libc/time/tz-how-to.html

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 1.3  christos <title>How to Read the tz Database</title>
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 1.1  christos <body>
 1.1  christos <h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
 1.1  christos Database</a> Source Files</h2>
 1.1  christos <h3>by Bill Seymour</h3>
 1.9  christos <p>This guide uses the <code>America/Chicago</code> and
 1.1  christos <code>Pacific/Honolulu</code> zones as examples of how to infer
 1.4  christos times of day from the <a href="tz-link.html">tz database</a>
 1.1  christos source files. It might be helpful, but not absolutely necessary,
 1.1  christos for the reader to have already downloaded the
 1.2  christos latest release of the database and become familiar with the basic layout
1.11  christos of the data files. The format is explained in the man page
1.11  christos for the zic compiler, <code>zic.8.txt</code>, in
 1.9  christos the <code>code</code> subdirectory.
 1.9  christos Although this guide covers many of the common cases, it is not a
 1.9  christos complete summary of what zic accepts; the man page is the
 1.9  christos authoritative reference.</p>
 1.1  christos
1.11  christos <p>Well begin by talking about the rules for changing between standard
1.11  christos and daylight saving time since well need that information when we talk
 1.1  christos about the zones.</p>
 1.1  christos
1.11  christos <p>First, lets consider the special daylight saving time rules
 1.1  christos for Chicago (from the <code>northamerica</code> file in
 1.1  christos the <code>data</code> subdirectory):</p>
 1.1  christos
1.12  christos <pre>#Rule NAME    FROM TO    -   IN  ON      AT   SAVE LETTER
 1.1  christos Rule  Chicago 1920 only  -   Jun 13      2:00 1:00 D
 1.1  christos Rule  Chicago 1920 1921  -   Oct lastSun 2:00 0    S
 1.1  christos Rule  Chicago 1921 only  -   Mar lastSun 2:00 1:00 D
 1.1  christos Rule  Chicago 1922 1966  -   Apr lastSun 2:00 1:00 D
 1.1  christos Rule  Chicago 1922 1954  -   Sep lastSun 2:00 0    S
 1.1  christos Rule  Chicago 1955 1966  -   Oct lastSun 2:00 0    S
 1.1  christos </pre>
1.12  christos <table>
 1.1  christos <tr>
 1.1  christos   <th colspan="6">Reformatted a Bit</th>
 1.1  christos </tr>
 1.1  christos <tr>
 1.1  christos   <th>From</th>
 1.1  christos   <th>To</th>
 1.1  christos   <th colspan="2">On</th>
 1.1  christos   <th>At</th>
 1.1  christos   <th>Action</th>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td colspan="2">1920 only</td>
 1.1  christos   <td colspan="2">June 13<small><sup>th</sup></small></td>
 1.1  christos   <td rowspan="6">02:00 local</td>
 1.1  christos   <td>go to daylight saving time</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>1920</td>
 1.1  christos   <td>1921</td>
 1.1  christos   <td rowspan="5">last Sunday</td>
 1.1  christos   <td>in October</td>
 1.1  christos   <td>return to standard time</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td colspan="2">1921 only</td>
 1.1  christos   <td>in March</td>
 1.1  christos   <td rowspan="2">go to daylight saving time</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td rowspan="2">1922</td>
 1.1  christos   <td>1966</td>
 1.1  christos   <td>in April</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>1954</td>
 1.1  christos   <td>in September</td>
 1.1  christos   <td rowspan="2">return to standard time</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>1955</td>
 1.1  christos   <td>1966</td>
 1.1  christos   <td>in October</td>
 1.1  christos </tr>
 1.1  christos </table>
 1.1  christos
 1.8  christos <p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the
 1.8  christos first and last calendar years defining a contiguous range over which a specific
 1.8  christos Rule line is to apply.  The keyword <code>only</code> can be used in the
 1.8  christos <code>TO</code> field to repeat the value of the <code>FROM</code> field in the
 1.8  christos event that a rule should only apply to a single year.  Often, the keyword
1.11  christos <code>max</code> is used to extend a rules application into the
 1.8  christos indefinite future; it is a platform-agnostic stand-in for the largest
 1.8  christos representable year.
