[tz] web page "How to Read the tz Database Source Files"
Paul Eggert
eggert at cs.ucla.edu
Tue Mar 1 05:09:22 UTC 2016
Bill Seymour wrote:
> I've attached a version that says clearly that it's in the public domain.
Thanks. I finally got around to merging it into the tzcode web pages and
installed the attached patches into the experimental version on GitHub. The
first patch simply adds your web page; the second adapts it and the existing
pages to each other and makes it acceptable to our web page checker and so forth.
> I wrote that quite a while ago to help me understand what the files are saying,
> and I'm not totally confident that I got everything exactly right. Perhaps an
> expert -- and I'm not one -- should pour over it to make sure that all the
> statements in the paper are actually true. 8-)
Yes, that would be nice....
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From c00fb506f74f705933c5796adde73efa90bfc872 Mon Sep 17 00:00:00 2001
From: Bill Seymour <stdbill.h at pobox.com>
Date: Sun, 28 Feb 2016 17:12:59 -0800
Subject: [PATCH 1/3] New file tz-how-to.html
Contributed by Bill Seymour; see:
http://mm.icann.org/pipermail/tz/2015-October/022789.html
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+<html>
+<head><title>How to Read the tz Database</title></head>
+<body>
+<h2>How to Read the <a href="http://en.wikipedia.org/wiki/Zoneinfo">tz Database</a> Source Files</h2>
+<h3>by Bill Seymour</h3>
+<h4>This paper is in the public domain.</h4>
+
+This paper uses the <tt>America/Chicago</tt> and <tt>Pacific/Honolulu</tt> zones as examples of how to infer
+times of day from the <a href="http://www.twinsun.com/tz/tz-link.htm">tz database</a>
+source files. It might be helpful, but not absolutely necessary,
+for the reader to have already downloaded the database from
+<nobr><tt><a href="ftp://elsie.nci.nih.gov/pub/">ftp://elsie.nci.nih.gov/pub/</a></tt></nobr>
+and become familiar with the basic layout of the data files. The format is explained
+in the “man page” for the zic compiler, <nobr><tt>zic.8.txt</tt>,</nobr> in the <tt>code</tt>
+subdirectory.
+
+<p>We’ll begin by talking about the rules for changing between standard
+and daylight saving time since we’ll need that information when we talk
+about the zones.
+
+<p>First, let’s consider the special daylight saving time rules for Chicago
+(from the <tt>northamerica</tt> file in the <tt>data</tt> subdirectory):
+<p>
+<table border>
+<tr>
+ <th colspan=6>From the Source File</th>
+</tr>
+<tr>
+ <td colspan=6>
+<pre>
+
+# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER
+Rule Chicago 1920 only - Jun 13 2:00 1:00 D
+Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S
+Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D
+Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D
+Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S
+Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S
+</pre>
+ </td>
+</tr>
+<tr>
+ <th colspan=6>Reformatted a Bit</th>
+</tr>
+<tr>
+ <th>From</th>
+ <th>To</th>
+ <th colspan=2>On</th>
+ <th>At</th>
+ <th>Action</th>
+</tr>
+<tr align=center>
+ <td colspan=2>1920 only</td>
+ <td colspan=2>June 13<small><sup>th</sup></small></td>
+ <td rowspan=6>02:00 local</td>
+ <td>go to daylight saving time</td>
+</tr>
+<tr align=center>
+ <td>1920</td>
+ <td>1921</td>
+ <td rowspan=5>last Sunday</td>
+ <td>in October</td>
+ <td>return to standard time</td>
+</tr>
+<tr align=center>
+ <td colspan=2>1921 only</td>
+ <td>in March</td>
+ <td rowspan=2>go to daylight saving time</td>
+</tr>
+<tr align=center>
+ <td rowspan=2>1922</td>
+ <td>1966</td>
+ <td>in April</td>
+</tr>
+<tr align=center>
+ <td>1954</td>
+ <td>in September</td>
+ <td rowspan=2>return to standard time</td>
+</tr>
+<tr align=center>
+ <td>1955</td>
+ <td>1966</td>
+ <td>in October</td>
+</tr>
+</table>
+
+<p>We’ll basically just ignore the <tt>TYPE</tt> column.
+In the 2007j release, the most recent as of this writing, the
+<tt>TYPE</tt> column never contains anything but a hyphen,
+a kind of null value. (From the description in <tt>zic.8.txt</tt>,
+this appears to be a mechanism for removing years from a set
+in some localizable way. It’s used in the file, <tt>pacificnew</tt>,
+to determine whether a given year will have a US presidential election;
+but everything related to that use is commented out.)
+
+<p>The <tt>SAVE</tt> column contains the wall clock offset from local standard time.
+This is usually either zero for standard time or one hour for daylight
+saving time; but there’s no reason, in principle, why it can’t
+take on other values.
+
+<p>The <tt>LETTER</tt> (sometimes called <tt>LETTER/S</tt>) column can contain a variable
+part of the usual abbreviation of the time zone’s name, or it can just
+be a hyphen if there’s no variable part. For example, the abbreviation
+used in the central time zone will be either “CST” or
+“CDT”. The variable part is ‘S’ or ‘D’;
+and, sure enough, that’s just what we find in the <tt>LETTER</tt> column
+in the <tt>Chicago</tt> rules. More about this when we talk about
+“Zone” lines.
+
+<p>One important thing to notice is that “Rule” lines
+want at once to be both <i>transitions</i> and <i>steady states</i>:
+<ul>
+<li>On the one hand, they represent transitions between standard and daylight saving time;
+ and any number of Rule lines can be in effect during a given period (which will always
+ be a non-empty set of contiguous calendar years).
+<p>
+<li>On the other hand, the <tt>SAVE</tt> and <tt>LETTER</tt> columns contain
+ state that exists between transitions. More about this when we talk about
+ the US rules.
+</ul>
+
+<p>In the example above, the transition to daylight saving time happened on the
+13<small><sup>th</sup></small> of June in 1920, and on the last Sunday in March
+in 1921; but the return to standard time happened on the last Sunday in October
+in both of those years. Similarly, the rule for changing to daylight saving time
+was the same from 1922 to 1966; but the rule for returning to standard time
+changed in 1955. Got it?
+
+<p>OK, now for the somewhat more interesting “US” rules:
+<p>
+<table border>
+<tr>
+ <th colspan=6>From the Source File</th>
+</tr>
+<tr>
+ <td colspan=6>
+<pre>
+
+# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S
+Rule US 1918 1919 - Mar lastSun 2:00 1:00 D
+Rule US 1918 1919 - Oct lastSun 2:00 0 S
+Rule US 1942 only - Feb 9 2:00 1:00 W # War
+Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
+Rule US 1945 only - Sep 30 2:00 0 S
+Rule US 1967 2006 - Oct lastSun 2:00 0 S
+Rule US 1967 1973 - Apr lastSun 2:00 1:00 D
+Rule US 1974 only - Jan 6 2:00 1:00 D
+Rule US 1975 only - Feb 23 2:00 1:00 D
+Rule US 1976 1986 - Apr lastSun 2:00 1:00 D
+Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D
+Rule US 2007 max - Mar Sun>=8 2:00 1:00 D
+Rule US 2007 max - Nov Sun>=1 2:00 0 S
+</pre>
+ </td>
+</tr>
+<tr>
+ <th colspan=6>Reformatted a Bit</th>
+</tr>
+<tr>
+ <th>From</th>
+ <th>To</th>
+ <th colspan=2>On</th>
+ <th>At</th>
+ <th>Action</th>
+</tr>
+<tr align=center>
+ <td rowspan=2>1918</td>
+ <td rowspan=2>1919</td>
+ <td rowspan=2>last Sunday</td>
+ <td>in March</td>
+ <td rowspan=3>02:00 local</td>
+ <td>go to daylight saving time</td>
+</tr>
+<tr align=center>
+ <td>in October</td>
+ <td>return to standard time</td>
+</tr>
+<tr align=center>
+ <td colspan=2>1942 only</td>
+ <td colspan=2>February 9<small><sup>th</sup></small></td>
+ <td>go to “war time”</td>
+</tr>
+<tr align=center>
+ <td colspan=2 rowspan=2>1945 only</td>
+ <td colspan=2>August 14<small><sup>th</sup></small></td>
+ <td>23:00 <a href="http://en.wikipedia.org/wiki/Universal_Time">UT</a></td>
+ <td>
+ rename “war time” to “peace<br>time;” clocks don’t change
+ </td>
+</tr>
+<tr align=center>
+ <td colspan=2>September 30<small><sup>th</sup></small></td>
+ <td rowspan=9>02:00 local</td>
+ <td rowspan=2>return to standard time</td>
+</tr>
+<tr align=center>
+ <td rowspan=2>1967</td>
+ <td>2006</td>
+ <td rowspan=2>last Sunday</td>
+ <td>in October</td>
+</tr>
+<tr align=center>
+ <td>1973</td>
+ <td>in April</td>
+ <td rowspan=6>go to daylight saving time</td>
+</tr>
+<tr align=center>
+ <td colspan=2>1974 only</td>
+ <td colspan=2>January 6<small><sup>th</sup></small></td>
+</tr>
+<tr align=center>
+ <td colspan=2>1975 only</td>
+ <td colspan=2>February 23<small><sup>rd</sup></small></td>
+</tr>
+<tr align=center>
+ <td>1976</td>
+ <td>1986</td>
+ <td>last Sunday</td>
+ <td rowspan=2>in April</td>
+</tr>
+<tr align=center>
+ <td>1987</td>
+ <td>2006</td>
+ <td>first Sunday</td>
+</tr>
+<tr align=center>
+ <td rowspan=2>2007</td>
+ <td rowspan=2>present</td>
+ <td colspan=2>second Sunday in March</td>
+</tr>
+<tr align=center>
+ <td colspan=2>first Sunday in November</td>
+ <td>return to standard time</td>
+</tr>
+</table>
+
+<p>There are two interesting things to note here.
