Home | History | Annotate | Line # | Download | only in time
tz-how-to.html revision 1.14
      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>
      8 pre {margin-left: 1.3rem; overflow: auto;}
      9 td {text-align: center;}
     10 table {border: 1px outset;}
     11 th, td {border: 1px inset;}
     12 table.rule {border: none; margin: auto;}
     13 td.footnote {text-align: left;}
     14 ul {padding-left: 1.3rem;}
     15 </style>
     16 </head>
     17 <body>
     18 <main>
     19 <h1>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
     20 Database</a> Source Files</h1>
     21 <h2>by Bill Seymour</h2>
     22 <p>This guide uses the <code>America/Chicago</code> and
     23 <code>Pacific/Honolulu</code> zones as examples of how to infer
     24 times of day from the <a href="tz-link.html">tz database</a>
     25 source files. It might be helpful, but not absolutely necessary,
     26 for the reader to have already downloaded the
     27 latest release of the database and become familiar with the basic layout
     28 of the data files. The format is explained in the man page
     29 for the zic compiler, <code>zic.8.txt</code>, in
     30 the <code>code</code> subdirectory.
     31 Although this guide covers many of the common cases, it is not a
     32 complete summary of what zic accepts; the man page is the
     33 authoritative reference.</p>
     34 
     35 <p>Well begin by talking about the rules for changing between standard
     36 and daylight saving time since well need that information when we talk
     37 about the zones.</p>
     38 
     39 <p>First, lets consider the special daylight saving time rules
     40 for Chicago (from the <code>northamerica</code> file in
     41 the <code>data</code> subdirectory):</p>
     42 
     43 <pre>#Rule NAME    FROM TO    -   IN  ON      AT   SAVE LETTER
     44 Rule  Chicago 1920 only  -   Jun 13      2:00 1:00 D
     45 Rule  Chicago 1920 1921  -   Oct lastSun 2:00 0    S
     46 Rule  Chicago 1921 only  -   Mar lastSun 2:00 1:00 D
     47 Rule  Chicago 1922 1966  -   Apr lastSun 2:00 1:00 D
     48 Rule  Chicago 1922 1954  -   Sep lastSun 2:00 0    S
     49 Rule  Chicago 1955 1966  -   Oct lastSun 2:00 0    S
     50 </pre>
     51 <table>
     52 <tr>
     53   <th colspan="6">Reformatted a Bit</th>
     54 </tr>
     55 <tr>
     56   <th>From</th>
     57   <th>To</th>
     58   <th colspan="2">On</th>
     59   <th>At</th>
     60   <th>Action</th>
     61 </tr>
     62 <tr>
     63   <td colspan="2">1920 only</td>
     64   <td colspan="2">June 13<small><sup>th</sup></small></td>
     65   <td rowspan="6">02:00 local</td>
     66   <td>go to daylight saving time</td>
     67 </tr>
     68 <tr>
     69   <td>1920</td>
     70   <td>1921</td>
     71   <td rowspan="5">last Sunday</td>
     72   <td>in October</td>
     73   <td>return to standard time</td>
     74 </tr>
     75 <tr>
     76   <td colspan="2">1921 only</td>
     77   <td>in March</td>
     78   <td rowspan="2">go to daylight saving time</td>
     79 </tr>
     80 <tr>
     81   <td rowspan="2">1922</td>
     82   <td>1966</td>
     83   <td>in April</td>
     84 </tr>
     85 <tr>
     86   <td>1954</td>
     87   <td>in September</td>
     88   <td rowspan="2">return to standard time</td>
     89 </tr>
     90 <tr>
     91   <td>1955</td>
     92   <td>1966</td>
     93   <td>in October</td>
     94 </tr>
     95 </table>
     96 
     97 <p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the
     98 first and last calendar years defining a contiguous range over which a specific
     99 Rule line is to apply.  The keyword <code>only</code> can be used in the
    100 <code>TO</code> field to repeat the value of the <code>FROM</code> field in the
    101 event that a rule should only apply to a single year.  Often, the keyword
    102 <code>max</code> is used to extend a rules application into the
    103 indefinite future; it is a platform-agnostic stand-in for the largest
    104 representable year.