 1.8  christos
 1.8  christos <p>The next column, <code>-</code>, is reserved; for compatibility with earlier
 1.8  christos releases, it always contains a hyphen, which acts as a kind of null value.
 1.8  christos Prior to the 2020b release, it was called the <code>TYPE</code> field, though
 1.9  christos it had not been used in the main data since the 2000e release.
 1.9  christos An obsolescent supplementary file used the
 1.8  christos field as a proof-of-concept to allow <code>zic</code> to apply a given Rule
1.11  christos line only to certain types of years within the specified range as
 1.8  christos dictated by the output of a separate script, such as: only years which would
1.11  christos have a US presidential election, or only years which wouldnt.
 1.1  christos
 1.7  christos <p>The <code>SAVE</code> column contains the local (wall clock) offset from
 1.1  christos local standard time.
 1.1  christos This is usually either zero for standard time or one hour for daylight
1.11  christos saving time; but theres no reason, in principle, why it cant
 1.1  christos take on other values.
 1.1  christos
 1.1  christos <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>)
 1.1  christos column can contain a variable
1.11  christos part of the usual abbreviation of the time zones name, or it can just
1.11  christos be a hyphen if theres no variable part. For example, the abbreviation
1.11  christos used in the central time zone will be either CST or CDT.
1.11  christos The variable part is S or D;
1.11  christos and, sure enough, thats just what we find in
 1.1  christos the <code>LETTER</code> column
 1.1  christos in the <code>Chicago</code> rules. More about this when we talk about
1.11  christos Zone lines.
 1.1  christos
1.11  christos <p>One important thing to notice is that Rule lines
 1.1  christos want at once to be both <i>transitions</i> and <i>steady states</i>:
 1.1  christos <ul>
 1.1  christos <li>On the one hand, they represent transitions between standard and
 1.1  christos daylight saving time; and any number of Rule lines can be in effect
 1.1  christos during a given period (which will always be a non-empty set of
 1.1  christos contiguous calendar years).</li>
 1.1  christos <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code>
 1.1  christos columns contain state that exists between transitions. More about this
 1.1  christos when we talk about the US rules.</li>
 1.1  christos </ul>
 1.1  christos
 1.1  christos <p>In the example above, the transition to daylight saving time
 1.1  christos happened on the 13<small><sup>th</sup></small> of June in 1920, and on
 1.1  christos the last Sunday in March in 1921; but the return to standard time
 1.1  christos happened on the last Sunday in October in both of those
 1.1  christos years. Similarly, the rule for changing to daylight saving time was
 1.1  christos the same from 1922 to 1966; but the rule for returning to standard
 1.1  christos time changed in 1955. Got it?</p>
 1.1  christos
1.11  christos <p>OK, now for the somewhat more interesting US rules:</p>
 1.1  christos
1.12  christos <pre>#Rule NAME FROM TO    -   IN  ON        AT   SAVE LETTER/S
 1.1  christos Rule  US   1918 1919  -   Mar lastSun  2:00  1:00 D
 1.1  christos Rule  US   1918 1919  -   Oct lastSun  2:00  0    S
 1.1  christos Rule  US   1942 only  -   Feb 9        2:00  1:00 W # War
 1.1  christos Rule  US   1945 only  -   Aug 14      23:00u 1:00 P # Peace
 1.1  christos Rule  US   1945 only  -   Sep 30       2:00  0    S
 1.1  christos Rule  US   1967 2006  -   Oct lastSun  2:00  0    S
 1.1  christos Rule  US   1967 1973  -   Apr lastSun  2:00  1:00 D
 1.1  christos Rule  US   1974 only  -   Jan 6        2:00  1:00 D
 1.1  christos Rule  US   1975 only  -   Feb 23       2:00  1:00 D
 1.1  christos Rule  US   1976 1986  -   Apr lastSun  2:00  1:00 D
 1.1  christos Rule  US   1987 2006  -   Apr Sun&gt;=1   2:00  1:00 D
 1.1  christos Rule  US   2007 max   -   Mar Sun&gt;=8   2:00  1:00 D