+
+<p>First, the time that something happens (in the <tt>AT</tt> column) is not necessarily
+the local wall clock time. The time can be suffixed with ‘s’ (for
+“standard”) to mean local standard time (different from wall clock
+time when observing daylight saving time); or it can be suffixed with ‘g’,
+‘u’, or ‘z’, all three of which mean the standard time at the
+<a href="http://en.wikipedia.org/wiki/Prime_Meridian">prime meridan</a>. ‘g’
+stands for “<a href="http://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”;
+‘u’
+stands for “<a href="http://en.wikipedia.org/wiki/Universal_Time">UT</a>” or
+“<a href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>”
+(whichever was official at the time);
+‘z’ stands for the
+<a href="http://aa.usno.navy.mil/graphics/TimeZoneMap2007.pdf">nautical time zone</a> Z
+(a.k.a. “Zulu” which, in turn, stands for ‘Z’).
+<!--<tt><rant></tt>(Don’t say “Z stands for Zulu;”
+that’s <a href="http://www.urbandictionary.com/define.php?term=Bass+Ackwards">bass
+ackwards</a>.)<tt></rant></tt>-->
+The time can also be suffixed with
+‘w’ meaning “wall clock time;” but it usually isn’t
+because that’s the default.
+
+<p>Second, the day in the <tt>ON</tt> column, in addition to “<tt>lastSun</tt>” or a particular
+day of the month, can have the form, <nobr>“<tt>Sun>=</tt><i>x</i>”</nobr>
+or <nobr>“<tt>Sun<=</tt><i>x</i>,”</nobr>
+where <i>x</i> is a day of the month. For example, <nobr>“<tt>Sun>=8</tt>”</nobr>
+means “the first Sunday on or after the eighth of the month,” in other words, the second Sunday
+of the month. Furthermore, although there are no examples above, the weekday needn’t be
+“<tt>Sun</tt>” in either form, but can be the usual three-character
+English abbreviation for any day of the week.
+
+<p>And the US rules give us more examples of a couple of things already mentioned:
+<ul>
+<li>The rules for changing to and from daylight saving time are actually <i>different sets</i>
+ of rules; and the two sets can change independently. Consider, for example, that the rule
+ for the return to standard time stayed the same from 1967 to 2006; but the rule for
+ the transition to daylight saving time changed several times in the same period.
+ There can also be periods, 1946 to 1966 for example, when no rule from this group
+ is in effect, and so either no transition happened in those years, or some other
+ rule is in effect (perhaps a state or other more local rule).
+<p>
+<li>The <tt>SAVE</tt> and <tt>LETTER</tt> columns contain <i>steady state</i>, not transitions.
+ Consider, for example, the transition from “war time” to “peace time”
+ that happened on August 14, 1945. The “1:00” in the <tt>SAVE</tt> column
+ is <i>not</i> an instruction to advance the clock an hour. It means that clocks should
+ <i>be</i> one hour ahead of standard time, which they already are because of the previous rule,
+ so there should be no change.
+</ul>
+
+<p>OK, now let’s look at a Zone record:
+<p>
+<table border>
+<tr>
+ <th colspan=5>From the Source File</th>
+</tr>
+<tr>
+ <td colspan=5>
+<pre>
+
+# Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24
+ -6:00 US C%sT 1920
+ -6:00 Chicago C%sT 1936 Mar 1 2:00
+ -5:00 - EST 1936 Nov 15 2:00
+ -6:00 Chicago C%sT 1942
+ -6:00 US C%sT 1946
+ -6:00 Chicago C%sT 1967
+ -6:00 US C%sT
+</pre>
+ </td>
+</tr>
+<tr>
+ <th colspan=5>Columns Renamed</th>
+</tr>
+<tr>
+ <th rowspan=2>Standard Offset<br>from <a href="http://en.wikipedia.org/wiki/Prime_Meridian">Prime Meridian</a></th>
+ <th rowspan=2>Daylight<br>Saving Time</th>
+ <th rowspan=2>Abbreviation(s)</th>
+ <th colspan=2>Ending at Local Time</th>
+</tr>
+<tr>
+ <th>Date</th>
+ <th>Time</th>
+</tr>
+<tr align=center>
+ <td>−5:50:36</td>
+ <td>not observed</td>
+ <td>LMT</td>
+ <td>1883-11-18</td>
+ <td>12:09:24</td>
+</tr>
+<tr align=center>
+ <td rowspan=2>−6:00:00</td>
+ <td>US rules</td>
+ <td rowspan=2>CST or CDT</td>
+ <td>1920-01-01</td>
+ <td>00:00:00</td>
+</tr>
+<tr align=center>
+ <td>Chicago rules</td>
+ <td>1936-03-01</td>
+ <td rowspan=2>02:00:00</td>
+</tr>
+<tr align=center>
+ <td>−5:00:00</td>
+ <td>not observed</td>
+ <td>EST</td>
+ <td>1936-11-15</td>
+</tr>
+<tr align=center>
+ <td rowspan=4>−6:00:00</td>
+ <td>Chicago rules</td>
+ <td>CST or CDT</td>
+ <td>1942-01-01</td>
+ <td rowspan=3>00:00:00</td>
+</tr>
+<tr align=center>
+ <td>US rules</td>
+ <td>CST, CWT or CPT</td>
+ <td>1946-01-01</td>
+</tr>
+<tr align=center>
+ <td>Chicago rules</td>
+ <td rowspan=2>CST or CDT</td>
+ <td>1967-01-01</td>
+</tr>
+<tr align=center>
+ <td>US rules</td>
+ <td colspan=2>—</td>
+</tr>
+</table>
+
+<p>There are a couple of interesting differences between Zones and Rules.
+
+<p>First, and somewhat trivially, whereas Rules are considered to contain one or more
+records, a Zone is considered to be a single record with zero or more <i>continuation
+lines</i>. Thus, the keyword, <nobr>“<tt>Zone</tt>,”</nobr> and the zone name
+are not repeated. The last line is the one without anything in the <tt>[UNTIL]</tt> column.
+
+<p>Second, and more fundamentally, each line of a Zone represents
+a steady state, not a transition between states. The state exists from the date and time
+in the previous line’s <tt>[UNTIL]</tt> column up to the
+date and time in the current line’s <tt>[UNTIL]</tt> column. In other words,
+the date and time in the <tt>[UNTIL]</tt> column is the instant that separates
+this state from the next. Where that would be ambiguous because we’re
+setting our clocks back, the <tt>[UNTIL]</tt> column specifies the first occurrence of
+the instant. The state specified by the last line, the one without
+anything in the <tt>[UNTIL]</tt> column, continues to the present.
+
+<p>The first line typically specifies the mean solar time observed before the introduction
+of standard time. Since there’s no line before that, it has no <nobr>beginning. <tt>8-) </tt></nobr>
+For some places near the
+<a href="http://en.wikipedia.org/wiki/International_Date_Line">International Date Line</a>,
+the first <i>two</i> lines will show solar times differing by 24 hours; this corresponds to
+a movement of the Date Line.
+For example:
+<pre>
+# Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone America/Juneau 15:02:19 - LMT 1867 Oct 18
+ -8:57:41 - LMT ...
+</pre>
+
+When Alaska was purchased from Russia in 1867, the Date Line moved from
+the Alaska/Canada border to
+the Bering Strait; and the time in Alaska was then 24 hours earlier than it had been.
+<nobr><tt><aside></tt>(6 October</nobr> in the Julian calendar,
+which Russia was still using then for religious reasons, was followed by <i>a second instance
+of the same day with a different name</i>, <nobr>18 October</nobr>
+in the Gregorian calendar. Isn’t civil time
+<nobr>wonderful? <tt>8-)</tt>)<tt></aside></tt></nobr>
+
+<p>The abbreviation, “LMT,” stands for “local mean time,”
+which is an invention of the <a href="http://en.wikipedia.org/wiki/Zoneinfo">tz database</a>
+and was probably never actually used during the period. Furthermore, the value
+is almost certainly wrong except in the archetypal place after which the zone
+is named. (The tz database usually doesn’t provide a separate Zone record
+for places where nothing significant happened after 1970.)
