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      4   1.3  christos <title>How to Read the tz Database</title>
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     18  1.14  christos <main>
     19  1.13  christos <h1>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
     20  1.13  christos Database</a> Source Files</h1>
     21  1.13  christos <h2>by Bill Seymour</h2>
     22   1.9  christos <p>This guide uses the <code>America/Chicago</code> and
     23   1.1  christos <code>Pacific/Honolulu</code> zones as examples of how to infer
     24   1.4  christos times of day from the <a href="tz-link.html">tz database</a>
     25   1.1  christos source files. It might be helpful, but not absolutely necessary,
     26   1.1  christos for the reader to have already downloaded the
     27   1.2  christos latest release of the database and become familiar with the basic layout
     28  1.11  christos of the data files. The format is explained in the man page
     29  1.11  christos for the zic compiler, <code>zic.8.txt</code>, in
     30   1.9  christos the <code>code</code> subdirectory.
     31   1.9  christos Although this guide covers many of the common cases, it is not a
     32   1.9  christos complete summary of what zic accepts; the man page is the
     33   1.9  christos authoritative reference.</p>
     34   1.1  christos 
     35  1.11  christos <p>Well begin by talking about the rules for changing between standard
     36  1.11  christos and daylight saving time since well need that information when we talk
     37   1.1  christos about the zones.</p>
     38   1.1  christos 
     39  1.11  christos <p>First, lets consider the special daylight saving time rules
     40   1.1  christos for Chicago (from the <code>northamerica</code> file in
     41   1.1  christos the <code>data</code> subdirectory):</p>
     42   1.1  christos 
     43  1.12  christos <pre>#Rule NAME    FROM TO    -   IN  ON      AT   SAVE LETTER
     44   1.1  christos Rule  Chicago 1920 only  -   Jun 13      2:00 1:00 D
     45   1.1  christos Rule  Chicago 1920 1921  -   Oct lastSun 2:00 0    S
     46   1.1  christos Rule  Chicago 1921 only  -   Mar lastSun 2:00 1:00 D
     47   1.1  christos Rule  Chicago 1922 1966  -   Apr lastSun 2:00 1:00 D
     48   1.1  christos Rule  Chicago 1922 1954  -   Sep lastSun 2:00 0    S
     49   1.1  christos Rule  Chicago 1955 1966  -   Oct lastSun 2:00 0    S
     50   1.1  christos </pre>
     51  1.12  christos <table>
     52   1.1  christos <tr>
     53   1.1  christos   <th colspan="6">Reformatted a Bit</th>
     54   1.1  christos </tr>
     55   1.1  christos <tr>
     56   1.1  christos   <th>From</th>
     57   1.1  christos   <th>To</th>
     58   1.1  christos   <th colspan="2">On</th>
     59   1.1  christos   <th>At</th>
     60   1.1  christos   <th>Action</th>
     61   1.1  christos </tr>
     62   1.8  christos <tr>
     63   1.1  christos   <td colspan="2">1920 only</td>
     64   1.1  christos   <td colspan="2">June 13<small><sup>th</sup></small></td>
     65   1.1  christos   <td rowspan="6">02:00 local</td>
     66   1.1  christos   <td>go to daylight saving time</td>
     67   1.1  christos </tr>
     68   1.8  christos <tr>
     69   1.1  christos   <td>1920</td>
     70   1.1  christos   <td>1921</td>
     71   1.1  christos   <td rowspan="5">last Sunday</td>
     72   1.1  christos   <td>in October</td>
     73   1.1  christos   <td>return to standard time</td>
     74   1.1  christos </tr>
     75   1.8  christos <tr>
     76   1.1  christos   <td colspan="2">1921 only</td>
     77   1.1  christos   <td>in March</td>
     78   1.1  christos   <td rowspan="2">go to daylight saving time</td>
     79   1.1  christos </tr>
     80   1.8  christos <tr>
     81   1.1  christos   <td rowspan="2">1922</td>
     82   1.1  christos   <td>1966</td>
     83   1.1  christos   <td>in April</td>
     84   1.1  christos </tr>
     85   1.8  christos <tr>
     86   1.1  christos   <td>1954</td>
     87   1.1  christos   <td>in September</td>
     88   1.1  christos   <td rowspan="2">return to standard time</td>
     89   1.1  christos </tr>
     90   1.8  christos <tr>
     91   1.1  christos   <td>1955</td>
     92   1.1  christos   <td>1966</td>
     93   1.1  christos   <td>in October</td>
     94   1.1  christos </tr>
     95   1.1  christos </table>
     96   1.1  christos 
     97   1.8  christos <p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the
     98   1.8  christos first and last calendar years defining a contiguous range over which a specific
     99   1.8  christos Rule line is to apply.  The keyword <code>only</code> can be used in the
    100   1.8  christos <code>TO</code> field to repeat the value of the <code>FROM</code> field in the
    101   1.8  christos event that a rule should only apply to a single year.  Often, the keyword
    102  1.11  christos <code>max</code> is used to extend a rules application into the
    103   1.8  christos indefinite future; it is a platform-agnostic stand-in for the largest
    104   1.8  christos representable year.