 1.1  christos Rule  US   2007 max   -   Nov Sun&gt;=1   2:00  0    S
 1.1  christos </pre>
1.12  christos <table>
 1.1  christos <tr>
 1.1  christos   <th colspan="6">Reformatted a Bit</th>
 1.1  christos </tr>
 1.1  christos <tr>
 1.1  christos   <th>From</th>
 1.1  christos   <th>To</th>
 1.1  christos   <th colspan="2">On</th>
 1.1  christos   <th>At</th>
 1.1  christos   <th>Action</th>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td rowspan="2">1918</td>
 1.1  christos   <td rowspan="2">1919</td>
 1.1  christos   <td rowspan="2">last Sunday</td>
 1.1  christos   <td>in March</td>
 1.1  christos   <td rowspan="3">02:00 local</td>
 1.1  christos   <td>go to daylight saving time</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>in October</td>
 1.1  christos   <td>return to standard time</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td colspan="2">1942 only</td>
 1.1  christos   <td colspan="2">February 9<small><sup>th</sup></small></td>
1.11  christos   <td>go to war time</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td colspan="2" rowspan="2">1945 only</td>
 1.1  christos   <td colspan="2">August 14<small><sup>th</sup></small></td>
 1.1  christos   <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td>
 1.1  christos   <td>
1.11  christos     rename war time to peace<br>time;
1.11  christos     clocks dont change
 1.1  christos   </td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td colspan="2">September 30<small><sup>th</sup></small></td>
 1.1  christos   <td rowspan="9">02:00 local</td>
 1.1  christos   <td rowspan="2">return to standard time</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td rowspan="2">1967</td>
 1.1  christos   <td>2006</td>
 1.1  christos   <td rowspan="2">last Sunday</td>
 1.1  christos   <td>in October</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>1973</td>
 1.1  christos   <td>in April</td>
 1.1  christos   <td rowspan="6">go to daylight saving time</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td colspan="2">1974 only</td>
 1.1  christos   <td colspan="2">January 6<small><sup>th</sup></small></td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td colspan="2">1975 only</td>
 1.1  christos   <td colspan="2">February 23<small><sup>rd</sup></small></td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>1976</td>
 1.1  christos   <td>1986</td>
 1.1  christos   <td>last Sunday</td>
 1.1  christos   <td rowspan="2">in April</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>1987</td>
 1.1  christos   <td>2006</td>
 1.1  christos   <td>first Sunday</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td rowspan="2">2007</td>
 1.1  christos   <td rowspan="2">present</td>
 1.1  christos   <td colspan="2">second Sunday in March</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td colspan="2">first Sunday in November</td>
 1.1  christos   <td>return to standard time</td>
 1.1  christos </tr>
 1.1  christos </table>
 1.1  christos
 1.1  christos <p>There are two interesting things to note here.</p>
 1.1  christos
 1.1  christos <p>First, the time that something happens (in the <code>AT</code>
 1.7  christos column) is not necessarily the local (wall clock) time. The time can be
1.11  christos suffixed with s (for standard) to mean
 1.7  christos local standard time, different from local (wall clock) time when observing
1.11  christos daylight saving time; or it can be suffixed with g,
1.11  christos u, or z, all three of which mean the
 1.1  christos standard time at the
 1.2  christos <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>.
1.11  christos g stands for <a
1.11  christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>;
1.11  christos u stands for <a
1.11  christos href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> or <a
1.11  christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
1.11  christos (whichever was official at the time); z stands for the
 1.1  christos <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a>
1.11  christos Z (a.k.a. Zulu which, in turn, stands for Z).