    105 
    106 <p>The next column, <code>-</code>, is reserved; for compatibility with earlier
    107 releases, it always contains a hyphen, which acts as a kind of null value.
    108 Prior to the 2020b release, it was called the <code>TYPE</code> field, though
    109 it had not been used in the main data since the 2000e release.
    110 An obsolescent supplementary file used the
    111 field as a proof-of-concept to allow <code>zic</code> to apply a given Rule
    112 line only to certain types of years within the specified range as
    113 dictated by the output of a separate script, such as: only years which would
    114 have a US presidential election, or only years which wouldnt.
    115 
    116 <p>The <code>SAVE</code> column contains the local (wall clock) offset from
    117 local standard time.
    118 This is usually either zero for standard time or one hour for daylight
    119 saving time; but theres no reason, in principle, why it cant
    120 take on other values.
    121 
    122 <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>)
    123 column can contain a variable
    124 part of the usual abbreviation of the time zones name, or it can just
    125 be a hyphen if theres no variable part. For example, the abbreviation
    126 used in the central time zone will be either CST or CDT.
    127 The variable part is S or D;
    128 and, sure enough, thats just what we find in
    129 the <code>LETTER</code> column
    130 in the <code>Chicago</code> rules. More about this when we talk about
    131 Zone lines.
    132 
    133 <p>One important thing to notice is that Rule lines
    134 want at once to be both <i>transitions</i> and <i>steady states</i>:
    135 <ul>
    136 <li>On the one hand, they represent transitions between standard and
    137 daylight saving time; and any number of Rule lines can be in effect
    138 during a given period (which will always be a non-empty set of
    139 contiguous calendar years).</li>
    140 <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code>
    141 columns contain state that exists between transitions. More about this
    142 when we talk about the US rules.</li>
    143 </ul>
    144 
    145 <p>In the example above, the transition to daylight saving time
    146 happened on the 13<small><sup>th</sup></small> of June in 1920, and on
    147 the last Sunday in March in 1921; but the return to standard time
    148 happened on the last Sunday in October in both of those
    149 years. Similarly, the rule for changing to daylight saving time was
    150 the same from 1922 to 1966; but the rule for returning to standard
    151 time changed in 1955. Got it?</p>
    152 
    153 <p>OK, now for the somewhat more interesting US rules:</p>
    154 
    155 <pre>#Rule NAME FROM TO    -   IN  ON        AT   SAVE LETTER/S
    156 Rule  US   1918 1919  -   Mar lastSun  2:00  1:00 D
    157 Rule  US   1918 1919  -   Oct lastSun  2:00  0    S
    158 Rule  US   1942 only  -   Feb 9        2:00  1:00 W # War
    159 Rule  US   1945 only  -   Aug 14      23:00u 1:00 P # Peace
    160 Rule  US   1945 only  -   Sep 30       2:00  0    S
    161 Rule  US   1967 2006  -   Oct lastSun  2:00  0    S
    162 Rule  US   1967 1973  -   Apr lastSun  2:00  1:00 D
    163 Rule  US   1974 only  -   Jan 6        2:00  1:00 D
    164 Rule  US   1975 only  -   Feb 23       2:00  1:00 D