    105   1.8  christos 
    106   1.8  christos <p>The next column, <code>-</code>, is reserved; for compatibility with earlier
    107   1.8  christos releases, it always contains a hyphen, which acts as a kind of null value.
    108   1.8  christos Prior to the 2020b release, it was called the <code>TYPE</code> field, though
    109   1.9  christos it had not been used in the main data since the 2000e release.
    110   1.9  christos An obsolescent supplementary file used the
    111   1.8  christos field as a proof-of-concept to allow <code>zic</code> to apply a given Rule
    112  1.11  christos line only to certain types of years within the specified range as
    113   1.8  christos dictated by the output of a separate script, such as: only years which would
    114  1.11  christos have a US presidential election, or only years which wouldnt.
    115   1.1  christos 
    116   1.7  christos <p>The <code>SAVE</code> column contains the local (wall clock) offset from
    117   1.1  christos local standard time.
    118   1.1  christos This is usually either zero for standard time or one hour for daylight
    119  1.11  christos saving time; but theres no reason, in principle, why it cant
    120   1.1  christos take on other values.
    121   1.1  christos 
    122   1.1  christos <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>)
    123   1.1  christos column can contain a variable
    124  1.11  christos part of the usual abbreviation of the time zones name, or it can just
    125  1.11  christos be a hyphen if theres no variable part. For example, the abbreviation
    126  1.11  christos used in the central time zone will be either CST or CDT.
    127  1.11  christos The variable part is S or D;
    128  1.11  christos and, sure enough, thats just what we find in
    129   1.1  christos the <code>LETTER</code> column
    130   1.1  christos in the <code>Chicago</code> rules. More about this when we talk about
    131  1.11  christos Zone lines.
    132   1.1  christos 
    133  1.11  christos <p>One important thing to notice is that Rule lines
    134   1.1  christos want at once to be both <i>transitions</i> and <i>steady states</i>:
    135   1.1  christos <ul>
    136   1.1  christos <li>On the one hand, they represent transitions between standard and
    137   1.1  christos daylight saving time; and any number of Rule lines can be in effect
    138   1.1  christos during a given period (which will always be a non-empty set of
    139   1.1  christos contiguous calendar years).</li>
    140   1.1  christos <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code>
    141   1.1  christos columns contain state that exists between transitions. More about this
    142   1.1  christos when we talk about the US rules.</li>
    143   1.1  christos </ul>
    144   1.1  christos 
    145   1.1  christos <p>In the example above, the transition to daylight saving time
    146   1.1  christos happened on the 13<small><sup>th</sup></small> of June in 1920, and on
    147   1.1  christos the last Sunday in March in 1921; but the return to standard time
    148   1.1  christos happened on the last Sunday in October in both of those
    149   1.1  christos years. Similarly, the rule for changing to daylight saving time was
    150   1.1  christos the same from 1922 to 1966; but the rule for returning to standard
    151   1.1  christos time changed in 1955. Got it?</p>
    152   1.1  christos 
    153  1.11  christos <p>OK, now for the somewhat more interesting US rules:</p>
    154   1.1  christos 
    155  1.12  christos <pre>#Rule NAME FROM TO    -   IN  ON        AT   SAVE LETTER/S
    156   1.1  christos Rule  US   1918 1919  -   Mar lastSun  2:00  1:00 D
    157   1.1  christos Rule  US   1918 1919  -   Oct lastSun  2:00  0    S
    158   1.1  christos Rule  US   1942 only  -   Feb 9        2:00  1:00 W # War
    159   1.1  christos Rule  US   1945 only  -   Aug 14      23:00u 1:00 P # Peace
    160   1.1  christos Rule  US   1945 only  -   Sep 30       2:00  0    S
    161   1.1  christos Rule  US   1967 2006  -   Oct lastSun  2:00  0    S
    162   1.1  christos Rule  US   1967 1973  -   Apr lastSun  2:00  1:00 D
    163   1.1  christos Rule  US   1974 only  -   Jan 6        2:00  1:00 D
    164   1.1  christos Rule  US   1975 only  -   Feb 23       2:00  1:00 D
    165   1.1  christos Rule  US   1976 1986  -   Apr lastSun  2:00  1:00 D
    166   1.1  christos Rule  US   1987 2006  -   Apr Sun&gt;=1   2:00  1:00 D
    167   1.1  christos Rule  US   2007 max   -   Mar Sun&gt;=8   2:00  1:00 D