+
+<p>The <tt>RULES</tt> column tells us whether daylight saving time is being observed:
+<ul>
+<li>A hyphen, a kind of null value, means that we have not set our clocks ahead of standard time.
+<li>An amount of time (usually but not necessarily <nobr>“1:00”</nobr>
+ meaning one hour) means that we have set our clocks ahead by that amount.
+<li>Some alphabetic string means that we <i>might have</i> set our clocks ahead;
+ and we need to check the rule the name of which is the given alphabetic string.
+</ul>
+
+An example of a specific amount of time is:
+<pre>
+# Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone Pacific/Honolulu ... 1933 Apr 30 2:00
+ -10:30 1:00 HDT 1933 May 21 2:00
+ ...
+</pre>
+Hawaii tried daylight saving time for three weeks in 1933 and decided they didn’t
+like <nobr>it. <tt>8-) </tt></nobr>Note that the <tt>GMTOFF</tt> column always contains
+the standard time offset, so the wall clock time during this period was
+<nobr>GMT − 10:30 + 1:00 = GMT − 9:30.</nobr>
+
+<p>The <tt>FORMAT</tt> column specifies the usual abbreviation of the time zone name.
+It can have one of four forms:
+<ul>
+<li>the string, “<tt>zzz</tt>,” which is a kind of null value (don’t ask)
+<li>a single alphabetic string other than “<tt>zzz</tt>,”
+ in which case that’s the abbreviation
+<li>a pair of strings separated by a slash <nobr>(‘<tt>/</tt>’),</nobr>
+ in which case the first string is the abbreviation for the standard time name
+ and the second string is the abbreviation for the daylight saving time name
+<li>a string containing <nobr>“<tt>%s</tt>,”</nobr> in which case the
+ <nobr>“<tt>%s</tt>”</nobr> will be replaced by the text in the
+ appropriate Rule’s <nobr><tt>LETTER</tt></nobr> column
+</ul>
+The last two make sense only if there’s a named rule in effect.
+
+<p>An example of a slash is:
+<pre>
+# Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone Europe/London ... 1996
+ 0:00 EU GMT/BST
+</pre>
+The current time in the UK is called either Greenwich mean time or British summer time.
+
+<p>One wrinkle, not fully explained in <tt>zic.8.txt</tt>, is what happens when
+switching to a named rule. To what values should the <tt>SAVE</tt> and <tt>LETTER</tt>
+data be initialized?
+<ul>
+<li>If at least one transition has happened, use the <tt>SAVE</tt> and <tt>LETTER</tt>
+ data from the most recent.
+<p>
+<li>If switching to a named rule before any transition has happened, assume standard
+ time (<tt>SAVE</tt> zero), and use the <tt>LETTER</tt> data from the earliest
+ transition with a <tt>SAVE</tt> of zero.
+</ul>
+
+<p>And two last things about the <tt>FORMAT</tt> column:
+<ul>
+<li>The <a href="http://en.wikipedia.org/wiki/Zoneinfo">tz database</a> gives
+ abbreviations for time zone names in <i>popular usage</i>, which is not
+ necessarily “correct” by law. For example, the last line in
+ <nobr><tt>Zone</tt> <tt>Pacific/Honolulu</tt></nobr> (shown below) gives
+ “HST” for “Hawaii standard time” even though the
+ <a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a>
+ name for that time zone is “Hawaii-Aleutian standard time.”
+ This author has read that there are also some places in Australia where
+ popular time zone names differ from the legal ones.
+<p>
+<li>No attempt is made to
+ <a href="http://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a>
+ the abbreviations. They are intended to be the values returned through the
+ <nobr><tt>"%Z"</tt></nobr> format specifier to
+ <a href="http://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s
+ <a href="http://opengroup.org/onlinepubs/007908799/xsh/strftime.html"><tt>strftime</tt></a>
+ function in the
+ <a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>.
+</ul>
+
+<p>As a final example, here’s the complete history for Hawaii:
+
+<p>
+<table border>
+<tr>
+ <th colspan=6>Relevant Excerpts from the US Rules</th>
+</tr>
+<tr>
+ <td colspan=6>
+<pre>
+
+# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S
+Rule US 1918 1919 - Oct lastSun 2:00 0 S
+Rule US 1942 only - Feb 9 2:00 1:00 W # War
+Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
+Rule US 1945 only - Sep 30 2:00 0 S
+</pre>
+ </td>
+</tr>
+<tr>
+ <th colspan=6>The Zone Record</th>
+</tr>
+<tr>
+ <td colspan=6 align=center><table border=0><tr><td>
+<pre>
+
+# Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00
+ -10:30 - HST 1933 Apr 30 2:00
+ -10:30 1:00 HDT 1933 May 21 2:00
+ -10:30 US H%sT 1947 Jun 8 2:00
+ -10:00 - HST
+</pre>
+ </td></tr></table></td>
+</tr>
+<tr>
+ <th colspan=6>What We Infer</th>
+</tr>
+<tr>
+ <th rowspan=2>Wall-Clock<br>Offset from<br>Prime Meridian</th>
+ <th rowspan=2>Adjust<br>Clocks</th>
+ <th colspan=2>Time Zone</th>
+ <th colspan=2>Ending at Local Time</th>
+</tr>
+<tr>
+ <th>Abbrv.</th>
+ <th>Name</th>
+ <th>Date</th>
+ <th>Time</th>
+</tr>
+<tr align=center>
+ <td>−10:31:26</td>
+ <td>—</td>
+ <td>LMT</td>
+ <td>local mean time</td>
+ <td>1900-01-01</td>
+ <td>12:00</td>
+</tr>
+<tr align=center>
+ <td>−10:30</td>
+ <td>+0:01:26</td>
+ <td>HST</td>
+ <td>Hawaii standard time</td>
+ <td>1933-04-30</td>
+ <td rowspan=3>02:00</td>
+</tr>
+<tr align=center>
+ <td>−9:30</td>
+ <td>+1:00</td>
+ <td>HDT</td>
+ <td>Hawaii daylight time</td>
+ <td>1933-05-21</td>
+</tr>
+<tr align=center>
+ <td>−10:30¹</td>
+ <td>−1:00¹</td>
+ <td>HST¹</td>
+ <td>Hawaii standard time</td>
+ <td>1942-02-09</td>
+</tr>
+<tr align=center>
+ <td rowspan=2>−9:30</td>
+ <td>+1:00</td>
+ <td>HWT</td>
+ <td>Hawaii war time</td>
+ <td>1945-08-14</td>
+ <td>13:30²</td>
+</tr>
+<tr align=center>
+ <td>0</td>
+ <td>HPT</td>
+ <td>Hawaii peace time</td>
+ <td>1945-09-30</td>
+ <td rowspan=2>02:00</td>
+</tr>
+<tr align=center>
+ <td>−10:30</td>
+ <td>−1:00</td>
+ <td rowspan=2>HST</td>
+ <td rowspan=2>Hawaii standard time</td>
+ <td>1947-06-08</td>
+</tr>
+<tr align=center>
+ <td>−10:00³</td>
+ <td>+0:30³</td>
+ <td colspan=2>—</td>
+</tr>
+<tr>
+ <td colspan=6>
+ ¹Switching to US rules…most recent transition (in 1919) was to standard time
+ </td>
+</tr>
+<tr>
+ <td colspan=6>
+ ²23:00 <a href="http://en.wikipedia.org/wiki/Universal_Time">UT</a>
+ + (−9:30) = 13:30 local
+ </td>
+</tr>
+<tr>
+ <td colspan=6>
+ ³Since <a href="http://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>,
+ the civil time in Hawaii has been
+ <a href="http://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
+ − 10:00 year-round.
+ </td>
+</tr>
+</table>
+
+<p>There will be a short quiz <nobr>later. <tt>8-)</tt></nobr>
+
+<p><hr>
+All suggestions and corrections will be welcome; all flames will be amusing.
+Mail to was at pobox dot com.
+</body>
+</html>
--
2.5.0
-------------- next part --------------
From 8208572aae11a349eb269006f4ca72b880d48c42 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert at cs.ucla.edu>
Date: Mon, 29 Feb 2016 21:00:44 -0800
Subject: [PATCH 2/3] tz-how-to merge and fixups
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
* NEWS: Mention new web page.
* Makefile (WEB_PAGES): Add tz-how-to.html.
* tz-how-to.html: Make it pass ‘make check’.
Update URLs. Put public-domain notice at bottom.
Avoid long lines when possible.
* tz-link.htm: Link to it. Also, fix some URLs (thanks to
Sanjeev and to Brian Inglis).
