted a Bit</th> 164</tr> 165<tr> 166 <th>From</th> 167 <th>To</th> 168 <th colspan="2">On</th> 169 <th>At</th> 170 <th>Action</th> 171</tr> 172<tr align="center"> 173 <td rowspan="2">1918</td> 174 <td rowspan="2">1919</td> 175 <td rowspan="2">last Sunday</td> 176 <td>in March</td> 177 <td rowspan="3">02:00 local</td> 178 <td>go to daylight saving time</td> 179</tr> 180<tr align="center"> 181 <td>in October</td> 182 <td>return to standard time</td> 183</tr> 184<tr align="center"> 185 <td colspan="2">1942 only</td> 186 <td colspan="2">February 9<small><sup>th</sup></small></td> 187 <td>go to “war time”</td> 188</tr> 189<tr align="center"> 190 <td colspan="2" rowspan="2">1945 only</td> 191 <td colspan="2">August 14<small><sup>th</sup></small></td> 192 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 193 <td> 194 rename “war time” to “peace<br>time;” 195 clocks don’t change 196 </td> 197</tr> 198<tr align="center"> 199 <td colspan="2">September 30<small><sup>th</sup></small></td> 200 <td rowspan="9">02:00 local</td> 201 <td rowspan="2">return to standard time</td> 202</tr> 203<tr align="center"> 204 <td rowspan="2">1967</td> 205 <td>2006</td> 206 <td rowspan="2">last Sunday</td> 207 <td>in October</td> 208</tr> 209<tr align="center"> 210 <td>1973</td> 211 <td>in April</td> 212 <td rowspan="6">go to daylight saving time</td> 213</tr> 214<tr align="center"> 215 <td colspan="2">1974 only</td> 216 <td colspan="2">January 6<small><sup>th</sup></small></td> 217</tr> 218<tr align="center"> 219 <td colspan="2">1975 only</td> 220 <td colspan="2">February 23<small><sup>rd</sup></small></td> 221</tr> 222<tr align="center"> 223 <td>1976</td> 224 <td>1986</td> 225 <td>last Sunday</td> 226 <td rowspan="2">in April</td> 227</tr> 228<tr align="center"> 229 <td>1987</td> 230 <td>2006</td> 231 <td>first Sunday</td> 232</tr> 233<tr align="center"> 234 <td rowspan="2">2007</td> 235 <td rowspan="2">present</td> 236 <td colspan="2">second Sunday in March</td> 237</tr> 238<tr align="center"> 239 <td colspan="2">first Sunday in November</td> 240 <td>return to standard time</td> 241</tr> 242</table> 243 244<p>There are two interesting things to note here.</p> 245 246<p>First, the time that something happens (in the <code>AT</code> 247column) is not necessarily the local wall clock time. The time can be 248suffixed with ‘s’ (for “standard”) to mean 249local standard time (different from wall clock time when observing 250daylight saving time); or it can be suffixed with ‘g’, 251‘u’, or ‘z’, all three of which mean the 252standard time at the 253<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 254‘g’ stands for “<a 255href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 256‘u’ stands for “<a 257href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 258href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 259(whichever was official at the time); ‘z’ stands for the 260<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 261Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 262The time can also be suffixed with ‘w’ meaning “wall 263clock time;” but it usually isn’t because that’s the 264default.</p> 265 266<p>Second, the day in the <code>ON</code> column, in addition to 267“<code>lastSun</code>” or a particular day of the month, 268can have the form, “<code>Sun>=</code><i>x</i>” or 269“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 270of the month. For example, “<code>Sun>=8</code>” means 271“the first Sunday on or after the eighth of the month,” in 272other words, the second Sunday of the month. Furthermore, although 273there are no examples above, the weekday needn’t be 274“<code>Sun</code>” in either form, but can be the usual 275three-character English abbreviation for any day of the week.</p> 276 277<p>And the US rules give us more examples of a couple of things 278already mentioned:</p> 279 280<ul> 281<li>The rules for changing to and from daylight saving time are 282actually <i>different sets</i> of rules; and the two sets can change 283independently. Consider, for example, that the rule for the return to 284standard time stayed the same from 1967 to 2006; but the rule for the 285transition to daylight saving time changed several times in the same 286period. There can also be periods, 1946 to 1966 for example, when no 287rule from this group is in effect, and so either no transition 288happened in those years, or some other rule is in effect (perhaps a 289state or other more local rule).