    168   1.1  christos Rule  US   2007 max   -   Nov Sun&gt;=1   2:00  0    S
    169   1.1  christos </pre>
    170  1.12  christos <table>
    171   1.1  christos <tr>
    172   1.1  christos   <th colspan="6">Reformatted a Bit</th>
    173   1.1  christos </tr>
    174   1.1  christos <tr>
    175   1.1  christos   <th>From</th>
    176   1.1  christos   <th>To</th>
    177   1.1  christos   <th colspan="2">On</th>
    178   1.1  christos   <th>At</th>
    179   1.1  christos   <th>Action</th>
    180   1.1  christos </tr>
    181   1.8  christos <tr>
    182   1.1  christos   <td rowspan="2">1918</td>
    183   1.1  christos   <td rowspan="2">1919</td>
    184   1.1  christos   <td rowspan="2">last Sunday</td>
    185   1.1  christos   <td>in March</td>
    186   1.1  christos   <td rowspan="3">02:00 local</td>
    187   1.1  christos   <td>go to daylight saving time</td>
    188   1.1  christos </tr>
    189   1.8  christos <tr>
    190   1.1  christos   <td>in October</td>
    191   1.1  christos   <td>return to standard time</td>
    192   1.1  christos </tr>
    193   1.8  christos <tr>
    194   1.1  christos   <td colspan="2">1942 only</td>
    195   1.1  christos   <td colspan="2">February 9<small><sup>th</sup></small></td>
    196  1.11  christos   <td>go to war time</td>
    197   1.1  christos </tr>
    198   1.8  christos <tr>
    199   1.1  christos   <td colspan="2" rowspan="2">1945 only</td>
    200   1.1  christos   <td colspan="2">August 14<small><sup>th</sup></small></td>
    201   1.1  christos   <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td>
    202   1.1  christos   <td>
    203  1.11  christos     rename war time to peace<br>time;
    204  1.11  christos     clocks dont change
    205   1.1  christos   </td>
    206   1.1  christos </tr>
    207   1.8  christos <tr>
    208   1.1  christos   <td colspan="2">September 30<small><sup>th</sup></small></td>
    209   1.1  christos   <td rowspan="9">02:00 local</td>
    210   1.1  christos   <td rowspan="2">return to standard time</td>
    211   1.1  christos </tr>
    212   1.8  christos <tr>
    213   1.1  christos   <td rowspan="2">1967</td>
    214   1.1  christos   <td>2006</td>
    215   1.1  christos   <td rowspan="2">last Sunday</td>
    216   1.1  christos   <td>in October</td>
    217   1.1  christos </tr>
    218   1.8  christos <tr>
    219   1.1  christos   <td>1973</td>
    220   1.1  christos   <td>in April</td>
    221   1.1  christos   <td rowspan="6">go to daylight saving time</td>
    222   1.1  christos </tr>
    223   1.8  christos <tr>
    224   1.1  christos   <td colspan="2">1974 only</td>
    225   1.1  christos   <td colspan="2">January 6<small><sup>th</sup></small></td>
    226   1.1  christos </tr>
    227   1.8  christos <tr>
    228   1.1  christos   <td colspan="2">1975 only</td>
    229   1.1  christos   <td colspan="2">February 23<small><sup>rd</sup></small></td>
    230   1.1  christos </tr>
    231   1.8  christos <tr>
    232   1.1  christos   <td>1976</td>
    233   1.1  christos   <td>1986</td>
    234   1.1  christos   <td>last Sunday</td>
    235   1.1  christos   <td rowspan="2">in April</td>
    236   1.1  christos </tr>
    237   1.8  christos <tr>
    238   1.1  christos   <td>1987</td>
    239   1.1  christos   <td>2006</td>
    240   1.1  christos   <td>first Sunday</td>
    241   1.1  christos </tr>
    242   1.8  christos <tr>
    243   1.1  christos   <td rowspan="2">2007</td>
    244   1.1  christos   <td rowspan="2">present</td>
    245   1.1  christos   <td colspan="2">second Sunday in March</td>
    246   1.1  christos </tr>
    247   1.8  christos <tr>
    248   1.1  christos   <td colspan="2">first Sunday in November</td>
    249   1.1  christos   <td>return to standard time</td>
    250   1.1  christos </tr>
    251   1.1  christos </table>
    252   1.1  christos 
    253   1.1  christos <p>There are two interesting things to note here.</p>
    254   1.1  christos 
    255   1.1  christos <p>First, the time that something happens (in the <code>AT</code>
    256   1.7  christos column) is not necessarily the local (wall clock) time. The time can be
    257  1.11  christos suffixed with s (for standard) to mean
    258   1.7  christos local standard time, different from local (wall clock) time when observing
    259  1.11  christos daylight saving time; or it can be suffixed with g,
    260  1.11  christos u, or z, all three of which mean the
    261   1.1  christos standard time at the
    262   1.2  christos <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>.