1.11  christos The time can also be suffixed with w meaning local (wall
1.11  christos clock) time; but it usually isnt because thats the
 1.1  christos default.</p>
 1.1  christos
 1.1  christos <p>Second, the day in the <code>ON</code> column, in addition to
1.11  christos <code>lastSun</code> or a particular day of the month,
1.11  christos can have the form, <code>Sun&gt;=</code><i>x</i> or
1.11  christos <code>Sun&lt;=</code><i>x</i>, where <i>x</i> is a day
1.11  christos of the month. For example, <code>Sun&gt;=8</code> means
1.11  christos the first Sunday on or after the eighth of the month, in
 1.1  christos other words, the second Sunday of the month. Furthermore, although
1.11  christos there are no examples above, the weekday neednt be
1.11  christos <code>Sun</code> in either form, but can be the usual
 1.1  christos three-character English abbreviation for any day of the week.</p>
 1.1  christos
 1.1  christos <p>And the US rules give us more examples of a couple of things
 1.1  christos already mentioned:</p>
 1.1  christos
 1.1  christos <ul>
 1.1  christos <li>The rules for changing to and from daylight saving time are
 1.1  christos actually <i>different sets</i> of rules; and the two sets can change
 1.1  christos independently. Consider, for example, that the rule for the return to
 1.1  christos standard time stayed the same from 1967 to 2006; but the rule for the
 1.1  christos transition to daylight saving time changed several times in the same
 1.1  christos period.  There can also be periods, 1946 to 1966 for example, when no
 1.1  christos rule from this group is in effect, and so either no transition
 1.1  christos happened in those years, or some other rule is in effect (perhaps a
 1.1  christos state or other more local rule).</li>
 1.1  christos
 1.1  christos <li>The <code>SAVE</code> and <code>LETTER</code> columns
 1.1  christos contain <i>steady state</i>, not transitions. Consider, for example,
1.11  christos the transition from war time to peace time
1.11  christos that happened on August 14, 1945. The 1:00 in
 1.1  christos the <code>SAVE</code> column is <i>not</i> an instruction to advance
 1.1  christos the clock an hour. It means that clocks should <i>be</i> one hour
 1.1  christos ahead of standard time, which they already are because of the previous
 1.1  christos rule, so there should be no change.</li>
 1.1  christos
 1.1  christos </ul>
 1.1  christos
1.11  christos <p>OK, now lets look at a Zone record:</p>
 1.1  christos
1.12  christos <pre>
 1.7  christos #Zone       NAME      STDOFF   RULES FORMAT [UNTIL]
 1.1  christos Zone  America/Chicago -5:50:36 -       LMT  1883 Nov 18 12:09:24
 1.1  christos                       -6:00    US      C%sT 1920
 1.1  christos                       -6:00    Chicago C%sT 1936 Mar  1  2:00
 1.1  christos                       -5:00    -       EST  1936 Nov 15  2:00
 1.1  christos                       -6:00    Chicago C%sT 1942
 1.1  christos                       -6:00    US      C%sT 1946
 1.1  christos                       -6:00    Chicago C%sT 1967
 1.1  christos                       -6:00    US      C%sT
 1.1  christos </pre>
1.12  christos <table>
 1.1  christos <tr>
 1.1  christos   <th colspan="5">Columns Renamed</th>
 1.1  christos </tr>
 1.1  christos <tr>
 1.1  christos   <th rowspan="2">Standard Offset<br>
 1.1  christos     from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime
 1.1  christos     Meridian</a></th>
 1.1  christos   <th rowspan="2">Daylight<br>Saving Time</th>
 1.1  christos   <th rowspan="2">Abbreviation(s)</th>
 1.1  christos   <th colspan="2">Ending at Local Time</th>
 1.1  christos </tr>
 1.1  christos <tr>