</li> 290 291<li>The <code>SAVE</code> and <code>LETTER</code> columns 292contain <i>steady state</i>, not transitions. Consider, for example, 293the transition from “war time” to “peace time” 294that happened on August 14, 1945. The “1:00” in 295the <code>SAVE</code> column is <i>not</i> an instruction to advance 296the clock an hour. It means that clocks should <i>be</i> one hour 297ahead of standard time, which they already are because of the previous 298rule, so there should be no change.</li> 299 300</ul> 301 302<p>OK, now let’s look at a Zone record:</p> 303 304<table border="1"> 305<tr> 306 <th colspan="5">From the Source File</th> 307</tr> 308<tr> 309 <td colspan="6" align="center"><table><tr><td> 310<pre> 311#Zone NAME GMTOFF RULES FORMAT [UNTIL] 312Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 313 -6:00 US C%sT 1920 314 -6:00 Chicago C%sT 1936 Mar 1 2:00 315 -5:00 - EST 1936 Nov 15 2:00 316 -6:00 Chicago C%sT 1942 317 -6:00 US C%sT 1946 318 -6:00 Chicago C%sT 1967 319 -6:00 US C%sT 320</pre> 321 </td></tr></table></td> 322</tr> 323<tr> 324 <th colspan="5">Columns Renamed</th> 325</tr> 326<tr> 327 <th rowspan="2">Standard Offset<br> 328 from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 329 Meridian</a></th> 330 <th rowspan="2">Daylight<br>Saving Time</th> 331 <th rowspan="2">Abbreviation(s)</th> 332 <th colspan="2">Ending at Local Time</th> 333</tr> 334<tr> 335 <th>Date</th> 336 <th>Time</th> 337</tr> 338<tr align="center"> 339 <td>−5:50:36</td> 340 <td>not observed</td> 341 <td>LMT</td> 342 <td>1883-11-18</td> 343 <td>12:09:24</td> 344</tr> 345<tr align="center"> 346 <td rowspan="2">−6:00:00</td> 347 <td>US rules</td> 348 <td rowspan="2">CST or CDT</td> 349 <td>1920-01-01</td> 350 <td>00:00:00</td> 351</tr> 352<tr align="center"> 353 <td>Chicago rules</td> 354 <td>1936-03-01</td> 355 <td rowspan="2">02:00:00</td> 356</tr> 357<tr align="center"> 358 <td>−5:00:00</td> 359 <td>not observed</td> 360 <td>EST</td> 361 <td>1936-11-15</td> 362</tr> 363<tr align="center"> 364 <td rowspan="4">−6:00:00</td> 365 <td>Chicago rules</td> 366 <td>CST or CDT</td> 367 <td>1942-01-01</td> 368 <td rowspan="3">00:00:00</td> 369</tr> 370<tr align="center"> 371 <td>US rules</td> 372 <td>CST, CWT or CPT</td> 373 <td>1946-01-01</td> 374</tr> 375<tr align="center"> 376 <td>Chicago rules</td> 377 <td rowspan="2">CST or CDT</td> 378 <td>1967-01-01</td> 379</tr> 380<tr align="center"> 381 <td>US rules</td> 382 <td colspan="2">—</td> 383</tr> 384</table> 385 386<p>There are a couple of interesting differences between Zones and Rules.</p> 387 388<p>First, and somewhat trivially, whereas Rules are considered to 389contain one or more records, a Zone is considered to be a single 390record with zero or more <i>continuation lines</i>. Thus, the keyword, 391“<code>Zone</code>,” and the zone name are not 392repeated. The last line is the one without anything in 393the <code>[UNTIL]</code> column.</p> 394 395<p>Second, and more fundamentally, each line of a Zone represents a 396steady state, not a transition between states. The state exists from 397the date and time in the previous line’s <code>[UNTIL]</code> 398column up to the date and time in the current 399line’s <code>[UNTIL]</code> column. In other words, the date and 400time in the <code>[UNTIL]</code> column is the instant that separates 401this state from the next. Where that would be ambiguous because 402we’re setting our clocks back, the <code>[UNTIL]</code> column 403specifies the first occurrence of the instant. The state specified by 404the last line, the one without anything in the <code>[UNTIL]</code> 405column, continues to the present.</p> 406 407<p>The first line typically specifies the mean solar time observed 408before the introduction of standard time. Since there’s no line before 409that, it has no beginning. <code>8-) </code> For some places near the <a 410href="https://en.wikipedia.org/wiki/International_Date_Line">International 411Date Line</a>, the first <i>two</i> lines will show solar times 412differing by 24 hours; this corresponds to a movement of the Date 413Line. For example:</p> 414 415<pre> 416#Zone NAME GMTOFF RULES FORMAT [UNTIL] 417Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 418 -8:57:41 - LMT ... 