    263  1.11  christos g stands for <a
    264  1.11  christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>;
    265  1.11  christos u stands for <a
    266  1.11  christos href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> or <a
    267  1.11  christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
    268  1.11  christos (whichever was official at the time); z stands for the
    269   1.1  christos <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a>
    270  1.11  christos Z (a.k.a. Zulu which, in turn, stands for Z).
    271  1.11  christos The time can also be suffixed with w meaning local (wall
    272  1.11  christos clock) time; but it usually isnt because thats the
    273   1.1  christos default.</p>
    274   1.1  christos 
    275   1.1  christos <p>Second, the day in the <code>ON</code> column, in addition to
    276  1.11  christos <code>lastSun</code> or a particular day of the month,
    277  1.11  christos can have the form, <code>Sun&gt;=</code><i>x</i> or
    278  1.11  christos <code>Sun&lt;=</code><i>x</i>, where <i>x</i> is a day
    279  1.11  christos of the month. For example, <code>Sun&gt;=8</code> means
    280  1.11  christos the first Sunday on or after the eighth of the month, in
    281   1.1  christos other words, the second Sunday of the month. Furthermore, although
    282  1.11  christos there are no examples above, the weekday neednt be
    283  1.11  christos <code>Sun</code> in either form, but can be the usual
    284   1.1  christos three-character English abbreviation for any day of the week.</p>
    285   1.1  christos 
    286   1.1  christos <p>And the US rules give us more examples of a couple of things
    287   1.1  christos already mentioned:</p>
    288   1.1  christos 
    289   1.1  christos <ul>
    290   1.1  christos <li>The rules for changing to and from daylight saving time are
    291   1.1  christos actually <i>different sets</i> of rules; and the two sets can change
    292   1.1  christos independently. Consider, for example, that the rule for the return to
    293   1.1  christos standard time stayed the same from 1967 to 2006; but the rule for the
    294   1.1  christos transition to daylight saving time changed several times in the same
    295   1.1  christos period.  There can also be periods, 1946 to 1966 for example, when no
    296   1.1  christos rule from this group is in effect, and so either no transition
    297   1.1  christos happened in those years, or some other rule is in effect (perhaps a
    298   1.1  christos state or other more local rule).</li>
    299   1.1  christos 
    300   1.1  christos <li>The <code>SAVE</code> and <code>LETTER</code> columns
    301   1.1  christos contain <i>steady state</i>, not transitions. Consider, for example,
    302  1.11  christos the transition from war time to peace time
    303  1.11  christos that happened on August 14, 1945. The 1:00 in
    304   1.1  christos the <code>SAVE</code> column is <i>not</i> an instruction to advance
    305   1.1  christos the clock an hour. It means that clocks should <i>be</i> one hour
    306   1.1  christos ahead of standard time, which they already are because of the previous
    307   1.1  christos rule, so there should be no change.</li>
    308   1.1  christos 
    309   1.1  christos </ul>
    310   1.1  christos 
    311  1.11  christos <p>OK, now lets look at a Zone record:</p>
    312   1.1  christos 
    313  1.12  christos <pre>
    314   1.7  christos #Zone       NAME      STDOFF   RULES FORMAT [UNTIL]
    315   1.1  christos Zone  America/Chicago -5:50:36 -       LMT  1883 Nov 18 12:09:24
    316   1.1  christos                       -6:00    US      C%sT 1920
    317   1.1  christos                       -6:00    Chicago C%sT 1936 Mar  1  2:00
    318   1.1  christos                       -5:00    -       EST  1936 Nov 15  2:00
    319   1.1  christos                       -6:00    Chicago C%sT 1942
    320   1.1  christos                       -6:00    US      C%sT 1946
    321   1.1  christos                       -6:00    Chicago C%sT 1967
    322   1.1  christos                       -6:00    US      C%sT
    323   1.1  christos </pre>
    324  1.12  christos <table>
    325   1.1  christos <tr>
    326   1.1  christos   <th colspan="5">Columns Renamed</th>
    327   1.1  christos </tr>
    328   1.1  christos <tr>
    329   1.1  christos   <th rowspan="2">Standard Offset<br>
    330   1.1  christos     from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime
    331   1.1  christos     Meridian</a></th>
    332   1.1  christos   <th rowspan="2">Daylight<br>Saving Time</th>
    333   1.1  christos   <th rowspan="2">Abbreviation(s)</th>
