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<h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 19Database</a> Source Files</h2> 20<h3>by Bill Seymour</h3> 21<p>This guide uses the <code>America/Chicago</code> and 22<code>Pacific/Honolulu</code> zones as examples of how to infer 23times of day from the <a href="tz-link.html">tz database</a> 24source files. It might be helpful, but not absolutely necessary, 25for the reader to have already downloaded the 26latest release of the database and become familiar with the basic layout 27of the data files. The format is explained in the “man page” 28for the zic compiler, <code>zic.8.txt</code>, in 29the <code>code</code> subdirectory. 30Although this guide covers many of the common cases, it is not a 31complete summary of what zic accepts; the man page is the 32authoritative reference.</p> 33 34<p>We’ll begin by talking about the rules for changing between standard 35and daylight saving time since we’ll need that information when we talk 36about the zones.</p> 37 38<p>First, let’s consider the special daylight saving time rules 39for Chicago (from the <code>northamerica</code> file in 40the <code>data</code> subdirectory):</p> 41 42<pre>#Rule NAME FROM TO - IN ON AT SAVE LETTER 43Rule Chicago 1920 only - Jun 13 2:00 1:00 D 44Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 45Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 46Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 47Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 48Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 49</pre> 50<table> 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> 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> 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> 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> 80 <td rowspan="2">1922</td> 81 <td>1966</td> 82 <td>in April</td> 83</tr> 84<tr> 85 <td>1954</td> 86 <td>in September</td> 87 <td rowspan="2">return to standard time</td> 88</tr> 89<tr> 90 <td>1955</td> 91 <td>1966</td> 92 <td>in October</td> 93</tr> 94</table> 95 96<p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the 97first and last calendar years defining a contiguous range over which a specific 98Rule line is to apply. The keyword <code>only</code> can be used in the 99<code>TO</code> field to repeat the value of the <code>FROM</code> field in the 100event that a rule should only apply to a single year. Often, the keyword 101<code>max</code> is used to extend a rule’s application into the 102indefinite future; it is a platform-agnostic stand-in for the largest 103representable year. 104 105<p>The next column, <code>-</code>, is reserved; for compatibility with earlier 106releases, it always contains a hyphen, which acts as a kind of null value. 107Prior to the 2020b release, it was called the <code>TYPE</code> field, though 108it had not been used in the main data since the 2000e release. 109An obsolescent supplementary file used the 110field as a proof-of-concept to allow <code>zic</code> to apply a given Rule 111line only to certain “types” of years within the specified range as 112dictated by the output of a separate script, such as: only years which would 113have a US presidential election, or only years which wouldn’t. 114 115<p>The <code>SAVE</code> column contains the local (wall clock) offset from 116local standard time. 117This is usually either zero for standard time or one hour for daylight 118saving time; but there’s no reason, in principle, why it can’t 119take on other values. 120 121<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 122column can contain a variable 123part of the usual abbreviation of the time zone’s name, or it can just 124be a hyphen if there’s no variable part. For example, the abbreviation 125used in the central time zone will be either “CST” or “CDT”. 126The variable part is ‘S’ or ‘D’; 127and, sure enough, that’s just what we find in 128the <code>LETTER</code> column 129in the <code>Chicago</code> rules. More about this when we talk about 130“Zone” lines. 