---
Makefile | 2 +-
NEWS | 2 +
tz-how-to.html | 770 ++++++++++++++++++++++++++++++---------------------------
tz-link.htm | 16 +-
4 files changed, 420 insertions(+), 370 deletions(-)
diff --git a/Makefile b/Makefile
index 5e2fb52..720c6ef 100644
--- a/Makefile
+++ b/Makefile
@@ -364,7 +364,7 @@ MANTXTS= newctime.3.txt newstrftime.3.txt newtzset.3.txt \
tzfile.5.txt tzselect.8.txt zic.8.txt zdump.8.txt \
date.1.txt
COMMON= CONTRIBUTING LICENSE Makefile NEWS README Theory
-WEB_PAGES= tz-art.htm tz-link.htm
+WEB_PAGES= tz-art.htm tz-how-to.html tz-link.htm
DOCS= $(MANS) date.1 $(MANTXTS) $(WEB_PAGES)
PRIMARY_YDATA= africa antarctica asia australasia \
europe northamerica southamerica
diff --git a/NEWS b/NEWS
index e4dc2b5..c55d7af 100644
--- a/NEWS
+++ b/NEWS
@@ -33,6 +33,8 @@ Unreleased, experimental changes
tzselect again limits its menu comments so that menus fit on a
24x80 alphanumeric display.
+ A new web page tz-how-to.html. (Thanks to Bill Seymour.)
+
In the Theory file, the description of possible time zone abbreviations in
tzdata has been cleaned up, as the old description was unclear and
inconsistent. (Thanks to Alain Mouette for reporting the problem.)
diff --git a/tz-how-to.html b/tz-how-to.html
index 544ef7b..3fa7602 100644
--- a/tz-how-to.html
+++ b/tz-how-to.html
@@ -1,539 +1,582 @@
+<!DOCTYPE html
+ PUBLIC "-//W3C//DTD HTML 4.01//EN"
+ "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head><title>How to Read the tz Database</title></head>
<body>
-<h2>How to Read the <a href="http://en.wikipedia.org/wiki/Zoneinfo">tz Database</a> Source Files</h2>
+<h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
+Database</a> Source Files</h2>
<h3>by Bill Seymour</h3>
-<h4>This paper is in the public domain.</h4>
-
-This paper uses the <tt>America/Chicago</tt> and <tt>Pacific/Honolulu</tt> zones as examples of how to infer
-times of day from the <a href="http://www.twinsun.com/tz/tz-link.htm">tz database</a>
-source files. It might be helpful, but not absolutely necessary,
-for the reader to have already downloaded the database from
-<nobr><tt><a href="ftp://elsie.nci.nih.gov/pub/">ftp://elsie.nci.nih.gov/pub/</a></tt></nobr>
-and become familiar with the basic layout of the data files. The format is explained
-in the “man page” for the zic compiler, <nobr><tt>zic.8.txt</tt>,</nobr> in the <tt>code</tt>
-subdirectory.
+<p>This page uses the <code>America/Chicago</code> and
+<code>Pacific/Honolulu</code> zones as examples of how to infer
+times of day from the <a href="tz-link.htm">tz database</a>
+source files. It might be helpful, but not absolutely necessary,
+for the reader to have already downloaded the
+<a href="http://www.iana.org/time-zones/repository/tzdata-latest.tar.gz">latest
+release of the database</a> and become familiar with the basic layout
+of the data files. The format is explained in the “man
+page” for the zic compiler, <code>zic.8.txt</code>, in
+the <code>code</code> subdirectory.</p>
<p>We’ll begin by talking about the rules for changing between standard
and daylight saving time since we’ll need that information when we talk
-about the zones.
+about the zones.</p>
+
+<p>First, let’s consider the special daylight saving time rules
+for Chicago (from the <code>northamerica</code> file in
+the <code>data</code> subdirectory):</p>
-<p>First, let’s consider the special daylight saving time rules for Chicago
-(from the <tt>northamerica</tt> file in the <tt>data</tt> subdirectory):
-<p>
-<table border>
+<table border="1">
<tr>
- <th colspan=6>From the Source File</th>
+ <th colspan="6">From the Source File</th>
</tr>
<tr>
- <td colspan=6>
+ <td colspan="6" align="center"><table><tr><td>
<pre>
-
-# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER
-Rule Chicago 1920 only - Jun 13 2:00 1:00 D
-Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S
-Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D
-Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D
-Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S
-Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S
+#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER
+Rule Chicago 1920 only - Jun 13 2:00 1:00 D
+Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S
+Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D
+Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D
+Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S
+Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S
</pre>
- </td>
+ </td></tr></table></td>
</tr>
<tr>
- <th colspan=6>Reformatted a Bit</th>
+ <th colspan="6">Reformatted a Bit</th>
</tr>
<tr>
<th>From</th>
<th>To</th>
- <th colspan=2>On</th>
+ <th colspan="2">On</th>
<th>At</th>
<th>Action</th>
</tr>
-<tr align=center>
- <td colspan=2>1920 only</td>
- <td colspan=2>June 13<small><sup>th</sup></small></td>
- <td rowspan=6>02:00 local</td>
+<tr align="center">
+ <td colspan="2">1920 only</td>
+ <td colspan="2">June 13<small><sup>th</sup></small></td>
+ <td rowspan="6">02:00 local</td>
<td>go to daylight saving time</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>1920</td>
<td>1921</td>
- <td rowspan=5>last Sunday</td>
+ <td rowspan="5">last Sunday</td>
<td>in October</td>
<td>return to standard time</td>
</tr>
-<tr align=center>
- <td colspan=2>1921 only</td>
+<tr align="center">
+ <td colspan="2">1921 only</td>
<td>in March</td>
- <td rowspan=2>go to daylight saving time</td>
+ <td rowspan="2">go to daylight saving time</td>
</tr>
-<tr align=center>
- <td rowspan=2>1922</td>
+<tr align="center">
+ <td rowspan="2">1922</td>
<td>1966</td>
<td>in April</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>1954</td>
<td>in September</td>
- <td rowspan=2>return to standard time</td>
+ <td rowspan="2">return to standard time</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>1955</td>
<td>1966</td>
<td>in October</td>
</tr>
</table>
-<p>We’ll basically just ignore the <tt>TYPE</tt> column.
+<p>We’ll basically just ignore the <code>TYPE</code> column.
In the 2007j release, the most recent as of this writing, the
-<tt>TYPE</tt> column never contains anything but a hyphen,
-a kind of null value. (From the description in <tt>zic.8.txt</tt>,
+<code>TYPE</code> column never contains anything but a hyphen,
+a kind of null value. (From the description in <code>zic.8.txt</code>,
this appears to be a mechanism for removing years from a set
-in some localizable way. It’s used in the file, <tt>pacificnew</tt>,
+in some localizable way. It’s used in the file, <code>pacificnew</code>,
to determine whether a given year will have a US presidential election;
but everything related to that use is commented out.)
-<p>The <tt>SAVE</tt> column contains the wall clock offset from local standard time.
+<p>The <code>SAVE</code> column contains the wall clock offset from
+local standard time.
This is usually either zero for standard time or one hour for daylight
saving time; but there’s no reason, in principle, why it can’t
take on other values.
-<p>The <tt>LETTER</tt> (sometimes called <tt>LETTER/S</tt>) column can contain a variable
+<p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>)
+column can contain a variable
part of the usual abbreviation of the time zone’s name, or it can just
-be a hyphen if there’s no variable part. For example, the abbreviation
+be a hyphen if there’s no variable part. For example, the abbreviation
used in the central time zone will be either “CST” or
-“CDT”. The variable part is ‘S’ or ‘D’;
-and, sure enough, that’s just what we find in the <tt>LETTER</tt> column
-in the <tt>Chicago</tt> rules. More about this when we talk about
+“CDT”. The variable part is ‘S’ or ‘D’;
+and, sure enough, that’s just what we find in
+the <code>LETTER</code> column
+in the <code>Chicago</code> rules. More about this when we talk about
“Zone” lines.
<p>One important thing to notice is that “Rule” lines
want at once to be both <i>transitions</i> and <i>steady states</i>:
<ul>
-<li>On the one hand, they represent transitions between standard and daylight saving time;
- and any number of Rule lines can be in effect during a given period (which will always
- be a non-empty set of contiguous calendar years).
-<p>
-<li>On the other hand, the <tt>SAVE</tt> and <tt>LETTER</tt> columns contain
- state that exists between transitions. More about this when we talk about
- the US rules.
+<li>On the one hand, they represent transitions between standard and
+daylight saving time; and any number of Rule lines can be in effect
+during a given period (which will always be a non-empty set of
+contiguous calendar years).</li>
+<li>On the other hand, the <code>SAVE</code> and <code>LETTER</code>
+columns contain state that exists between transitions. More about this
+when we talk about the US rules.</li>
</ul>
-<p>In the example above, the transition to daylight saving time happened on the
-13<small><sup>th</sup></small> of June in 1920, and on the last Sunday in March
-in 1921; but the return to standard time happened on the last Sunday in October
-in both of those years. Similarly, the rule for changing to daylight saving time
-was the same from 1922 to 1966; but the rule for returning to standard time
-changed in 1955. Got it?