    334   1.1  christos   <th colspan="2">Ending at Local Time</th>
    335   1.1  christos </tr>
    336   1.1  christos <tr>
    337   1.1  christos   <th>Date</th>
    338   1.1  christos   <th>Time</th>
    339   1.1  christos </tr>
    340   1.8  christos <tr>
    341  1.11  christos   <td>5:50:36</td>
    342   1.1  christos   <td>not observed</td>
    343   1.1  christos   <td>LMT</td>
    344   1.1  christos   <td>1883-11-18</td>
    345   1.1  christos   <td>12:09:24</td>
    346   1.1  christos </tr>
    347   1.8  christos <tr>
    348  1.11  christos   <td rowspan="2">6:00:00</td>
    349   1.1  christos   <td>US rules</td>
    350   1.1  christos   <td rowspan="2">CST or CDT</td>
    351   1.1  christos   <td>1920-01-01</td>
    352   1.1  christos   <td>00:00:00</td>
    353   1.1  christos </tr>
    354   1.8  christos <tr>
    355   1.1  christos   <td>Chicago rules</td>
    356   1.1  christos   <td>1936-03-01</td>
    357   1.1  christos   <td rowspan="2">02:00:00</td>
    358   1.1  christos </tr>
    359   1.8  christos <tr>
    360  1.11  christos   <td>5:00:00</td>
    361   1.1  christos   <td>not observed</td>
    362   1.1  christos   <td>EST</td>
    363   1.1  christos   <td>1936-11-15</td>
    364   1.1  christos </tr>
    365   1.8  christos <tr>
    366  1.11  christos   <td rowspan="4">6:00:00</td>
    367   1.1  christos   <td>Chicago rules</td>
    368   1.1  christos   <td>CST or CDT</td>
    369   1.1  christos   <td>1942-01-01</td>
    370   1.1  christos   <td rowspan="3">00:00:00</td>
    371   1.1  christos </tr>
    372   1.8  christos <tr>
    373   1.1  christos   <td>US rules</td>
    374   1.1  christos   <td>CST, CWT or CPT</td>
    375   1.1  christos   <td>1946-01-01</td>
    376   1.1  christos </tr>
    377   1.8  christos <tr>
    378   1.1  christos   <td>Chicago rules</td>
    379   1.1  christos   <td rowspan="2">CST or CDT</td>
    380   1.1  christos   <td>1967-01-01</td>
    381   1.1  christos </tr>
    382   1.8  christos <tr>
    383   1.1  christos   <td>US rules</td>
    384  1.11  christos   <td colspan="2"></td>
    385   1.1  christos </tr>
    386   1.1  christos </table>
    387   1.1  christos 
    388   1.1  christos <p>There are a couple of interesting differences between Zones and Rules.</p>
    389   1.1  christos 
    390   1.1  christos <p>First, and somewhat trivially, whereas Rules are considered to
    391   1.1  christos contain one or more records, a Zone is considered to be a single
    392   1.1  christos record with zero or more <i>continuation lines</i>. Thus, the keyword,
    393  1.11  christos <code>Zone</code>, and the zone name are not
    394   1.1  christos repeated. The last line is the one without anything in
    395   1.1  christos the <code>[UNTIL]</code> column.</p>
    396   1.1  christos 
    397   1.1  christos <p>Second, and more fundamentally, each line of a Zone represents a
    398   1.1  christos steady state, not a transition between states. The state exists from
    399  1.11  christos the date and time in the previous lines <code>[UNTIL]</code>
    400   1.1  christos column up to the date and time in the current
    401  1.11  christos lines <code>[UNTIL]</code> column. In other words, the date and
    402   1.1  christos time in the <code>[UNTIL]</code> column is the instant that separates
    403   1.1  christos this state from the next. Where that would be ambiguous because
    404  1.11  christos were setting our clocks back, the <code>[UNTIL]</code> column
    405   1.1  christos specifies the first occurrence of the instant. The state specified by
    406   1.1  christos the last line, the one without anything in the <code>[UNTIL]</code>
    407   1.1  christos column, continues to the present.</p>
    408   1.1  christos 
    409   1.1  christos <p>The first line typically specifies the mean solar time observed
    410  1.11  christos before the introduction of standard time. Since theres no line before
    411   1.1  christos that, it has no beginning. <code>8-) </code> For some places near the <a
    412   1.1  christos href="https://en.wikipedia.org/wiki/International_Date_Line">International
    413   1.1  christos Date Line</a>, the first <i>two</i> lines will show solar times
    414   1.1  christos differing by 24 hours; this corresponds to a movement of the Date
    415   1.1  christos Line.  For example:</p>
    416   1.1  christos 
    417   1.1  christos <pre>
    418   1.7  christos #Zone NAME          STDOFF   RULES FORMAT [UNTIL]
    419  1.14  christos Zone America/Juneau 15:02:19 -     LMT    1867 Oct 19  0:31:13u
    420   1.1  christos                     -8:57:41 -     LMT    ...