    165 Rule  US   1976 1986  -   Apr lastSun  2:00  1:00 D
    166 Rule  US   1987 2006  -   Apr Sun&gt;=1   2:00  1:00 D
    167 Rule  US   2007 max   -   Mar Sun&gt;=8   2:00  1:00 D
    168 Rule  US   2007 max   -   Nov Sun&gt;=1   2:00  0    S
    169 </pre>
    170 <table>
    171 <tr>
    172   <th colspan="6">Reformatted a Bit</th>
    173 </tr>
    174 <tr>
    175   <th>From</th>
    176   <th>To</th>
    177   <th colspan="2">On</th>
    178   <th>At</th>
    179   <th>Action</th>
    180 </tr>
    181 <tr>
    182   <td rowspan="2">1918</td>
    183   <td rowspan="2">1919</td>
    184   <td rowspan="2">last Sunday</td>
    185   <td>in March</td>
    186   <td rowspan="3">02:00 local</td>
    187   <td>go to daylight saving time</td>
    188 </tr>
    189 <tr>
    190   <td>in October</td>
    191   <td>return to standard time</td>
    192 </tr>
    193 <tr>
    194   <td colspan="2">1942 only</td>
    195   <td colspan="2">February 9<small><sup>th</sup></small></td>
    196   <td>go to war time</td>
    197 </tr>
    198 <tr>
    199   <td colspan="2" rowspan="2">1945 only</td>
    200   <td colspan="2">August 14<small><sup>th</sup></small></td>
    201   <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td>
    202   <td>
    203     rename war time to peace<br>time;
    204     clocks dont change
    205   </td>
    206 </tr>
    207 <tr>
    208   <td colspan="2">September 30<small><sup>th</sup></small></td>
    209   <td rowspan="9">02:00 local</td>
    210   <td rowspan="2">return to standard time</td>
    211 </tr>
    212 <tr>
    213   <td rowspan="2">1967</td>
    214   <td>2006</td>
    215   <td rowspan="2">last Sunday</td>
    216   <td>in October</td>
    217 </tr>
    218 <tr>
    219   <td>1973</td>
    220   <td>in April</td>
    221   <td rowspan="6">go to daylight saving time</td>
    222 </tr>
    223 <tr>
    224   <td colspan="2">1974 only</td>
    225   <td colspan="2">January 6<small><sup>th</sup></small></td>
    226 </tr>
    227 <tr>
    228   <td colspan="2">1975 only</td>
    229   <td colspan="2">February 23<small><sup>rd</sup></small></td>
    230 </tr>
    231 <tr>
    232   <td>1976</td>
    233   <td>1986</td>
    234   <td>last Sunday</td>
    235   <td rowspan="2">in April</td>
    236 </tr>
    237 <tr>
    238   <td>1987</td>
    239   <td>2006</td>
    240   <td>first Sunday</td>
    241 </tr>
    242 <tr>
    243   <td rowspan="2">2007</td>
    244   <td rowspan="2">present</td>
    245   <td colspan="2">second Sunday in March</td>
    246 </tr>
    247 <tr>
    248   <td colspan="2">first Sunday in November</td>
    249   <td>return to standard time</td>
    250 </tr>
    251 </table>
    252 
    253 <p>There are two interesting things to note here.</p>
    254 
    255 <p>First, the time that something happens (in the <code>AT</code>
    256 column) is not necessarily the local (wall clock) time. The time can be
    257 suffixed with s (for standard) to mean
    258 local standard time, different from local (wall clock) time when observing
    259 daylight saving time; or it can be suffixed with g,
    260 u, or z, all three of which mean the
    261 standard time at the
    262 <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>.
    263 g stands for <a
    264 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>;