131 132<p>One important thing to notice is that “Rule” lines 133want at once to be both <i>transitions</i> and <i>steady states</i>: 134<ul> 135<li>On the one hand, they represent transitions between standard and 136daylight saving time; and any number of Rule lines can be in effect 137during a given period (which will always be a non-empty set of 138contiguous calendar years).</li> 139<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 140columns contain state that exists between transitions. More about this 141when we talk about the US rules.</li> 142</ul> 143 144<p>In the example above, the transition to daylight saving time 145happened on the 13<small><sup>th</sup></small> of June in 1920, and on 146the last Sunday in March in 1921; but the return to standard time 147happened on the last Sunday in October in both of those 148years. Similarly, the rule for changing to daylight saving time was 149the same from 1922 to 1966; but the rule for returning to standard 150time changed in 1955. Got it?</p> 151 152<p>OK, now for the somewhat more interesting “US” rules:</p> 153 154<pre>#Rule NAME FROM TO - IN ON AT SAVE LETTER/S 155Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 156Rule US 1918 1919 - Oct lastSun 2:00 0 S 157Rule US 1942 only - Feb 9 2:00 1:00 W # War 158Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 159Rule US 1945 only - Sep 30 2:00 0 S 160Rule US 1967 2006 - Oct lastSun 2:00 0 S 161Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 162Rule US 1974 only - Jan 6 2:00 1:00 D 163Rule US 1975 only - Feb 23 2:00 1:00 D 164Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 165Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 166Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 167Rule US 2007 max - Nov Sun>=1 2:00 0 S 168</pre> 169<table> 170<tr> 171 <th colspan="6">Reformatted a Bit</th> 172</tr> 173<tr> 174 <th>From</th> 175 <th>To</th> 176 <th colspan="2">On</th> 177 <th>At</th> 178 <th>Action</th> 179</tr> 180<tr> 181 <td rowspan="2">1918</td> 182 <td rowspan="2">1919</td> 183 <td rowspan="2">last Sunday</td> 184 <td>in March</td> 185 <td rowspan="3">02:00 local</td> 186 <td>go to daylight saving time</td> 187</tr> 188<tr> 189 <td>in October</td> 190 <td>return to standard time</td> 191</tr> 192<tr> 193 <td colspan="2">1942 only</td> 194 <td colspan="2">February 9<small><sup>th</sup></small></td> 195 <td>go to “war time”</td> 196</tr> 197<tr> 198 <td colspan="2" rowspan="2">1945 only</td> 199 <td colspan="2">August 14<small><sup>th</sup></small></td> 200 <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 201 <td> 202 rename “war time” to “peace<br>time;” 203 clocks don’t change 204 </td> 205</tr> 206<tr> 207 <td colspan="2">September 30<small><sup>th</sup></small></td> 208 <td rowspan="9">02:00 local</td> 209 <td rowspan="2">return to standard time</td> 210</tr> 211<tr> 212 <td rowspan="2">1967</td> 213 <td>2006</td> 214 <td rowspan="2">last Sunday</td> 215 <td>in October</td> 216</tr> 217<tr> 218 <td>1973</td> 219 <td>in April</td> 220 <td rowspan="6">go to daylight saving time</td> 221</tr> 222<tr> 223 <td colspan="2">1974 only</td> 224 <td colspan="2">January 6<small><sup>th</sup></small></td> 225</tr> 226<tr> 227 <td colspan="2">1975 only</td> 228 <td colspan="2">February 23<small><sup>rd</sup></small></td> 229</tr> 230<tr> 231 <td>1976</td> 232 <td>1986</td> 233 <td>last Sunday</td> 234 <td rowspan="2">in April</td> 235</tr> 236<tr> 237 <td>1987</td> 238 <td>2006</td> 239 <td>first Sunday</td> 240</tr> 241<tr> 242 <td rowspan="2">2007</td> 243 <td rowspan="2">present</td> 244 <td colspan="2">second Sunday in March</td> 245</tr> 246<tr> 247 <td colspan="2">first Sunday in November</td> 248 <td>return to standard time</td> 249</tr> 250</table> 251 252<p>There are two interesting things to note here.