    421   1.1  christos </pre>
    422   1.1  christos 
    423   1.1  christos <p>When Alaska was purchased from Russia in 1867, the Date Line moved
    424   1.1  christos from the Alaska/Canada border to the Bering Strait; and the time in
    425   1.1  christos Alaska was then 24 hours earlier than it had
    426  1.14  christos been. <code>&lt;aside&gt;</code>(This also means that, although the sun had set
    427  1.14  christos on Friday 6 October and risen on Saturday 7 October in the Julian calendar,
    428  1.14  christos which Russia was still using then for religious reasons, upon the transfer
    429  1.14  christos Saturday afternoon became Friday afternoon, and then the sun set and rose once
    430  1.14  christos more on <i>second instances of the same days with different names</i>,
    431  1.14  christos Friday 18 and Saturday 19 October in the Gregorian calendar. Isnt civil time
    432   1.1  christos wonderful? <code>8-)</code>)<code>&lt;/aside&gt;</code></p>
    433   1.1  christos 
    434  1.11  christos <p>The abbreviation, LMT stands for local mean
    435  1.11  christos time, which is an invention of
    436   1.1  christos the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
    437   1.1  christos database</a> and was probably never actually used during the
    438   1.1  christos period. Furthermore, the value is almost certainly wrong except in the
    439   1.1  christos archetypal place after which the zone is named. (The tz database
    440  1.11  christos usually doesnt provide a separate Zone record for places where
    441   1.1  christos nothing significant happened after 1970.)</p>
    442   1.1  christos 
    443   1.1  christos <p>The <code>RULES</code> column tells us whether daylight saving time is being observed:
    444   1.1  christos <ul>
    445   1.1  christos <li>A hyphen, a kind of null value, means that we have not set our
    446   1.1  christos clocks ahead of standard time.</li>
    447   1.1  christos 
    448  1.11  christos <li>An amount of time (usually but not necessarily 1:00
    449   1.1  christos meaning one hour) means that we have set our clocks ahead by that
    450   1.1  christos amount.</li>
    451   1.1  christos 
    452   1.1  christos <li>Some alphabetic string means that we <i>might have</i> set our
    453   1.1  christos clocks ahead; and we need to check the rule the name of which is the
    454   1.1  christos given alphabetic string.</li>
    455   1.1  christos </ul>
    456   1.1  christos 
    457   1.1  christos <p>An example of a specific amount of time is:</p>
    458   1.1  christos <pre>
    459   1.7  christos #Zone NAME            STDOFF RULES FORMAT [UNTIL]
    460   1.6  christos Zone Pacific/Honolulu ...                 1933 Apr 30  2:00
    461   1.6  christos                       -10:30 1:00  HDT    1933 May 21 12:00
    462   1.1  christos                       ...
    463   1.1  christos </pre>
    464   1.1  christos 
    465   1.1  christos <p>Hawaii tried daylight saving time for three weeks in 1933 and
    466  1.11  christos decided they didnt like it. <code>8-) </code>Note that
    467   1.7  christos the <code>STDOFF</code> column always contains the standard time
    468  1.11  christos offset, so the local (wall clock) time during this period was GMT 
    469  1.11  christos 10:30 + 1:00 = GMT  9:30.</p>
    470   1.1  christos 
    471   1.1  christos <p>The <code>FORMAT</code> column specifies the usual abbreviation of
    472   1.9  christos the time zone name. It should have one of four forms:</p>
    473   1.1  christos <ul>
    474   1.1  christos 
    475   1.9  christos <li>a time zone abbreviation that is a string of three or more
    476   1.9  christos characters that are either ASCII alphanumerics,
    477  1.11  christos <code>+</code>, or <code>-</code></li>
    478   1.9  christos 
    479  1.11  christos <li>the string %z, in which case the
    480  1.11  christos <code>%z</code> will be replaced by a numeric time zone
    481   1.9  christos abbreviation</li>
    482   1.1  christos 
    483   1.9  christos <li>a pair of time zone abbreviations separated by a slash
    484  1.11  christos (<code>/</code>), in which case the first string is the
    485   1.1  christos abbreviation for the standard time name and the second string is the
    486   1.1  christos abbreviation for the daylight saving time name</li>
    487   1.1  christos 
    488  1.11  christos <li>a string containing <code>%s</code>, in which case
    489  1.11  christos the <code>%s</code> will be replaced by the text in the
    490  1.11  christos appropriate Rules <code>LETTER</code> column, and the resulting
    491   1.9  christos string should be a time zone abbreviation</li>
    492   1.1  christos </ul>
    493   1.1  christos 
    494  1.11  christos <p>The last two make sense only if theres a named rule in effect.</p>
    495   1.1  christos 
    496   1.1  christos <p>An example of a slash is:</p>
    497   1.1  christos <pre>
    498   1.7  christos #Zone NAME          STDOFF RULES FORMAT  [UNTIL]
    499   1.1  christos Zone  Europe/London ...                  1996
    500   1.1  christos                     0:00   EU    GMT/BST
    501   1.1  christos </pre>
    502   1.1  christos 
    503   1.1  christos <p>The current time in the UK is called either Greenwich mean time or
    504   1.1  christos British summer time.</p>
    505   1.1  christos 
    506   1.1  christos <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what
    507   1.1  christos happens when switching to a named rule. To what values should
    508   1.1  christos the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p>
    509   1.1  christos 
    510   1.1  christos <ul>
    511   1.1  christos <li>If at least one transition has happened, use
    512   1.1  christos the <code>SAVE</code> and <code>LETTER</code> data from the most
    513   1.1  christos recent.</li>
    514   1.1  christos 
    515   1.1  christos <li>If switching to a named rule before any transition has happened,
    516   1.1  christos assume standard time (<code>SAVE</code> zero), and use
    517   1.1  christos the <code>LETTER</code> data from the earliest transition with
    518   1.1  christos a <code>SAVE</code> of zero.