    265 u stands for <a
    266 href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> or <a
    267 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
    268 (whichever was official at the time); z stands for the
    269 <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a>
    270 Z (a.k.a. Zulu which, in turn, stands for Z).
    271 The time can also be suffixed with w meaning local (wall
    272 clock) time; but it usually isnt because thats the
    273 default.</p>
    274 
    275 <p>Second, the day in the <code>ON</code> column, in addition to
    276 <code>lastSun</code> or a particular day of the month,
    277 can have the form, <code>Sun&gt;=</code><i>x</i> or
    278 <code>Sun&lt;=</code><i>x</i>, where <i>x</i> is a day
    279 of the month. For example, <code>Sun&gt;=8</code> means
    280 the first Sunday on or after the eighth of the month, in
    281 other words, the second Sunday of the month. Furthermore, although
    282 there are no examples above, the weekday neednt be
    283 <code>Sun</code> in either form, but can be the usual
    284 three-character English abbreviation for any day of the week.</p>
    285 
    286 <p>And the US rules give us more examples of a couple of things
    287 already mentioned:</p>
    288 
    289 <ul>
    290 <li>The rules for changing to and from daylight saving time are
    291 actually <i>different sets</i> of rules; and the two sets can change
    292 independently. Consider, for example, that the rule for the return to
    293 standard time stayed the same from 1967 to 2006; but the rule for the
    294 transition to daylight saving time changed several times in the same
    295 period.  There can also be periods, 1946 to 1966 for example, when no
    296 rule from this group is in effect, and so either no transition
    297 happened in those years, or some other rule is in effect (perhaps a
    298 state or other more local rule).</li>
    299 
    300 <li>The <code>SAVE</code> and <code>LETTER</code> columns
    301 contain <i>steady state</i>, not transitions. Consider, for example,
    302 the transition from war time to peace time
    303 that happened on August 14, 1945. The 1:00 in
    304 the <code>SAVE</code> column is <i>not</i> an instruction to advance
    305 the clock an hour. It means that clocks should <i>be</i> one hour
    306 ahead of standard time, which they already are because of the previous
    307 rule, so there should be no change.</li>
    308 
    309 </ul>
    310 
    311 <p>OK, now lets look at a Zone record:</p>
    312 
    313 <pre>
    314 #Zone       NAME      STDOFF   RULES FORMAT [UNTIL]
    315 Zone  America/Chicago -5:50:36 -       LMT  1883 Nov 18 12:09:24
    316                       -6:00    US      C%sT 1920
    317                       -6:00    Chicago C%sT 1936 Mar  1  2:00
    318                       -5:00    -       EST  1936 Nov 15  2:00
    319                       -6:00    Chicago C%sT 1942
    320                       -6:00    US      C%sT 1946
    321                       -6:00    Chicago C%sT 1967
    322                       -6:00    US      C%sT
    323 </pre>
    324 <table>
    325 <tr>
    326   <th colspan="5">Columns Renamed</th>
    327 </tr>
    328 <tr>
    329   <th rowspan="2">Standard Offset<br>
    330     from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime
    331     Meridian</a></th>
    332   <th rowspan="2">Daylight<br>Saving Time</th>
    333   <th rowspan="2">Abbreviation(s)</th>
    334   <th colspan="2">Ending at Local Time</th>
    335 </tr>
    336 <tr>
    337   <th>Date</th>
    338   <th>Time</th>
    339 </tr>
    340 <tr>
    341   <td>5:50:36</td>
    342   <td>not observed</td>
    343   <td>LMT</td>
    344   <td>1883-11-18</td>
    345   <td>12:09:24</td>
    346 </tr>
    347 <tr>
    348   <td rowspan="2">6:00:00</td>
    349   <td>US rules</td>
    350   <td rowspan="2">CST or CDT</td>
    351   <td>1920-01-01</td>
    352   <td>00:00:00</td>
    353 </tr>
    354 <tr>
    355   <td>Chicago rules</td>
    356   <td>1936-03-01</td>
    357   <td rowspan="2">02:00:00</td>
    358 </tr>
    359 <tr>
    360   <td>5:00:00</td>
    361   <td>not observed</td>
    362   <td>EST</td>
    363   <td>1936-11-15</td>
    364 </tr>
    365 <tr>
    366   <td rowspan="4">6:00:00</td>
    367   <td>Chicago rules</td>
    368   <td>CST or CDT</td>
    369   <td>1942-01-01</td>
    370   <td rowspan="3">00:00:00</td>
    371 </tr>
    372 <tr>
    373   <td>US rules</td>