</p> 253 254<p>First, the time that something happens (in the <code>AT</code> 255column) is not necessarily the local (wall clock) time. The time can be 256suffixed with ‘s’ (for “standard”) to mean 257local standard time, different from local (wall clock) time when observing 258daylight saving time; or it can be suffixed with ‘g’, 259‘u’, or ‘z’, all three of which mean the 260standard time at the 261<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 262‘g’ stands for “<a 263href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 264‘u’ stands for “<a 265href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 266href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 267(whichever was official at the time); ‘z’ stands for the 268<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 269Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 270The time can also be suffixed with ‘w’ meaning local (wall 271clock) time; but it usually isn’t because that’s the 272default.</p> 273 274<p>Second, the day in the <code>ON</code> column, in addition to 275“<code>lastSun</code>” or a particular day of the month, 276can have the form, “<code>Sun>=</code><i>x</i>” or 277“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 278of the month. For example, “<code>Sun>=8</code>” means 279“the first Sunday on or after the eighth of the month,” in 280other words, the second Sunday of the month. Furthermore, although 281there are no examples above, the weekday needn’t be 282“<code>Sun</code>” in either form, but can be the usual 283three-character English abbreviation for any day of the week.</p> 284 285<p>And the US rules give us more examples of a couple of things 286already mentioned:</p> 287 288<ul> 289<li>The rules for changing to and from daylight saving time are 290actually <i>different sets</i> of rules; and the two sets can change 291independently. Consider, for example, that the rule for the return to 292standard time stayed the same from 1967 to 2006; but the rule for the 293transition to daylight saving time changed several times in the same 294period. There can also be periods, 1946 to 1966 for example, when no 295rule from this group is in effect, and so either no transition 296happened in those years, or some other rule is in effect (perhaps a 297state or other more local rule).</li> 298 299<li>The <code>SAVE</code> and <code>LETTER</code> columns 300contain <i>steady state</i>, not transitions. Consider, for example, 301the transition from “war time” to “peace time” 302that happened on August 14, 1945. The “1:00” in 303the <code>SAVE</code> column is <i>not</i> an instruction to advance 304the clock an hour. It means that clocks should <i>be</i> one hour 305ahead of standard time, which they already are because of the previous 306rule, so there should be no change.</li> 307 308</ul> 309 310<p>OK, now let’s look at a Zone record:</p> 311 312<pre> 313#Zone NAME STDOFF RULES FORMAT [UNTIL] 314Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 315 -6:00 US C%sT 1920 316 -6:00 Chicago C%sT 1936 Mar 1 2:00 317 -5:00 - EST 1936 Nov 15 2:00 318 -6:00 Chicago C%sT 1942 319 -6:00 US C%sT 1946 320 -6:00 Chicago C%sT 1967 321 -6:00 US C%sT 322</pre> 323<table> 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> 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> 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> 354 <td>Chicago rules</td> 355 <td>1936-03-01</td> 356 <td rowspan="2">02:00:00</td> 357</tr> 358<tr> 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> 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> 372 <td>US rules</td> 373 <td>CST, CWT or CPT</td> 374 <td>1946-01-01</td> 375</tr> 376<tr> 377 <td>Chicago rules</td> 378 <td rowspan="2">CST or CDT</td> 379 <td>1967-01-01</td> 380</tr> 381<tr> 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 STDOFF 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 STDOFF RULES FORMAT [UNTIL] 457Zone Pacific/Honolulu ... 1933 Apr 30 2:00 458 -10:30 1:00 HDT 1933 May 21 12:00 459 ... 