 1.1  christos   <th>Date</th>
 1.1  christos   <th>Time</th>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td>5:50:36</td>
 1.1  christos   <td>not observed</td>
 1.1  christos   <td>LMT</td>
 1.1  christos   <td>1883-11-18</td>
 1.1  christos   <td>12:09:24</td>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td rowspan="2">6:00:00</td>
 1.1  christos   <td>US rules</td>
 1.1  christos   <td rowspan="2">CST or CDT</td>
 1.1  christos   <td>1920-01-01</td>
 1.1  christos   <td>00:00:00</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>Chicago rules</td>
 1.1  christos   <td>1936-03-01</td>
 1.1  christos   <td rowspan="2">02:00:00</td>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td>5:00:00</td>
 1.1  christos   <td>not observed</td>
 1.1  christos   <td>EST</td>
 1.1  christos   <td>1936-11-15</td>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td rowspan="4">6:00:00</td>
 1.1  christos   <td>Chicago rules</td>
 1.1  christos   <td>CST or CDT</td>
 1.1  christos   <td>1942-01-01</td>
 1.1  christos   <td rowspan="3">00:00:00</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>US rules</td>
 1.1  christos   <td>CST, CWT or CPT</td>
 1.1  christos   <td>1946-01-01</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>Chicago rules</td>
 1.1  christos   <td rowspan="2">CST or CDT</td>
 1.1  christos   <td>1967-01-01</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>US rules</td>
1.11  christos   <td colspan="2"></td>
 1.1  christos </tr>
 1.1  christos </table>
 1.1  christos
 1.1  christos <p>There are a couple of interesting differences between Zones and Rules.</p>
 1.1  christos
 1.1  christos <p>First, and somewhat trivially, whereas Rules are considered to
 1.1  christos contain one or more records, a Zone is considered to be a single
 1.1  christos record with zero or more <i>continuation lines</i>. Thus, the keyword,
1.11  christos <code>Zone</code>, and the zone name are not
 1.1  christos repeated. The last line is the one without anything in
 1.1  christos the <code>[UNTIL]</code> column.</p>
 1.1  christos
 1.1  christos <p>Second, and more fundamentally, each line of a Zone represents a
 1.1  christos steady state, not a transition between states. The state exists from
1.11  christos the date and time in the previous lines <code>[UNTIL]</code>
 1.1  christos column up to the date and time in the current
1.11  christos lines <code>[UNTIL]</code> column. In other words, the date and
 1.1  christos time in the <code>[UNTIL]</code> column is the instant that separates
 1.1  christos this state from the next. Where that would be ambiguous because
1.11  christos were setting our clocks back, the <code>[UNTIL]</code> column
 1.1  christos specifies the first occurrence of the instant. The state specified by
 1.1  christos the last line, the one without anything in the <code>[UNTIL]</code>
 1.1  christos column, continues to the present.</p>
 1.1  christos
 1.1  christos <p>The first line typically specifies the mean solar time observed
1.11  christos before the introduction of standard time. Since theres no line before
 1.1  christos that, it has no beginning. <code>8-) </code> For some places near the <a
 1.1  christos href="https://en.wikipedia.org/wiki/International_Date_Line">International
 1.1  christos Date Line</a>, the first <i>two</i> lines will show solar times
 1.1  christos differing by 24 hours; this corresponds to a movement of the Date
 1.1  christos Line.  For example:</p>
 1.1  christos
 1.1  christos <pre>
 1.7  christos #Zone NAME          STDOFF   RULES FORMAT [UNTIL]
 1.1  christos Zone America/Juneau 15:02:19 -     LMT    1867 Oct 18
 1.1  christos                     -8:57:41 -     LMT    ...