    374   <td>CST, CWT or CPT</td>
    375   <td>1946-01-01</td>
    376 </tr>
    377 <tr>
    378   <td>Chicago rules</td>
    379   <td rowspan="2">CST or CDT</td>
    380   <td>1967-01-01</td>
    381 </tr>
    382 <tr>
    383   <td>US rules</td>
    384   <td colspan="2"></td>
    385 </tr>
    386 </table>
    387 
    388 <p>There are a couple of interesting differences between Zones and Rules.</p>
    389 
    390 <p>First, and somewhat trivially, whereas Rules are considered to
    391 contain one or more records, a Zone is considered to be a single
    392 record with zero or more <i>continuation lines</i>. Thus, the keyword,
    393 <code>Zone</code>, and the zone name are not
    394 repeated. The last line is the one without anything in
    395 the <code>[UNTIL]</code> column.</p>
    396 
    397 <p>Second, and more fundamentally, each line of a Zone represents a
    398 steady state, not a transition between states. The state exists from
    399 the date and time in the previous lines <code>[UNTIL]</code>
    400 column up to the date and time in the current
    401 lines <code>[UNTIL]</code> column. In other words, the date and
    402 time in the <code>[UNTIL]</code> column is the instant that separates
    403 this state from the next. Where that would be ambiguous because
    404 were setting our clocks back, the <code>[UNTIL]</code> column
    405 specifies the first occurrence of the instant. The state specified by
    406 the last line, the one without anything in the <code>[UNTIL]</code>
    407 column, continues to the present.</p>
    408 
    409 <p>The first line typically specifies the mean solar time observed
    410 before the introduction of standard time. Since theres no line before
    411 that, it has no beginning. <code>8-) </code> For some places near the <a
    412 href="https://en.wikipedia.org/wiki/International_Date_Line">International
    413 Date Line</a>, the first <i>two</i> lines will show solar times
    414 differing by 24 hours; this corresponds to a movement of the Date
    415 Line.  For example:</p>
    416 
    417 <pre>
    418 #Zone NAME          STDOFF   RULES FORMAT [UNTIL]
    419 Zone America/Juneau 15:02:19 -     LMT    1867 Oct 19  0:31:13u
    420                     -8:57:41 -     LMT    ...
    421 </pre>
    422 
    423 <p>When Alaska was purchased from Russia in 1867, the Date Line moved
    424 from the Alaska/Canada border to the Bering Strait; and the time in
    425 Alaska was then 24 hours earlier than it had
    426 been. <code>&lt;aside&gt;</code>(This also means that, although the sun had set
    427 on Friday 6 October and risen on Saturday 7 October in the Julian calendar,
    428 which Russia was still using then for religious reasons, upon the transfer
    429 Saturday afternoon became Friday afternoon, and then the sun set and rose once
    430 more on <i>second instances of the same days with different names</i>,
    431 Friday 18 and Saturday 19 October in the Gregorian calendar. Isnt civil time
    432 wonderful? <code>8-)</code>)<code>&lt;/aside&gt;</code></p>
    433 
    434 <p>The abbreviation, LMT stands for local mean
    435 time, which is an invention of
    436 the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
    437 database</a> and was probably never actually used during the
    438 period. Furthermore, the value is almost certainly wrong except in the
    439 archetypal place after which the zone is named. (The tz database
    440 usually doesnt provide a separate Zone record for places where
    441 nothing significant happened after 1970.)</p>
    442 
    443 <p>The <code>RULES</code> column tells us whether daylight saving time is being observed:
    444 <ul>
    445 <li>A hyphen, a kind of null value, means that we have not set our
    446 clocks ahead of standard time.</li>
    447 
    448 <li>An amount of time (usually but not necessarily 1:00
    449 meaning one hour) means that we have set our clocks ahead by that
    450 amount.</li>
    451 
    452 <li>Some alphabetic string means that we <i>might have</i> set our
    453 clocks ahead; and we need to check the rule the name of which is the
    454 given alphabetic string.</li>
    455 </ul>
    456 
    457 <p>An example of a specific amount of time is:</p>
    458 <pre>
    459 #Zone NAME            STDOFF RULES FORMAT [UNTIL]
    460 Zone Pacific/Honolulu ...                 1933 Apr 30  2:00
    461                       -10:30 1:00  HDT    1933 May 21 12:00
    462                       ...