+<p>In the example above, the transition to daylight saving time
+happened on the 13<small><sup>th</sup></small> of June in 1920, and on
+the last Sunday in March in 1921; but the return to standard time
+happened on the last Sunday in October in both of those
+years. Similarly, the rule for changing to daylight saving time was
+the same from 1922 to 1966; but the rule for returning to standard
+time changed in 1955. Got it?</p>
-<p>OK, now for the somewhat more interesting “US” rules:
-<p>
-<table border>
+<p>OK, now for the somewhat more interesting “US” rules:</p>
+
+<table border="1">
<tr>
- <th colspan=6>From the Source File</th>
+ <th colspan="6">From the Source File</th>
</tr>
<tr>
- <td colspan=6>
+ <td colspan="6" align="center"><table><tr><td>
<pre>
-
-# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S
-Rule US 1918 1919 - Mar lastSun 2:00 1:00 D
-Rule US 1918 1919 - Oct lastSun 2:00 0 S
-Rule US 1942 only - Feb 9 2:00 1:00 W # War
-Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
-Rule US 1945 only - Sep 30 2:00 0 S
-Rule US 1967 2006 - Oct lastSun 2:00 0 S
-Rule US 1967 1973 - Apr lastSun 2:00 1:00 D
-Rule US 1974 only - Jan 6 2:00 1:00 D
-Rule US 1975 only - Feb 23 2:00 1:00 D
-Rule US 1976 1986 - Apr lastSun 2:00 1:00 D
-Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D
-Rule US 2007 max - Mar Sun>=8 2:00 1:00 D
-Rule US 2007 max - Nov Sun>=1 2:00 0 S
+#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S
+Rule US 1918 1919 - Mar lastSun 2:00 1:00 D
+Rule US 1918 1919 - Oct lastSun 2:00 0 S
+Rule US 1942 only - Feb 9 2:00 1:00 W # War
+Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
+Rule US 1945 only - Sep 30 2:00 0 S
+Rule US 1967 2006 - Oct lastSun 2:00 0 S
+Rule US 1967 1973 - Apr lastSun 2:00 1:00 D
+Rule US 1974 only - Jan 6 2:00 1:00 D
+Rule US 1975 only - Feb 23 2:00 1:00 D
+Rule US 1976 1986 - Apr lastSun 2:00 1:00 D
+Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D
+Rule US 2007 max - Mar Sun>=8 2:00 1:00 D
+Rule US 2007 max - Nov Sun>=1 2:00 0 S
</pre>
- </td>
+ </td></tr></table></td>
</tr>
<tr>
- <th colspan=6>Reformatted a Bit</th>
+ <th colspan="6">Reformatted a Bit</th>
</tr>
<tr>
<th>From</th>
<th>To</th>
- <th colspan=2>On</th>
+ <th colspan="2">On</th>
<th>At</th>
<th>Action</th>
</tr>
-<tr align=center>
- <td rowspan=2>1918</td>
- <td rowspan=2>1919</td>
- <td rowspan=2>last Sunday</td>
+<tr align="center">
+ <td rowspan="2">1918</td>
+ <td rowspan="2">1919</td>
+ <td rowspan="2">last Sunday</td>
<td>in March</td>
- <td rowspan=3>02:00 local</td>
+ <td rowspan="3">02:00 local</td>
<td>go to daylight saving time</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>in October</td>
<td>return to standard time</td>
</tr>
-<tr align=center>
- <td colspan=2>1942 only</td>
- <td colspan=2>February 9<small><sup>th</sup></small></td>
+<tr align="center">
+ <td colspan="2">1942 only</td>
+ <td colspan="2">February 9<small><sup>th</sup></small></td>
<td>go to “war time”</td>
</tr>
-<tr align=center>
- <td colspan=2 rowspan=2>1945 only</td>
- <td colspan=2>August 14<small><sup>th</sup></small></td>
- <td>23:00 <a href="http://en.wikipedia.org/wiki/Universal_Time">UT</a></td>
+<tr align="center">
+ <td colspan="2" rowspan="2">1945 only</td>
+ <td colspan="2">August 14<small><sup>th</sup></small></td>
+ <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td>
<td>
- rename “war time” to “peace<br>time;” clocks don’t change
+ rename “war time” to “peace<br>time;”
+ clocks don’t change
</td>
</tr>
-<tr align=center>
- <td colspan=2>September 30<small><sup>th</sup></small></td>
- <td rowspan=9>02:00 local</td>
- <td rowspan=2>return to standard time</td>
+<tr align="center">
+ <td colspan="2">September 30<small><sup>th</sup></small></td>
+ <td rowspan="9">02:00 local</td>
+ <td rowspan="2">return to standard time</td>
</tr>
-<tr align=center>
- <td rowspan=2>1967</td>
+<tr align="center">
+ <td rowspan="2">1967</td>
<td>2006</td>
- <td rowspan=2>last Sunday</td>
+ <td rowspan="2">last Sunday</td>
<td>in October</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>1973</td>
<td>in April</td>
- <td rowspan=6>go to daylight saving time</td>
+ <td rowspan="6">go to daylight saving time</td>
</tr>
-<tr align=center>
- <td colspan=2>1974 only</td>
- <td colspan=2>January 6<small><sup>th</sup></small></td>
+<tr align="center">
+ <td colspan="2">1974 only</td>
+ <td colspan="2">January 6<small><sup>th</sup></small></td>
</tr>
-<tr align=center>
- <td colspan=2>1975 only</td>
- <td colspan=2>February 23<small><sup>rd</sup></small></td>
+<tr align="center">
+ <td colspan="2">1975 only</td>
+ <td colspan="2">February 23<small><sup>rd</sup></small></td>
</tr>
-<tr align=center>
+<tr align="center">
<td>1976</td>
<td>1986</td>
<td>last Sunday</td>
- <td rowspan=2>in April</td>
+ <td rowspan="2">in April</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>1987</td>
<td>2006</td>
<td>first Sunday</td>
</tr>
-<tr align=center>
- <td rowspan=2>2007</td>
- <td rowspan=2>present</td>
- <td colspan=2>second Sunday in March</td>
+<tr align="center">
+ <td rowspan="2">2007</td>
+ <td rowspan="2">present</td>
+ <td colspan="2">second Sunday in March</td>
</tr>
-<tr align=center>
- <td colspan=2>first Sunday in November</td>
+<tr align="center">
+ <td colspan="2">first Sunday in November</td>
<td>return to standard time</td>
</tr>
</table>
-<p>There are two interesting things to note here.
-
-<p>First, the time that something happens (in the <tt>AT</tt> column) is not necessarily
-the local wall clock time. The time can be suffixed with ‘s’ (for
-“standard”) to mean local standard time (different from wall clock
-time when observing daylight saving time); or it can be suffixed with ‘g’,
-‘u’, or ‘z’, all three of which mean the standard time at the
-<a href="http://en.wikipedia.org/wiki/Prime_Meridian">prime meridan</a>. ‘g’
-stands for “<a href="http://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”;
-‘u’
-stands for “<a href="http://en.wikipedia.org/wiki/Universal_Time">UT</a>” or
-“<a href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>”
-(whichever was official at the time);
-‘z’ stands for the
-<a href="http://aa.usno.navy.mil/graphics/TimeZoneMap2007.pdf">nautical time zone</a> Z
-(a.k.a. “Zulu” which, in turn, stands for ‘Z’).
-<!--<tt><rant></tt>(Don’t say “Z stands for Zulu;”
-that’s <a href="http://www.urbandictionary.com/define.php?term=Bass+Ackwards">bass
-ackwards</a>.)<tt></rant></tt>-->
-The time can also be suffixed with
-‘w’ meaning “wall clock time;” but it usually isn’t
-because that’s the default.
-
-<p>Second, the day in the <tt>ON</tt> column, in addition to “<tt>lastSun</tt>” or a particular
-day of the month, can have the form, <nobr>“<tt>Sun>=</tt><i>x</i>”</nobr>
-or <nobr>“<tt>Sun<=</tt><i>x</i>,”</nobr>
-where <i>x</i> is a day of the month. For example, <nobr>“<tt>Sun>=8</tt>”</nobr>
-means “the first Sunday on or after the eighth of the month,” in other words, the second Sunday
-of the month. Furthermore, although there are no examples above, the weekday needn’t be
-“<tt>Sun</tt>” in either form, but can be the usual three-character
-English abbreviation for any day of the week.
-
-<p>And the US rules give us more examples of a couple of things already mentioned:
+<p>There are two interesting things to note here.</p>
+
+<p>First, the time that something happens (in the <code>AT</code>
+column) is not necessarily the local wall clock time. The time can be
+suffixed with ‘s’ (for “standard”) to mean
+local standard time (different from wall clock time when observing
+daylight saving time); or it can be suffixed with ‘g’,
+‘u’, or ‘z’, all three of which mean the
+standard time at the
+<a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridan</a>.
+‘g’ stands for “<a
+href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”;
+‘u’ stands for “<a
+href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a
+href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>”
+(whichever was official at the time); ‘z’ stands for the
+<a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a>
+Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’).