 1.1  christos </pre>
 1.1  christos
 1.1  christos <p>When Alaska was purchased from Russia in 1867, the Date Line moved
 1.1  christos from the Alaska/Canada border to the Bering Strait; and the time in
 1.1  christos Alaska was then 24 hours earlier than it had
 1.1  christos been. <code>&lt;aside&gt;</code>(6 October in the Julian calendar,
 1.1  christos which Russia was still using then for religious reasons, was followed
 1.1  christos by <i>a second instance of the same day with a different name</i>, 18
1.11  christos October in the Gregorian calendar. Isnt civil time
 1.1  christos wonderful? <code>8-)</code>)<code>&lt;/aside&gt;</code></p>
 1.1  christos
1.11  christos <p>The abbreviation, LMT stands for local mean
1.11  christos time, which is an invention of
 1.1  christos the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
 1.1  christos database</a> and was probably never actually used during the
 1.1  christos period. Furthermore, the value is almost certainly wrong except in the
 1.1  christos archetypal place after which the zone is named. (The tz database
1.11  christos usually doesnt provide a separate Zone record for places where
 1.1  christos nothing significant happened after 1970.)</p>
 1.1  christos
 1.1  christos <p>The <code>RULES</code> column tells us whether daylight saving time is being observed:
 1.1  christos <ul>
 1.1  christos <li>A hyphen, a kind of null value, means that we have not set our
 1.1  christos clocks ahead of standard time.</li>
 1.1  christos
1.11  christos <li>An amount of time (usually but not necessarily 1:00
 1.1  christos meaning one hour) means that we have set our clocks ahead by that
 1.1  christos amount.</li>
 1.1  christos
 1.1  christos <li>Some alphabetic string means that we <i>might have</i> set our
 1.1  christos clocks ahead; and we need to check the rule the name of which is the
 1.1  christos given alphabetic string.</li>
 1.1  christos </ul>
 1.1  christos
 1.1  christos <p>An example of a specific amount of time is:</p>
 1.1  christos <pre>
 1.7  christos #Zone NAME            STDOFF RULES FORMAT [UNTIL]
 1.6  christos Zone Pacific/Honolulu ...                 1933 Apr 30  2:00
 1.6  christos                       -10:30 1:00  HDT    1933 May 21 12:00
 1.1  christos                       ...
 1.1  christos </pre>
 1.1  christos
 1.1  christos <p>Hawaii tried daylight saving time for three weeks in 1933 and
1.11  christos decided they didnt like it. <code>8-) </code>Note that
 1.7  christos the <code>STDOFF</code> column always contains the standard time
1.11  christos offset, so the local (wall clock) time during this period was GMT
1.11  christos 10:30 + 1:00 = GMT  9:30.</p>
 1.1  christos
 1.1  christos <p>The <code>FORMAT</code> column specifies the usual abbreviation of
 1.9  christos the time zone name. It should have one of four forms:</p>
 1.1  christos <ul>
 1.1  christos
 1.9  christos <li>a time zone abbreviation that is a string of three or more
 1.9  christos characters that are either ASCII alphanumerics,
1.11  christos <code>+</code>, or <code>-</code></li>
 1.9  christos
1.11  christos <li>the string %z, in which case the
1.11  christos <code>%z</code> will be replaced by a numeric time zone
 1.9  christos abbreviation</li>
 1.1  christos
 1.9  christos <li>a pair of time zone abbreviations separated by a slash
1.11  christos (<code>/</code>), in which case the first string is the
 1.1  christos abbreviation for the standard time name and the second string is the
 1.1  christos abbreviation for the daylight saving time name</li>
 1.1  christos
1.11  christos <li>a string containing <code>%s</code>, in which case
1.11  christos the <code>%s</code> will be replaced by the text in the
1.11  christos appropriate Rules <code>LETTER</code> column, and the resulting
 1.9  christos string should be a time zone abbreviation</li>
 1.1  christos </ul>
 1.1  christos
1.11  christos <p>The last two make sense only if theres a named rule in effect.</p>
 1.1  christos
 1.1  christos <p>An example of a slash is:</p>
 1.1  christos <pre>
 1.7  christos #Zone NAME          STDOFF RULES FORMAT  [UNTIL]
 1.1  christos Zone  Europe/London ...                  1996
 1.1  christos                     0:00   EU    GMT/BST
 1.1  christos </pre>
 1.1  christos
 1.1  christos <p>The current time in the UK is called either Greenwich mean time or
 1.1  christos British summer time.</p>
 1.1  christos
 1.1  christos <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what
 1.1  christos happens when switching to a named rule. To what values should
 1.1  christos the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p>
 1.1  christos
 1.1  christos <ul>
 1.1  christos <li>If at least one transition has happened, use
 1.1  christos the <code>SAVE</code> and <code>LETTER</code> data from the most
 1.1  christos recent.</li>
 1.1  christos
 1.1  christos <li>If switching to a named rule before any transition has happened,
 1.1  christos assume standard time (<code>SAVE</code> zero), and use
 1.1  christos the <code>LETTER</code> data from the earliest transition with
 1.1  christos a <code>SAVE</code> of zero.