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>STDOFF</code> column always contains the standard time 465offset, so the local (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 should have one of four forms:</p> 470<ul> 471 472<li>a time zone abbreviation that is a string of three or more 473characters that are either ASCII alphanumerics, 474“<code>+</code>”, or “<code>-</code>”</li> 475 476<li>the string “%z”, in which case the 477“<code>%z</code>” will be replaced by a numeric time zone 478abbreviation</li> 479 480<li>a pair of time zone abbreviations separated by a slash 481(‘<code>/</code>’), in which case the first string is the 482abbreviation for the standard time name and the second string is the 483abbreviation for the daylight saving time name</li> 484 485<li>a string containing “<code>%s</code>”, in which case 486the “<code>%s</code>” will be replaced by the text in the 487appropriate Rule’s <code>LETTER</code> column, and the resulting 488string should be a time zone abbreviation</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 STDOFF 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 524in popular English-language usage. 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="https://pubs.opengroup.org/onlinepubs/9799919799/functions/strftime.html"><code>strftime</code></a> 539function in the 540<a href="https://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<pre># Relevant Excerpts from the US Rules 552#Rule NAME FROM TO - 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 lastSun 2:00 0 S 557 558# The Zone Record 559#Zone NAME STDOFF RULES FORMAT [UNTIL] 560Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 561 -10:30 - HST 1933 Apr 30 2:00 562 -10:30 1:00 HDT 1933 May 21 2:00 563 -10:30 US H%sT 1947 Jun 8 2:00 564 -10:00 - HST 565</pre> 566<table> 567<tr> 568 <th colspan="6">What We Infer</th> 569</tr> 570<tr> 571 <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 572 <th rowspan="2">Adjust<br>Clocks</th> 573 <th colspan="2">Time Zone</th> 574 <th colspan="2">Ending at Local Time</th> 575</tr> 576<tr> 577 <th>Abbrv.</th> 578 <th>Name</th> 579 <th>Date</th> 580 <th>Time</th> 581</tr> 582<tr> 583 <td>−10:31:26</td> 584 <td>–</td> 585 <td>LMT</td> 586 <td>local mean time</td> 587 <td>1896-01-13</td> 588 <td>12:00</td> 589</tr> 590<tr> 591 <td>−10:30</td> 592 <td>+0:01:26</td> 593 <td>HST</td> 594 <td>Hawaii standard time</td> 595 <td>1933-04-30</td> 596 <td>02:00</td> 597</tr> 598<tr> 599 <td>−9:30</td> 600 <td>+1:00</td> 601 <td>HDT</td> 602 <td>Hawaii daylight time</td> 603 <td>1933-05-21</td> 604 <td>12:00</td> 605</tr> 606<tr> 607 <td>−10:30¹</td> 608 <td>−1:00¹</td> 609 <td>HST¹</td> 610 <td>Hawaii standard time</td> 611 <td>1942-02-09</td> 612 <td>02:00</td> 613</tr> 614<tr> 615 <td rowspan="2">−9:30</td> 616 <td>+1:00</td> 617 <td>HWT</td> 618 <td>Hawaii war time</td> 619 <td>1945-08-14</td> 620 <td>13:30²</td> 621</tr> 622<tr> 623 <td>0</td> 624 <td>HPT</td> 625 <td>Hawaii peace time</td> 626 <td>1945-09-30</td> 627 <td rowspan="2">02:00</td> 628</tr> 629<tr> 630 <td>−10:30</td> 631 <td>−1:00</td> 632 <td rowspan="2">HST</td> 633 <td rowspan="2">Hawaii standard time</td> 634 <td>1947-06-08</td> 635</tr> 636<tr> 637 <td>−10:00³</td> 638 <td>+0:30³</td> 639 <td colspan="2">–</td> 640</tr> 641<tr> 642 <td colspan="6" class="footnote"> 643 ¹Switching to US rules...most recent transition (in 1919) was to standard time 644 </td> 645</tr> 646<tr> 647 <td colspan="6" class="footnote"> 648 ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 649 + (−9:30) = 13:30 local 650 </td> 651</tr> 652<tr> 653 <td colspan="6" class="footnote"> 654 ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947-06-08T12:30Z</a>, 655 the civil time in Hawaii has been 656 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 657 −10:00 year-round. 658 </td> 659</tr> 660</table> 661 662<p>There will be a short quiz later. <code>8-)</code></p> 663 664<hr> 665<address> 666This web page is in the public domain, so clarified as of 6672015-10-20 by Bill Seymour. 668<br> 669All suggestions and corrections will be welcome; all flames will be amusing. 670Mail to was at pobox dot com. 671</address> 672</body> 673</html> 674