+The time can also be suffixed with ‘w’ meaning “wall
+clock time;” but it usually isn’t because that’s the
+default.</p>
+
+<p>Second, the day in the <code>ON</code> column, in addition to
+“<code>lastSun</code>” or a particular day of the month,
+can have the form, “<code>Sun>=</code><i>x</i>” or
+“<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day
+of the month. For example, “<code>Sun>=8</code>” means
+“the first Sunday on or after the eighth of the month,” in
+other words, the second Sunday of the month. Furthermore, although
+there are no examples above, the weekday needn’t be
+“<code>Sun</code>” in either form, but can be the usual
+three-character English abbreviation for any day of the week.</p>
+
+<p>And the US rules give us more examples of a couple of things
+already mentioned:</p>
+
<ul>
-<li>The rules for changing to and from daylight saving time are actually <i>different sets</i>
- of rules; and the two sets can change independently. Consider, for example, that the rule
- for the return to standard time stayed the same from 1967 to 2006; but the rule for
- the transition to daylight saving time changed several times in the same period.
- There can also be periods, 1946 to 1966 for example, when no rule from this group
- is in effect, and so either no transition happened in those years, or some other
- rule is in effect (perhaps a state or other more local rule).
-<p>
-<li>The <tt>SAVE</tt> and <tt>LETTER</tt> columns contain <i>steady state</i>, not transitions.
- Consider, for example, the transition from “war time” to “peace time”
- that happened on August 14, 1945. The “1:00” in the <tt>SAVE</tt> column
- is <i>not</i> an instruction to advance the clock an hour. It means that clocks should
- <i>be</i> one hour ahead of standard time, which they already are because of the previous rule,
- so there should be no change.
+<li>The rules for changing to and from daylight saving time are
+actually <i>different sets</i> of rules; and the two sets can change
+independently. Consider, for example, that the rule for the return to
+standard time stayed the same from 1967 to 2006; but the rule for the
+transition to daylight saving time changed several times in the same
+period. There can also be periods, 1946 to 1966 for example, when no
+rule from this group is in effect, and so either no transition
+happened in those years, or some other rule is in effect (perhaps a
+state or other more local rule).</li>
+
+<li>The <code>SAVE</code> and <code>LETTER</code> columns
+contain <i>steady state</i>, not transitions. Consider, for example,
+the transition from “war time” to “peace time”
+that happened on August 14, 1945. The “1:00” in
+the <code>SAVE</code> column is <i>not</i> an instruction to advance
+the clock an hour. It means that clocks should <i>be</i> one hour
+ahead of standard time, which they already are because of the previous
+rule, so there should be no change.</li>
+
</ul>
-<p>OK, now let’s look at a Zone record:
-<p>
-<table border>
+<p>OK, now let’s look at a Zone record:</p>
+
+<table border="1">
<tr>
- <th colspan=5>From the Source File</th>
+ <th colspan="5">From the Source File</th>
</tr>
<tr>
- <td colspan=5>
+ <td colspan="6" align="center"><table><tr><td>
<pre>
-
-# Zone NAME GMTOFF RULES FORMAT [UNTIL]
-Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24
- -6:00 US C%sT 1920
- -6:00 Chicago C%sT 1936 Mar 1 2:00
- -5:00 - EST 1936 Nov 15 2:00
- -6:00 Chicago C%sT 1942
- -6:00 US C%sT 1946
- -6:00 Chicago C%sT 1967
- -6:00 US C%sT
+#Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24
+ -6:00 US C%sT 1920
+ -6:00 Chicago C%sT 1936 Mar 1 2:00
+ -5:00 - EST 1936 Nov 15 2:00
+ -6:00 Chicago C%sT 1942
+ -6:00 US C%sT 1946
+ -6:00 Chicago C%sT 1967
+ -6:00 US C%sT
</pre>
- </td>
+ </td></tr></table></td>
</tr>
<tr>
- <th colspan=5>Columns Renamed</th>
+ <th colspan="5">Columns Renamed</th>
</tr>
<tr>
- <th rowspan=2>Standard Offset<br>from <a href="http://en.wikipedia.org/wiki/Prime_Meridian">Prime Meridian</a></th>
- <th rowspan=2>Daylight<br>Saving Time</th>
- <th rowspan=2>Abbreviation(s)</th>
- <th colspan=2>Ending at Local Time</th>
+ <th rowspan="2">Standard Offset<br>
+ from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime
+ Meridian</a></th>
+ <th rowspan="2">Daylight<br>Saving Time</th>
+ <th rowspan="2">Abbreviation(s)</th>
+ <th colspan="2">Ending at Local Time</th>
</tr>
<tr>
<th>Date</th>
<th>Time</th>
</tr>
-<tr align=center>
+<tr align="center">
<td>−5:50:36</td>
<td>not observed</td>
<td>LMT</td>
<td>1883-11-18</td>
<td>12:09:24</td>
</tr>
-<tr align=center>
- <td rowspan=2>−6:00:00</td>
+<tr align="center">
+ <td rowspan="2">−6:00:00</td>
<td>US rules</td>
- <td rowspan=2>CST or CDT</td>
+ <td rowspan="2">CST or CDT</td>
<td>1920-01-01</td>
<td>00:00:00</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>Chicago rules</td>
<td>1936-03-01</td>
- <td rowspan=2>02:00:00</td>
+ <td rowspan="2">02:00:00</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>−5:00:00</td>
<td>not observed</td>
<td>EST</td>
<td>1936-11-15</td>
</tr>
-<tr align=center>
- <td rowspan=4>−6:00:00</td>
+<tr align="center">
+ <td rowspan="4">−6:00:00</td>
<td>Chicago rules</td>
<td>CST or CDT</td>
<td>1942-01-01</td>
- <td rowspan=3>00:00:00</td>
+ <td rowspan="3">00:00:00</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>US rules</td>
<td>CST, CWT or CPT</td>
<td>1946-01-01</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>Chicago rules</td>
- <td rowspan=2>CST or CDT</td>
+ <td rowspan="2">CST or CDT</td>
<td>1967-01-01</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>US rules</td>
- <td colspan=2>—</td>
+ <td colspan="2">—</td>
</tr>
</table>
-<p>There are a couple of interesting differences between Zones and Rules.
-
-<p>First, and somewhat trivially, whereas Rules are considered to contain one or more
-records, a Zone is considered to be a single record with zero or more <i>continuation
-lines</i>. Thus, the keyword, <nobr>“<tt>Zone</tt>,”</nobr> and the zone name
-are not repeated. The last line is the one without anything in the <tt>[UNTIL]</tt> column.
-
-<p>Second, and more fundamentally, each line of a Zone represents
-a steady state, not a transition between states. The state exists from the date and time
-in the previous line’s <tt>[UNTIL]</tt> column up to the
-date and time in the current line’s <tt>[UNTIL]</tt> column. In other words,
-the date and time in the <tt>[UNTIL]</tt> column is the instant that separates
-this state from the next. Where that would be ambiguous because we’re
-setting our clocks back, the <tt>[UNTIL]</tt> column specifies the first occurrence of
-the instant. The state specified by the last line, the one without
-anything in the <tt>[UNTIL]</tt> column, continues to the present.
-
-<p>The first line typically specifies the mean solar time observed before the introduction
-of standard time. Since there’s no line before that, it has no <nobr>beginning. <tt>8-) </tt></nobr>
-For some places near the
-<a href="http://en.wikipedia.org/wiki/International_Date_Line">International Date Line</a>,
-the first <i>two</i> lines will show solar times differing by 24 hours; this corresponds to
-a movement of the Date Line.
-For example:
+<p>There are a couple of interesting differences between Zones and Rules.</p>
+
+<p>First, and somewhat trivially, whereas Rules are considered to
+contain one or more records, a Zone is considered to be a single
+record with zero or more <i>continuation lines</i>. Thus, the keyword,
+“<code>Zone</code>,” and the zone name are not
+repeated. The last line is the one without anything in
+the <code>[UNTIL]</code> column.</p>
+
+<p>Second, and more fundamentally, each line of a Zone represents a
+steady state, not a transition between states. The state exists from
+the date and time in the previous line’s <code>[UNTIL]</code>
+column up to the date and time in the current
+line’s <code>[UNTIL]</code> column. In other words, the date and
+time in the <code>[UNTIL]</code> column is the instant that separates
+this state from the next. Where that would be ambiguous because
+we’re setting our clocks back, the <code>[UNTIL]</code> column
+specifies the first occurrence of the instant. The state specified by
+the last line, the one without anything in the <code>[UNTIL]</code>
+column, continues to the present.</p>
+
+<p>The first line typically specifies the mean solar time observed
+before the introduction of standard time. Since there’s no line before
+that, it has no beginning. <code>8-) </code> For some places near the <a
+href="https://en.wikipedia.org/wiki/International_Date_Line">International
+Date Line</a>, the first <i>two</i> lines will show solar times
+differing by 24 hours; this corresponds to a movement of the Date
+Line. For example:</p>
+
<pre>
-# Zone NAME GMTOFF RULES FORMAT [UNTIL]
-Zone America/Juneau 15:02:19 - LMT 1867 Oct 18
- -8:57:41 - LMT ...