 1.1  christos
 1.1  christos </ul>
 1.1  christos
 1.1  christos <p>And three last things about the <code>FORMAT</code> column:</p>
 1.1  christos <ul>
 1.1  christos
 1.1  christos <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz
1.10  christos database</a> gives abbreviations for time zones
1.10  christos in popular English-language usage. For
 1.1  christos example, the last line in
 1.1  christos <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives
1.11  christos HST for Hawaii standard time even though the
 1.3  christos <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a>
1.11  christos name for that time zone is HawaiiAleutian standard time.
 1.1  christos This author has read that there are also some places in Australia where
 1.1  christos popular time zone names differ from the legal ones.
 1.1  christos
 1.1  christos <li>No attempt is made to <a
 1.1  christos href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a>
 1.1  christos the abbreviations. They are intended to be the values returned through the
 1.1  christos <code>"%Z"</code> format specifier to
1.11  christos <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>s
1.11  christos <a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/strftime.html"><code>strftime</code></a>
 1.1  christos function in the
1.11  christos <a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">C locale</a>.
 1.1  christos
1.10  christos <li>If there is no generally accepted abbreviation for a time zone,
 1.1  christos a numeric offset is used instead, e.g., <code>+07</code> for 7 hours
 1.1  christos ahead of Greenwich. By convention, <code>-00</code> is used in a
 1.1  christos zone while uninhabited, where the offset is zero but in some sense
 1.1  christos the true offset is undefined.
 1.1  christos </ul>
 1.1  christos
1.11  christos <p>As a final example, heres the complete history for Hawaii:</p>
 1.1  christos
1.12  christos <pre># Relevant Excerpts from the US Rules
 1.8  christos #Rule NAME FROM TO   -    IN  ON      AT     SAVE LETTER/S
 1.1  christos Rule  US   1918 1919 -    Oct lastSun  2:00  0    S
 1.1  christos Rule  US   1942 only -    Feb  9       2:00  1:00 W # War
 1.1  christos Rule  US   1945 only -    Aug 14      23:00u 1:00 P # Peace
 1.6  christos Rule  US   1945 only -    Sep lastSun  2:00  0    S
1.12  christos
1.12  christos # The Zone Record
 1.7  christos #Zone NAME            STDOFF    RULES FORMAT [UNTIL]
 1.6  christos Zone Pacific/Honolulu -10:31:26 -     LMT    1896 Jan 13 12:00
 1.1  christos                       -10:30    -     HST    1933 Apr 30  2:00
 1.1  christos                       -10:30    1:00  HDT    1933 May 21  2:00
 1.1  christos                       -10:30    US    H%sT   1947 Jun  8  2:00
 1.1  christos                       -10:00    -     HST
 1.1  christos </pre>
1.12  christos <table>
 1.1  christos <tr>
 1.1  christos   <th colspan="6">What We Infer</th>
 1.1  christos </tr>
 1.1  christos <tr>
 1.1  christos   <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th>
 1.1  christos   <th rowspan="2">Adjust<br>Clocks</th>
 1.1  christos   <th colspan="2">Time Zone</th>
 1.1  christos   <th colspan="2">Ending at Local Time</th>
 1.1  christos </tr>
 1.1  christos <tr>
 1.1  christos   <th>Abbrv.</th>