    463 </pre>
    464 
    465 <p>Hawaii tried daylight saving time for three weeks in 1933 and
    466 decided they didnt like it. <code>8-) </code>Note that
    467 the <code>STDOFF</code> column always contains the standard time
    468 offset, so the local (wall clock) time during this period was GMT 
    469 10:30 + 1:00 = GMT  9:30.</p>
    470 
    471 <p>The <code>FORMAT</code> column specifies the usual abbreviation of
    472 the time zone name. It should have one of four forms:</p>
    473 <ul>
    474 
    475 <li>a time zone abbreviation that is a string of three or more
    476 characters that are either ASCII alphanumerics,
    477 <code>+</code>, or <code>-</code></li>
    478 
    479 <li>the string %z, in which case the
    480 <code>%z</code> will be replaced by a numeric time zone
    481 abbreviation</li>
    482 
    483 <li>a pair of time zone abbreviations separated by a slash
    484 (<code>/</code>), in which case the first string is the
    485 abbreviation for the standard time name and the second string is the
    486 abbreviation for the daylight saving time name</li>
    487 
    488 <li>a string containing <code>%s</code>, in which case
    489 the <code>%s</code> will be replaced by the text in the
    490 appropriate Rules <code>LETTER</code> column, and the resulting
    491 string should be a time zone abbreviation</li>
    492 </ul>
    493 
    494 <p>The last two make sense only if theres a named rule in effect.</p>
    495 
    496 <p>An example of a slash is:</p>
    497 <pre>
    498 #Zone NAME          STDOFF RULES FORMAT  [UNTIL]
    499 Zone  Europe/London ...                  1996
    500                     0:00   EU    GMT/BST
    501 </pre>
    502 
    503 <p>The current time in the UK is called either Greenwich mean time or
    504 British summer time.</p>
    505 
    506 <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what
    507 happens when switching to a named rule. To what values should
    508 the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p>
    509 
    510 <ul>
    511 <li>If at least one transition has happened, use
    512 the <code>SAVE</code> and <code>LETTER</code> data from the most
    513 recent.</li>
    514 
    515 <li>If switching to a named rule before any transition has happened,
    516 assume standard time (<code>SAVE</code> zero), and use
    517 the <code>LETTER</code> data from the earliest transition with
    518 a <code>SAVE</code> of zero.
    519 
    520 </ul>
    521 
    522 <p>And three last things about the <code>FORMAT</code> column:</p>
    523 <ul>
    524 
    525 <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz
    526 database</a> gives abbreviations for time zones
    527 in popular English-language usage. For
    528 example, the last line in
    529 <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives
    530 HST for Hawaii standard time even though the
    531 <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a>
    532 name for that time zone is HawaiiAleutian standard time.
    533 This author has read that there are also some places in Australia where
    534 popular time zone names differ from the legal ones.
    535 
    536 <li>No attempt is made to <a
    537 href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a>
    538 the abbreviations. They are intended to be the values returned through the
    539 <code>"%Z"</code> format specifier to
    540 <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>s
    541 <a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/strftime.html"><code>strftime</code></a>
    542 function in the
    543 <a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">C locale</a>.
    544 
    545 <li>If there is no generally accepted abbreviation for a time zone,
    546 a numeric offset is used instead, e.g., <code>+07</code> for 7 hours
    547 ahead of Greenwich. By convention, <code>-00</code> is used in a
    548 zone while uninhabited, where the offset is zero but in some sense
    549 the true offset is undefined.