419</pre> 420 421<p>When Alaska was purchased from Russia in 1867, the Date Line moved 422from the Alaska/Canada border to the Bering Strait; and the time in 423Alaska was then 24 hours earlier than it had 424been. <code><aside></code>(6 October in the Julian calendar, 425which Russia was still using then for religious reasons, was followed 426by <i>a second instance of the same day with a different name</i>, 18 427October in the Gregorian calendar. Isn’t civil time 428wonderful? <code>8-)</code>)<code></aside></code></p> 429 430<p>The abbreviation, “LMT” stands for “local mean 431time”, which is an invention of 432the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 433database</a> and was probably never actually used during the 434period. Furthermore, the value is almost certainly wrong except in the 435archetypal place after which the zone is named. (The tz database 436usually doesn’t provide a separate Zone record for places where 437nothing significant happened after 1970.)</p> 438 439<p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 440<ul> 441<li>A hyphen, a kind of null value, means that we have not set our 442clocks ahead of standard time.</li> 443 444<li>An amount of time (usually but not necessarily “1:00” 445meaning one hour) means that we have set our clocks ahead by that 446amount.</li> 447 448<li>Some alphabetic string means that we <i>might have</i> set our 449clocks ahead; and we need to check the rule the name of which is the 450given alphabetic string.</li> 451</ul> 452 453<p>An example of a specific amount of time is:</p> 454<pre> 455#Zone NAME GMTOFF RULES FORMAT [UNTIL] 456Zone Pacific/Honolulu ... 1933 Apr 30 2:00 457 -10:30 1:00 HDT 1933 May 21 2:00 458 ... 459</pre> 460 461<p>Hawaii tried daylight saving time for three weeks in 1933 and 462decided they didn’t like it. <code>8-) </code>Note that 463the <code>GMTOFF</code> column always contains the standard time 464offset, so the wall clock time during this period was GMT − 46510:30 + 1:00 = GMT − 9:30.</p> 466 467<p>The <code>FORMAT</code> column specifies the usual abbreviation of 468the time zone name. It can have one of three forms:</p> 469<ul> 470 471<li>a string of three or more characters that are either ASCII alphanumerics, 472“<code>+</code>”, or “<code>-</code>”, 473in which case that’s the abbreviation</li> 474 475<li>a pair of strings separated by a slash 476(‘<code>/</code>’), in which case the first string is the 477abbreviation for the standard time name and the second string is the 478abbreviation for the daylight saving time name</li> 479 480<li>a string containing “<code>%s</code>,” in which case 481the “<code>%s</code>” will be replaced by the text in the 482appropriate Rule’s <code>LETTER</code> column</li> 483</ul> 484 485<p>The last two make sense only if there’s a named rule in effect.</p> 486 487<p>An example of a slash is:</p> 488<pre> 489#Zone NAME GMTOFF RULES FORMAT [UNTIL] 490Zone Europe/London ... 1996 491 0:00 EU GMT/BST 492</pre> 493 494<p>The current time in the UK is called either Greenwich mean time or 495British summer time.</p> 496 497<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 498happens when switching to a named rule. To what values should 499the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 500 501<ul> 502<li>If at least one transition has happened, use 503the <code>SAVE</code> and <code>LETTER</code> data from the most 504recent.</li> 505 506<li>If switching to a named rule before any transition has happened, 507assume standard time (<code>SAVE</code> zero), and use 508the <code>LETTER</code> data from the earliest transition with 509a <code>SAVE</code> of zero. 510 511</ul> 512 513<p>And three last things about the <code>FORMAT</code> column:</p> 514<ul> 515 516<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 517database</a> gives abbreviations for time zone names in <i>popular 518usage</i>, which is not necessarily “correct” by law. For 519example, the last line in 520<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 521“HST” for “Hawaii standard time” even though the 522<a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a> 