    519   1.1  christos 
    520   1.1  christos </ul>
    521   1.1  christos 
    522   1.1  christos <p>And three last things about the <code>FORMAT</code> column:</p>
    523   1.1  christos <ul>
    524   1.1  christos 
    525   1.1  christos <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz
    526  1.10  christos database</a> gives abbreviations for time zones
    527  1.10  christos in popular English-language usage. For
    528   1.1  christos example, the last line in
    529   1.1  christos <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives
    530  1.11  christos HST for Hawaii standard time even though the
    531   1.3  christos <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a>
    532  1.11  christos name for that time zone is HawaiiAleutian standard time.
    533   1.1  christos This author has read that there are also some places in Australia where
    534   1.1  christos popular time zone names differ from the legal ones.
    535   1.1  christos 
    536   1.1  christos <li>No attempt is made to <a
    537   1.1  christos href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a>
    538   1.1  christos the abbreviations. They are intended to be the values returned through the
    539   1.1  christos <code>"%Z"</code> format specifier to
    540  1.11  christos <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>s
    541  1.11  christos <a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/strftime.html"><code>strftime</code></a>
    542   1.1  christos function in the
    543  1.11  christos <a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">C locale</a>.
    544   1.1  christos 
    545  1.10  christos <li>If there is no generally accepted abbreviation for a time zone,
    546   1.1  christos a numeric offset is used instead, e.g., <code>+07</code> for 7 hours
    547   1.1  christos ahead of Greenwich. By convention, <code>-00</code> is used in a
    548   1.1  christos zone while uninhabited, where the offset is zero but in some sense
    549   1.1  christos the true offset is undefined.
    550   1.1  christos </ul>
    551   1.1  christos 
    552  1.11  christos <p>As a final example, heres the complete history for Hawaii:</p>
    553   1.1  christos 
    554  1.12  christos <pre># Relevant Excerpts from the US Rules
    555   1.8  christos #Rule NAME FROM TO   -    IN  ON      AT     SAVE LETTER/S
    556   1.1  christos Rule  US   1918 1919 -    Oct lastSun  2:00  0    S
    557   1.1  christos Rule  US   1942 only -    Feb  9       2:00  1:00 W # War
    558   1.1  christos Rule  US   1945 only -    Aug 14      23:00u 1:00 P # Peace
    559   1.6  christos Rule  US   1945 only -    Sep lastSun  2:00  0    S
    560  1.12  christos 
    561  1.12  christos # The Zone Record
    562   1.7  christos #Zone NAME            STDOFF    RULES FORMAT [UNTIL]
    563   1.6  christos Zone Pacific/Honolulu -10:31:26 -     LMT    1896 Jan 13 12:00
    564   1.1  christos                       -10:30    -     HST    1933 Apr 30  2:00
    565   1.1  christos                       -10:30    1:00  HDT    1933 May 21  2:00
    566   1.1  christos                       -10:30    US    H%sT   1947 Jun  8  2:00
    567   1.1  christos                       -10:00    -     HST
    568   1.1  christos </pre>
    569  1.12  christos <table>
    570   1.1  christos <tr>
    571   1.1  christos   <th colspan="6">What We Infer</th>
    572   1.1  christos </tr>
    573   1.1  christos <tr>
    574   1.1  christos   <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th>
    575   1.1  christos   <th rowspan="2">Adjust<br>Clocks</th>
    576   1.1  christos   <th colspan="2">Time Zone</th>
    577   1.1  christos   <th colspan="2">Ending at Local Time</th>
    578   1.1  christos </tr>
    579   1.1  christos <tr>
    580   1.1  christos   <th>Abbrv.</th>
    581   1.1  christos   <th>Name</th>
    582   1.1  christos   <th>Date</th>
    583   1.1  christos   <th>Time</th>
    584   1.1  christos </tr>
    585   1.8  christos <tr>
    586  1.11  christos   <td>10:31:26</td>