 1.1  christos   <th>Name</th>
 1.1  christos   <th>Date</th>
 1.1  christos   <th>Time</th>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td>10:31:26</td>
1.11  christos   <td></td>
 1.1  christos   <td>LMT</td>
 1.1  christos   <td>local mean time</td>
 1.6  christos   <td>1896-01-13</td>
 1.1  christos   <td>12:00</td>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td>10:30</td>
 1.1  christos   <td>+0:01:26</td>
 1.1  christos   <td>HST</td>
 1.1  christos   <td>Hawaii standard time</td>
 1.1  christos   <td>1933-04-30</td>
 1.6  christos   <td>02:00</td>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td>9:30</td>
 1.1  christos   <td>+1:00</td>
 1.1  christos   <td>HDT</td>
 1.1  christos   <td>Hawaii daylight time</td>
 1.1  christos   <td>1933-05-21</td>
 1.6  christos   <td>12:00</td>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td>10:30&sup1;</td>
1.11  christos   <td>1:00&sup1;</td>
 1.1  christos   <td>HST&sup1;</td>
 1.1  christos   <td>Hawaii standard time</td>
 1.1  christos   <td>1942-02-09</td>
 1.6  christos   <td>02:00</td>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td rowspan="2">9:30</td>
 1.1  christos   <td>+1:00</td>
 1.1  christos   <td>HWT</td>
 1.1  christos   <td>Hawaii war time</td>
 1.1  christos   <td>1945-08-14</td>
 1.1  christos   <td>13:30&sup2;</td>
 1.1  christos </tr>
 1.8  christos <tr>
 1.1  christos   <td>0</td>
 1.1  christos   <td>HPT</td>
 1.1  christos   <td>Hawaii peace time</td>
 1.1  christos   <td>1945-09-30</td>
 1.1  christos   <td rowspan="2">02:00</td>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td>10:30</td>
1.11  christos   <td>1:00</td>
 1.1  christos   <td rowspan="2">HST</td>
 1.1  christos   <td rowspan="2">Hawaii standard time</td>
 1.1  christos   <td>1947-06-08</td>
 1.1  christos </tr>
 1.8  christos <tr>
1.11  christos   <td>10:00&sup3;</td>
 1.1  christos   <td>+0:30&sup3;</td>
1.11  christos   <td colspan="2"></td>
 1.1  christos </tr>
 1.1  christos <tr>
 1.8  christos   <td colspan="6" class="footnote">
1.11  christos     &sup1;Switching to US rules...most recent transition (in 1919) was to standard time
 1.1  christos   </td>
 1.1  christos </tr>
 1.1  christos <tr>
 1.8  christos   <td colspan="6" class="footnote">
 1.1  christos     &sup2;23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>
1.11  christos     + (9:30) = 13:30 local
 1.1  christos   </td>
 1.1  christos </tr>
 1.1  christos <tr>
 1.8  christos   <td colspan="6" class="footnote">
1.11  christos     &sup3;Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947-06-08T12:30Z</a>,
 1.1  christos     the civil time in Hawaii has been
 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>
1.11  christos     10:00 year-round.
 1.1  christos   </td>
 1.1  christos </tr>
 1.1  christos </table>
 1.1  christos
 1.1  christos <p>There will be a short quiz later. <code>8-)</code></p>
 1.1  christos
 1.1  christos <hr>
 1.1  christos <address>
 1.1  christos This web page is in the public domain, so clarified as of
 1.1  christos 2015-10-20 by Bill Seymour.
 1.1  christos <br>
 1.1  christos All suggestions and corrections will be welcome; all flames will be amusing.
 1.1  christos Mail to was at pobox dot com.
 1.1  christos </address>
 1.1  christos </body>
 1.1  christos </html>