+#Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone America/Juneau 15:02:19 - LMT 1867 Oct 18
+ -8:57:41 - LMT ...
</pre>
-When Alaska was purchased from Russia in 1867, the Date Line moved from
-the Alaska/Canada border to
-the Bering Strait; and the time in Alaska was then 24 hours earlier than it had been.
-<nobr><tt><aside></tt>(6 October</nobr> in the Julian calendar,
-which Russia was still using then for religious reasons, was followed by <i>a second instance
-of the same day with a different name</i>, <nobr>18 October</nobr>
-in the Gregorian calendar. Isn’t civil time
-<nobr>wonderful? <tt>8-)</tt>)<tt></aside></tt></nobr>
-
-<p>The abbreviation, “LMT,” stands for “local mean time,”
-which is an invention of the <a href="http://en.wikipedia.org/wiki/Zoneinfo">tz database</a>
-and was probably never actually used during the period. Furthermore, the value
-is almost certainly wrong except in the archetypal place after which the zone
-is named. (The tz database usually doesn’t provide a separate Zone record
-for places where nothing significant happened after 1970.)
-
-<p>The <tt>RULES</tt> column tells us whether daylight saving time is being observed:
+<p>When Alaska was purchased from Russia in 1867, the Date Line moved
+from the Alaska/Canada border to the Bering Strait; and the time in
+Alaska was then 24 hours earlier than it had
+been. <code><aside></code>(6 October in the Julian calendar,
+which Russia was still using then for religious reasons, was followed
+by <i>a second instance of the same day with a different name</i>, 18
+October in the Gregorian calendar. Isn’t civil time
+wonderful? <code>8-)</code>)<code></aside></code></p>
+
+<p>The abbreviation, “LMT” stands for “local mean
+time”, which is an invention of
+the <a href="https://en.wikipedia.org/wiki/Tz_database">tz
+database</a> and was probably never actually used during the
+period. Furthermore, the value is almost certainly wrong except in the
+archetypal place after which the zone is named. (The tz database
+usually doesn’t provide a separate Zone record for places where
+nothing significant happened after 1970.)</p>
+
+<p>The <code>RULES</code> column tells us whether daylight saving time is being observed:
<ul>
-<li>A hyphen, a kind of null value, means that we have not set our clocks ahead of standard time.
-<li>An amount of time (usually but not necessarily <nobr>“1:00”</nobr>
- meaning one hour) means that we have set our clocks ahead by that amount.
-<li>Some alphabetic string means that we <i>might have</i> set our clocks ahead;
- and we need to check the rule the name of which is the given alphabetic string.
+<li>A hyphen, a kind of null value, means that we have not set our
+clocks ahead of standard time.</li>
+
+<li>An amount of time (usually but not necessarily “1:00”
+meaning one hour) means that we have set our clocks ahead by that
+amount.</li>
+
+<li>Some alphabetic string means that we <i>might have</i> set our
+clocks ahead; and we need to check the rule the name of which is the
+given alphabetic string.</li>
</ul>
-An example of a specific amount of time is:
+<p>An example of a specific amount of time is:</p>
<pre>
-# Zone NAME GMTOFF RULES FORMAT [UNTIL]
-Zone Pacific/Honolulu ... 1933 Apr 30 2:00
- -10:30 1:00 HDT 1933 May 21 2:00
- ...
+#Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone Pacific/Honolulu ... 1933 Apr 30 2:00
+ -10:30 1:00 HDT 1933 May 21 2:00
+ ...
</pre>
-Hawaii tried daylight saving time for three weeks in 1933 and decided they didn’t
-like <nobr>it. <tt>8-) </tt></nobr>Note that the <tt>GMTOFF</tt> column always contains
-the standard time offset, so the wall clock time during this period was
-<nobr>GMT − 10:30 + 1:00 = GMT − 9:30.</nobr>
-<p>The <tt>FORMAT</tt> column specifies the usual abbreviation of the time zone name.
-It can have one of four forms:
+<p>Hawaii tried daylight saving time for three weeks in 1933 and
+decided they didn’t like it. <code>8-) </code>Note that
+the <code>GMTOFF</code> column always contains the standard time
+offset, so the wall clock time during this period was GMT −
+10:30 + 1:00 = GMT − 9:30.</p>
+
+<p>The <code>FORMAT</code> column specifies the usual abbreviation of
+the time zone name. It can have one of four forms:</p>
<ul>
-<li>the string, “<tt>zzz</tt>,” which is a kind of null value (don’t ask)
-<li>a single alphabetic string other than “<tt>zzz</tt>,”
- in which case that’s the abbreviation
-<li>a pair of strings separated by a slash <nobr>(‘<tt>/</tt>’),</nobr>
- in which case the first string is the abbreviation for the standard time name
- and the second string is the abbreviation for the daylight saving time name
-<li>a string containing <nobr>“<tt>%s</tt>,”</nobr> in which case the
- <nobr>“<tt>%s</tt>”</nobr> will be replaced by the text in the
- appropriate Rule’s <nobr><tt>LETTER</tt></nobr> column
+
+<li>the string, “<code>zzz</code>,” which is a kind of
+null value (don’t ask)</li>
+
+<li>a single alphabetic string other than “<code>zzz</code>,”
+in which case that’s the abbreviation</li>
+
+<li>a pair of strings separated by a slash
+(‘<code>/</code>’), in which case the first string is the
+abbreviation for the standard time name and the second string is the
+abbreviation for the daylight saving time name</li>
+
+<li>a string containing “<code>%s</code>,” in which case
+the “<code>%s</code>” will be replaced by the text in the
+appropriate Rule’s <code>LETTER</code> column</li>
</ul>
-The last two make sense only if there’s a named rule in effect.
-<p>An example of a slash is:
+<p>The last two make sense only if there’s a named rule in effect.</p>
+
+<p>An example of a slash is:</p>
<pre>
-# Zone NAME GMTOFF RULES FORMAT [UNTIL]
-Zone Europe/London ... 1996
- 0:00 EU GMT/BST
+#Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone Europe/London ... 1996
+ 0:00 EU GMT/BST
</pre>
-The current time in the UK is called either Greenwich mean time or British summer time.
-<p>One wrinkle, not fully explained in <tt>zic.8.txt</tt>, is what happens when
-switching to a named rule. To what values should the <tt>SAVE</tt> and <tt>LETTER</tt>
-data be initialized?
+<p>The current time in the UK is called either Greenwich mean time or
+British summer time.</p>
+
+<p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what
+happens when switching to a named rule. To what values should
+the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p>
+
<ul>
-<li>If at least one transition has happened, use the <tt>SAVE</tt> and <tt>LETTER</tt>
- data from the most recent.
-<p>
-<li>If switching to a named rule before any transition has happened, assume standard
- time (<tt>SAVE</tt> zero), and use the <tt>LETTER</tt> data from the earliest
- transition with a <tt>SAVE</tt> of zero.
+<li>If at least one transition has happened, use
+the <code>SAVE</code> and <code>LETTER</code> data from the most
+recent.</li>
+
+<li>If switching to a named rule before any transition has happened,
+assume standard time (<code>SAVE</code> zero), and use
+the <code>LETTER</code> data from the earliest transition with
+a <code>SAVE</code> of zero.
+
</ul>
-<p>And two last things about the <tt>FORMAT</tt> column:
+<p>And two last things about the <code>FORMAT</code> column:</p>
<ul>
-<li>The <a href="http://en.wikipedia.org/wiki/Zoneinfo">tz database</a> gives
- abbreviations for time zone names in <i>popular usage</i>, which is not
- necessarily “correct” by law. For example, the last line in
- <nobr><tt>Zone</tt> <tt>Pacific/Honolulu</tt></nobr> (shown below) gives
- “HST” for “Hawaii standard time” even though the
- <a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a>
- name for that time zone is “Hawaii-Aleutian standard time.”
- This author has read that there are also some places in Australia where
- popular time zone names differ from the legal ones.
-<p>
-<li>No attempt is made to
- <a href="http://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a>
- the abbreviations. They are intended to be the values returned through the
- <nobr><tt>"%Z"</tt></nobr> format specifier to
- <a href="http://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s
- <a href="http://opengroup.org/onlinepubs/007908799/xsh/strftime.html"><tt>strftime</tt></a>
- function in the
- <a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>.
+
+<li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz
+database</a> gives abbreviations for time zone names in <i>popular
+usage</i>, which is not necessarily “correct” by law. For
+example, the last line in
+<code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives
+“HST” for “Hawaii standard time” even though the
+<a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a>
+name for that time zone is “Hawaii-Aleutian standard time.”
+This author has read that there are also some places in Australia where
+popular time zone names differ from the legal ones.
+
+<li>No attempt is made to <a
+href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a>
+the abbreviations. They are intended to be the values returned through the
+<code>"%Z"</code> format specifier to
+<a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s
+<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a>
+function in the
+<a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>.