    587  1.11  christos   <td></td>
    588   1.1  christos   <td>LMT</td>
    589   1.1  christos   <td>local mean time</td>
    590   1.6  christos   <td>1896-01-13</td>
    591   1.1  christos   <td>12:00</td>
    592   1.1  christos </tr>
    593   1.8  christos <tr>
    594  1.11  christos   <td>10:30</td>
    595   1.1  christos   <td>+0:01:26</td>
    596   1.1  christos   <td>HST</td>
    597   1.1  christos   <td>Hawaii standard time</td>
    598   1.1  christos   <td>1933-04-30</td>
    599   1.6  christos   <td>02:00</td>
    600   1.1  christos </tr>
    601   1.8  christos <tr>
    602  1.11  christos   <td>9:30</td>
    603   1.1  christos   <td>+1:00</td>
    604   1.1  christos   <td>HDT</td>
    605   1.1  christos   <td>Hawaii daylight time</td>
    606   1.1  christos   <td>1933-05-21</td>
    607   1.6  christos   <td>12:00</td>
    608   1.1  christos </tr>
    609   1.8  christos <tr>
    610  1.11  christos   <td>10:30&sup1;</td>
    611  1.11  christos   <td>1:00&sup1;</td>
    612   1.1  christos   <td>HST&sup1;</td>
    613   1.1  christos   <td>Hawaii standard time</td>
    614   1.1  christos   <td>1942-02-09</td>
    615   1.6  christos   <td>02:00</td>
    616   1.1  christos </tr>
    617   1.8  christos <tr>
    618  1.11  christos   <td rowspan="2">9:30</td>
    619   1.1  christos   <td>+1:00</td>
    620   1.1  christos   <td>HWT</td>
    621   1.1  christos   <td>Hawaii war time</td>
    622   1.1  christos   <td>1945-08-14</td>
    623   1.1  christos   <td>13:30&sup2;</td>
    624   1.1  christos </tr>
    625   1.8  christos <tr>
    626   1.1  christos   <td>0</td>
    627   1.1  christos   <td>HPT</td>
    628   1.1  christos   <td>Hawaii peace time</td>
    629   1.1  christos   <td>1945-09-30</td>
    630   1.1  christos   <td rowspan="2">02:00</td>
    631   1.1  christos </tr>
    632   1.8  christos <tr>
    633  1.11  christos   <td>10:30</td>
    634  1.11  christos   <td>1:00</td>
    635   1.1  christos   <td rowspan="2">HST</td>
    636   1.1  christos   <td rowspan="2">Hawaii standard time</td>
    637   1.1  christos   <td>1947-06-08</td>
    638   1.1  christos </tr>
    639   1.8  christos <tr>
    640  1.11  christos   <td>10:00&sup3;</td>
    641   1.1  christos   <td>+0:30&sup3;</td>
    642  1.11  christos   <td colspan="2"></td>
    643   1.1  christos </tr>
    644   1.1  christos <tr>
    645   1.8  christos   <td colspan="6" class="footnote">
    646  1.11  christos     &sup1;Switching to US rules...most recent transition (in 1919) was to standard time
    647   1.1  christos   </td>
    648   1.1  christos </tr>
    649   1.1  christos <tr>
    650   1.8  christos   <td colspan="6" class="footnote">
    651   1.1  christos     &sup2;23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>
    652  1.11  christos     + (9:30) = 13:30 local
    653   1.1  christos   </td>
    654   1.1  christos </tr>
    655   1.1  christos <tr>
    656   1.8  christos   <td colspan="6" class="footnote">
    657  1.11  christos     &sup3;Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947-06-08T12:30Z</a>,
    658   1.1  christos     the civil time in Hawaii has been
    659   1.1  christos     <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
    660  1.11  christos     10:00 year-round.
    661   1.1  christos   </td>
    662   1.1  christos </tr>
    663   1.1  christos </table>
    664   1.1  christos 
    665   1.1  christos <p>There will be a short quiz later. <code>8-)</code></p>
    666  1.14  christos </main>
    667   1.1  christos 
    668  1.14  christos <footer>
    669   1.1  christos <hr>
    670   1.1  christos This web page is in the public domain, so clarified as of
    671   1.1  christos 2015-10-20 by Bill Seymour.
    672   1.1  christos <br>
    673   1.1  christos All suggestions and corrections will be welcome; all flames will be amusing.
    674   1.1  christos Mail to was at pobox dot com.
    675  1.14  christos </footer>
    676   1.1  christos </body>
    677   1.1  christos </html>
    678