    550 </ul>
    551 
    552 <p>As a final example, heres the complete history for Hawaii:</p>
    553 
    554 <pre># Relevant Excerpts from the US Rules
    555 #Rule NAME FROM TO   -    IN  ON      AT     SAVE LETTER/S
    556 Rule  US   1918 1919 -    Oct lastSun  2:00  0    S
    557 Rule  US   1942 only -    Feb  9       2:00  1:00 W # War
    558 Rule  US   1945 only -    Aug 14      23:00u 1:00 P # Peace
    559 Rule  US   1945 only -    Sep lastSun  2:00  0    S
    560 
    561 # The Zone Record
    562 #Zone NAME            STDOFF    RULES FORMAT [UNTIL]
    563 Zone Pacific/Honolulu -10:31:26 -     LMT    1896 Jan 13 12:00
    564                       -10:30    -     HST    1933 Apr 30  2:00
    565                       -10:30    1:00  HDT    1933 May 21  2:00
    566                       -10:30    US    H%sT   1947 Jun  8  2:00
    567                       -10:00    -     HST
    568 </pre>
    569 <table>
    570 <tr>
    571   <th colspan="6">What We Infer</th>
    572 </tr>
    573 <tr>
    574   <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th>
    575   <th rowspan="2">Adjust<br>Clocks</th>
    576   <th colspan="2">Time Zone</th>
    577   <th colspan="2">Ending at Local Time</th>
    578 </tr>
    579 <tr>
    580   <th>Abbrv.</th>
    581   <th>Name</th>
    582   <th>Date</th>
    583   <th>Time</th>
    584 </tr>
    585 <tr>
    586   <td>10:31:26</td>
    587   <td></td>
    588   <td>LMT</td>
    589   <td>local mean time</td>
    590   <td>1896-01-13</td>
    591   <td>12:00</td>
    592 </tr>
    593 <tr>
    594   <td>10:30</td>
    595   <td>+0:01:26</td>
    596   <td>HST</td>
    597   <td>Hawaii standard time</td>
    598   <td>1933-04-30</td>
    599   <td>02:00</td>
    600 </tr>
    601 <tr>
    602   <td>9:30</td>
    603   <td>+1:00</td>
    604   <td>HDT</td>
    605   <td>Hawaii daylight time</td>
    606   <td>1933-05-21</td>
    607   <td>12:00</td>
    608 </tr>
    609 <tr>
    610   <td>10:30&sup1;</td>
    611   <td>1:00&sup1;</td>
    612   <td>HST&sup1;</td>
    613   <td>Hawaii standard time</td>
    614   <td>1942-02-09</td>
    615   <td>02:00</td>
    616 </tr>
    617 <tr>
    618   <td rowspan="2">9:30</td>
    619   <td>+1:00</td>
    620   <td>HWT</td>
    621   <td>Hawaii war time</td>
    622   <td>1945-08-14</td>
    623   <td>13:30&sup2;</td>
    624 </tr>
    625 <tr>
    626   <td>0</td>
    627   <td>HPT</td>
    628   <td>Hawaii peace time</td>
    629   <td>1945-09-30</td>
    630   <td rowspan="2">02:00</td>
    631 </tr>
    632 <tr>
    633   <td>10:30</td>
    634   <td>1:00</td>
    635   <td rowspan="2">HST</td>
    636   <td rowspan="2">Hawaii standard time</td>
    637   <td>1947-06-08</td>
    638 </tr>
    639 <tr>
    640   <td>10:00&sup3;</td>
    641   <td>+0:30&sup3;</td>
    642   <td colspan="2"></td>
    643 </tr>
    644 <tr>
    645   <td colspan="6" class="footnote">
    646     &sup1;Switching to US rules...most recent transition (in 1919) was to standard time
    647   </td>
    648 </tr>
    649 <tr>
    650   <td colspan="6" class="footnote">
    651     &sup2;23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>
    652     + (9:30) = 13:30 local
    653   </td>
    654 </tr>
    655 <tr>
    656   <td colspan="6" class="footnote">
    657     &sup3;Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947-06-08T12:30Z</a>,
    658     the civil time in Hawaii has been
    659     <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
    660     10:00 year-round.
    661   </td>
    662 </tr>
    663 </table>
    664 
    665 <p>There will be a short quiz later. <code>8-)</code></p>
    666 </main>
    667 
    668 <footer>
    669 <hr>
    670 This web page is in the public domain, so clarified as of
    671 2015-10-20 by Bill Seymour.
    672 <br>
    673 All suggestions and corrections will be welcome; all flames will be amusing.
    674 Mail to was at pobox dot com.
    675 </footer>
    676 </body>
    677 </html>
    678