</ul>
-<p>As a final example, here’s the complete history for Hawaii:
+<p>As a final example, here’s the complete history for Hawaii:</p>
-<p>
-<table border>
+<table border="1">
<tr>
- <th colspan=6>Relevant Excerpts from the US Rules</th>
+ <th colspan="6">Relevant Excerpts from the US Rules</th>
</tr>
<tr>
- <td colspan=6>
+ <td colspan="6" align="center"><table><tr><td>
<pre>
-
-# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S
-Rule US 1918 1919 - Oct lastSun 2:00 0 S
-Rule US 1942 only - Feb 9 2:00 1:00 W # War
-Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
-Rule US 1945 only - Sep 30 2:00 0 S
+#Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S
+Rule US 1918 1919 - Oct lastSun 2:00 0 S
+Rule US 1942 only - Feb 9 2:00 1:00 W # War
+Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace
+Rule US 1945 only - Sep 30 2:00 0 S
</pre>
- </td>
+ </td></tr></table></td>
</tr>
<tr>
- <th colspan=6>The Zone Record</th>
+ <th colspan="6">The Zone Record</th>
</tr>
<tr>
- <td colspan=6 align=center><table border=0><tr><td>
+ <td colspan="6" align="center"><table><tr><td>
<pre>
-
-# Zone NAME GMTOFF RULES FORMAT [UNTIL]
-Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00
- -10:30 - HST 1933 Apr 30 2:00
- -10:30 1:00 HDT 1933 May 21 2:00
- -10:30 US H%sT 1947 Jun 8 2:00
- -10:00 - HST
+#Zone NAME GMTOFF RULES FORMAT [UNTIL]
+Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00
+ -10:30 - HST 1933 Apr 30 2:00
+ -10:30 1:00 HDT 1933 May 21 2:00
+ -10:30 US H%sT 1947 Jun 8 2:00
+ -10:00 - HST
</pre>
</td></tr></table></td>
</tr>
<tr>
- <th colspan=6>What We Infer</th>
+ <th colspan="6">What We Infer</th>
</tr>
<tr>
- <th rowspan=2>Wall-Clock<br>Offset from<br>Prime Meridian</th>
- <th rowspan=2>Adjust<br>Clocks</th>
- <th colspan=2>Time Zone</th>
- <th colspan=2>Ending at Local Time</th>
+ <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th>
+ <th rowspan="2">Adjust<br>Clocks</th>
+ <th colspan="2">Time Zone</th>
+ <th colspan="2">Ending at Local Time</th>
</tr>
<tr>
<th>Abbrv.</th>
@@ -541,7 +584,7 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00
<th>Date</th>
<th>Time</th>
</tr>
-<tr align=center>
+<tr align="center">
<td>−10:31:26</td>
<td>—</td>
<td>LMT</td>
@@ -549,80 +592,85 @@ Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00
<td>1900-01-01</td>
<td>12:00</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>−10:30</td>
<td>+0:01:26</td>
<td>HST</td>
<td>Hawaii standard time</td>
<td>1933-04-30</td>
- <td rowspan=3>02:00</td>
+ <td rowspan="3">02:00</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>−9:30</td>
<td>+1:00</td>
<td>HDT</td>
<td>Hawaii daylight time</td>
<td>1933-05-21</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>−10:30¹</td>
<td>−1:00¹</td>
<td>HST¹</td>
<td>Hawaii standard time</td>
<td>1942-02-09</td>
</tr>
-<tr align=center>
- <td rowspan=2>−9:30</td>
+<tr align="center">
+ <td rowspan="2">−9:30</td>
<td>+1:00</td>
<td>HWT</td>
<td>Hawaii war time</td>
<td>1945-08-14</td>
<td>13:30²</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>0</td>
<td>HPT</td>
<td>Hawaii peace time</td>
<td>1945-09-30</td>
- <td rowspan=2>02:00</td>
+ <td rowspan="2">02:00</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>−10:30</td>
<td>−1:00</td>
- <td rowspan=2>HST</td>
- <td rowspan=2>Hawaii standard time</td>
+ <td rowspan="2">HST</td>
+ <td rowspan="2">Hawaii standard time</td>
<td>1947-06-08</td>
</tr>
-<tr align=center>
+<tr align="center">
<td>−10:00³</td>
<td>+0:30³</td>
- <td colspan=2>—</td>
+ <td colspan="2">—</td>
</tr>
<tr>
- <td colspan=6>
+ <td colspan="6">
¹Switching to US rules…most recent transition (in 1919) was to standard time
</td>
</tr>
<tr>
- <td colspan=6>
- ²23:00 <a href="http://en.wikipedia.org/wiki/Universal_Time">UT</a>
+ <td colspan="6">
+ ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>
+ (−9:30) = 13:30 local
</td>
</tr>
<tr>
- <td colspan=6>
- ³Since <a href="http://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>,
+ <td colspan="6">
+ ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>,
the civil time in Hawaii has been
- <a href="http://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
+ <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>
− 10:00 year-round.
</td>
</tr>
</table>
-<p>There will be a short quiz <nobr>later. <tt>8-)</tt></nobr>
+<p>There will be a short quiz later. <code>8-)</code></p>
-<p><hr>
+<hr>
+<address>
+This web page is in the public domain, so clarified as of
+2015-10-20 by Bill Seymour.
+<br>
All suggestions and corrections will be welcome; all flames will be amusing.
Mail to was at pobox dot com.
+</address>
</body>
</html>
diff --git a/tz-link.htm b/tz-link.htm
index bd92f64..f0bbf70 100644
--- a/tz-link.htm
+++ b/tz-link.htm
@@ -10,7 +10,7 @@
content="Sources for time zone and daylight saving time data">
<meta name="DC.Creator" content="Eggert, Paul">
<meta name="DC.Contributor" content="Olson, Arthur David">
-<meta name="DC.Date" content="2015-12-13">
+<meta name="DC.Date" content="2016-02-29">
<meta name="DC.Description"
content="Sources of information about time zones and daylight saving time">
<meta name="DC.Identifier"
@@ -162,7 +162,7 @@ Here are some links that may be of interest.
<li>The article
<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
an encyclopedic summary.</li>
-<li><a href="https://web.archive.org/web/20140221080916/http://www.cstdbill.com/tzdb/tz-how-to.html">How to Read the
+<li><a href="tz-how-to.html">How to Read the
tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
database format.</li>
<li><a
@@ -201,7 +201,7 @@ time zone converter.</li>
href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
<li><a
-href="http://convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
+href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
Time in 1000 Places</a> uses descriptions of the values.</li>
<li><a href="http://www.timezoneconverter.com/cgi-bin/tzc.tzc">Time Zone
Converter</a>
@@ -209,7 +209,7 @@ uses a pulldown menu.</li>
<li><a href="http://home.kpn.nl/vanadovv/time/TZworld.html">Complete
timezone information for all countries</a> displays tables of DST rules.
<li><a href="http://www.timeanddate.com/worldclock/">The World Clock –
-Time Zones</a> lets you sort zone names and convert times.</li>
+Worldwide</a> lets you sort zone names and convert times.</li>
<li><a href="http://www.zeitverschiebung.net/en/">Time Difference</a>
calculates the current time difference between locations.</li>
<li><a href="http://www.wx-now.com">Weather Now</a> lists the weather too.</li>
@@ -642,7 +642,7 @@ is Singapore in the "Wrong" Time Zone?</a> details the
history of legal time in Singapore and Malaysia.</dd>
<dt>United Kingdom</dt>
<dd><a
-href="http://www.polyomino.org.uk/british-time/">History of
+href="https://www.polyomino.org.uk/british-time/">History of
legal time in Britain</a> discusses in detail the country
with perhaps the best-documented history of clock adjustments.
The National Physical Laboratory also maintains an <a
@@ -650,7 +650,7 @@ href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-d
of Summer time dates</a>.</dd>
<dt>United States</dt>
<dd>The Department of Transportation's <a
-href="http://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
+href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
</dl>
<h2>Precision timekeeping</h2>
@@ -699,7 +699,7 @@ code for converting among time scales like
<abbr title="International Atomic Time">TAI</abbr>,
<abbr>TDB</abbr>, <abbr>TDT</abbr> and
<abbr>UTC</abbr>.</li>
-<li><a href="http://www2.jpl.nasa.gov/basics/bsf2-3.php">Basics of
+<li><a href="http://solarsystem.nasa.gov/basics/bsf2-3.php">Basics of
Space Flight – Reference Systems – Time Conventions</a>
briefly explains interplanetary space flight timekeeping.</li>
<li><a
@@ -770,7 +770,7 @@ specifies an <abbr>ISO</abbr> 8601
profile for use in new Internet
protocols.</li>
<li>
-<a href="http://www.hackcraft.net/web/datetime/">Date & Time
+<a href="https://www.hackcraft.net/web/datetime/">Date & Time
Formats on the Web</a> surveys web- and Internet-oriented date and time
formats.</li>
<li>
--
2.5.0
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