523name for that time zone is “Hawaii-Aleutian standard time.” 524This author has read that there are also some places in Australia where 525popular time zone names differ from the legal ones. 526 527<li>No attempt is made to <a 528href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 529the abbreviations. They are intended to be the values returned through the 530<code>"%Z"</code> format specifier to 531<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 532<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 533function in the 534<a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 535 536<li>If there is no generally-accepted abbreviation for a time zone, 537a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 538ahead of Greenwich. By convention, <code>-00</code> is used in a 539zone while uninhabited, where the offset is zero but in some sense 540the true offset is undefined. 541</ul> 542 543<p>As a final example, here’s the complete history for Hawaii:</p> 544 545<table border="1"> 546<tr> 547 <th colspan="6">Relevant Excerpts from the US Rules</th> 548</tr> 549<tr> 550 <td colspan="6" align="center"><table><tr><td> 551<pre> 552#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 553Rule US 1918 1919 - Oct lastSun 2:00 0 S 554Rule US 1942 only - Feb 9 2:00 1:00 W # War 555Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 556Rule US 1945 only - Sep 30 2:00 0 S 557</pre> 558 </td></tr></table></td> 559</tr> 560<tr> 561 <th colspan="6">The Zone Record</th> 562</tr> 563<tr> 564 <td colspan="6" align="center"><table><tr><td> 565<pre> 566#Zone NAME GMTOFF RULES FORMAT [UNTIL] 567Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 568 -10:30 - HST 1933 Apr 30 2:00 569 -10:30 1:00 HDT 1933 May 21 2:00 570 -10:30 US H%sT 1947 Jun 8 2:00 571 -10:00 - HST 572</pre> 573 </td></tr></table></td> 574</tr> 575<tr> 576 <th colspan="6">What We Infer</th> 577</tr> 578<tr> 579 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 580 <th rowspan="2">Adjust<br>Clocks</th> 581 <th colspan="2">Time Zone</th> 582 <th colspan="2">Ending at Local Time</th> 583</tr> 584<tr> 585 <th>Abbrv.</th> 586 <th>Name</th> 587 <th>Date</th> 588 <th>Time</th> 589</tr> 590<tr align="center"> 591 <td>−10:31:26</td> 592 <td>—</td> 593 <td>LMT</td> 594 <td>local mean time</td> 595 <td>1900-01-01</td> 596 <td>12:00</td> 597</tr> 598<tr align="center"> 599 <td>−10:30</td> 600 <td>+0:01:26</td> 601 <td>HST</td> 602 <td>Hawaii standard time</td> 603 <td>1933-04-30</td> 604 <td rowspan="3">02:00</td> 605</tr> 606<tr align="center"> 607 <td>−9:30</td> 608 <td>+1:00</td> 609 <td>HDT</td> 610 <td>Hawaii daylight time</td> 611 <td>1933-05-21</td> 612</tr> 613<tr align="center"> 614 <td>−10:30¹</td> 615 <td>−1:00¹</td> 616 <td>HST¹</td> 617 <td>Hawaii standard time</td> 618 <td>1942-02-09</td> 619</tr> 620<tr align="center"> 621 <td rowspan="2">−9:30</td> 622 <td>+1:00</td> 623 <td>HWT</td> 624 <td>Hawaii war time</td> 625 <td>1945-08-14</td> 626 <td>13:30²</td> 627</tr> 628<tr align="center"> 629 <td>0</td> 630 <td>HPT</td> 631 <td>Hawaii peace time</td> 632 <td>1945-09-30</td> 633 <td rowspan="2">02:00</td> 634</tr> 635<tr align="center"> 636 <td>−10:30</td> 637 <td>−1:00</td> 638 <td rowspan="2">HST</td> 639 <td rowspan="2">Hawaii standard time</td> 640 <td>1947-06-08</td> 641</tr> 642<tr align="center"> 643 <td>−10:00³</td> 644 <td>+0:30³</td> 645 <td colspan="2">—</td> 646</tr> 647<tr> 648 <td colspan="6"> 649 ¹Switching to US rules…most recent transition (in 1919) was to standard time 650 </td> 651</tr> 652<tr> 653 <td colspan="6"> 654 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 655 + (−9:30) = 13:30 local 656 </td> 657</tr> 658<tr> 659 <td colspan="6"> 660 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 661 the civil time in Hawaii has been 662 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 663 − 10:00 year-round. 664 </td> 665</tr> 666</table> 667 668<p>There will be a short quiz later. <code>8-)</code></p> 669 670<hr> 671<address> 672This web page is in the public domain, so clarified as of 6732015-10-20 by Bill Seymour. 674<br> 675All suggestions and corrections will be welcome; all flames will be amusing. 676Mail to was at pobox dot com. 677</address> 678</body> 679</html> 680