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diff --git a/contrib/tzcode/theory.html b/contrib/tzcode/theory.html index 8f76aaf0b3c8..d56e1a79df46 100644 --- a/contrib/tzcode/theory.html +++ b/contrib/tzcode/theory.html @@ -12,6 +12,7 @@ </head> <body> +<main> <h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1> <nav> <ul> @@ -26,10 +27,10 @@ <li><a href="#leapsec">Leap seconds</a></li> <li><a href="#calendar">Calendrical issues</a></li> <li><a href="#planets">Time and time zones off earth</a></li> + <li><a href="#see-also">See also</a></li> </ul> </nav> -<section> <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2> <p> The <a @@ -112,9 +113,6 @@ It does not always make sense to talk about a timezone’s “base offset”, which is not necessarily a single number. </p> -</section> - -<section> <h2 id="naming">Timezone identifiers</h2> <p> Each timezone has a name that uniquely identifies the timezone. @@ -345,8 +343,7 @@ These legacy names are mostly defined in the file "<code>etcetera</code>". Also, the file "<code>backward</code>" defines the legacy names <code>Etc/GMT0</code>, <code>Etc/GMT-0</code>, <code>Etc/GMT+0</code>, -<code>GMT0</code>, <code>GMT-0</code> and <code>GMT+0</code>, -and the file "<code>northamerica</code>" defines the legacy names +<code>GMT0</code>, <code>GMT-0</code>, <code>GMT+0</code>, <code>EST5EDT</code>, <code>CST6CDT</code>, <code>MST7MDT</code>, and <code>PST8PDT</code>. </li> @@ -387,11 +384,11 @@ and <code>zone.tab</code>. <p> The database defines each timezone name to be a zone, or a link to a zone. -The source file <code>backward</code> defines links for backward -compatibility; it does not define zones. +The source file <code>backward</code> defines links +and a few zones for backward compatibility. Although <code>backward</code> was originally designed to be optional, nowadays distributions typically use it -and no great weight should be attached to whether a link +and no great weight should be attached to whether a link or zone is defined in <code>backward</code> or in some other file. The source file <code>etcetera</code> defines names that may be useful on platforms that do not support proleptic <code>TZ</code> strings @@ -404,9 +401,7 @@ second information on platforms that support leap seconds. Another <code>etcetera</code> name, <code>GMT</code>, is used by older code releases. </p> -</section> -<section> <h2 id="abbreviations">Time zone abbreviations</h2> <p> When this package is installed, it generates time zone abbreviations @@ -610,9 +605,7 @@ Israel. To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like <code>-</code>0600 instead of time zone abbreviations like CST. </p> -</section> -<section> <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2> <p> The <code><abbr>tz</abbr></code> database is not authoritative, and it @@ -876,9 +869,7 @@ merely because two locations differ in <abbr>LMT</abbr> or transitioned to standard time at different dates. </p> -</section> -<section> <h2 id="functions">Time and date functions</h2> <p> The <code><abbr>tz</abbr></code> code contains time and date functions @@ -1284,9 +1275,7 @@ The vestigial <abbr>API</abbr>s are: much the better. </li> </ul> -</section> -<section> <h2 id="stability">Interface stability</h2> <p> The <code><abbr>tz</abbr></code> code and data supply the following interfaces: @@ -1353,9 +1342,7 @@ than Bangkok by putting <samp>Asia/Bangkok</samp> in the event’s record, the application should be robust in the presence of timezone splits between now and the future time. </p> -</section> -<section> <h2 id="leapsec">Leap seconds</h2> <p> Leap seconds were introduced in 1972 to accommodate the @@ -1430,9 +1417,7 @@ Alternatively, you can install just one set of <abbr>TZif</abbr> files in the first place; see the <code>REDO</code> variable in this package’s <a href="https://en.wikipedia.org/wiki/Makefile">makefile</a>. </p> -</section> -<section> <h2 id="calendar">Calendrical issues</h2> <p> Calendrical issues are a bit out of scope for a time zone database, @@ -1446,22 +1431,30 @@ Other information and sources are given in the file "<code>calendars</code>" in the <code><abbr>tz</abbr></code> distribution. They sometimes disagree. </p> -</section> -<section> <h2 id="planets">Time and time zones off Earth</h2> <p> -The European Space Agency is <a -href="https://www.esa.int/Applications/Satellite_navigation/Telling_time_on_the_Moon">considering</a> -the establishment of a reference timescale for the Moon, which has +Efforts are underway to establish a reference timescale for the Moon, which has days roughly equivalent to 29.5 Earth days, and where relativistic effects cause clocks to tick slightly faster than on Earth. -Also, <abbr title="National Aeronautics and Space Administration">NASA</abbr> -has been <a +The European Space Agency <a +href="https://www.esa.int/Applications/Satellite_navigation/Telling_time_on_the_Moon">began +considering</a> the problem in 2023, and in 2024 +<abbr title="National Aeronautics and Space Administration">NASA</abbr> +was <a href="https://bidenwhitehouse.archives.gov/wp-content/uploads/2024/04/Celestial-Time-Standardization-Policy.pdf">ordered</a> to consider the establishment of Coordinated Lunar Time (<abbr>LTC</abbr>). -It is not yet known whether the US and European efforts will result in -multiple timescales on the Moon. +In 2025 a team at China’s +<a href="https://en.wikipedia.org/wiki/Purple_Mountain_Observatory">Purple +Mountain Observatory</a> published +<a href="https://doi.org/10.1051/0004-6361/202557345">Lunar Time Ephemeris +440</a>, the first formal proposal for a standard +for Temps Coordonné Lunaire (<abbr>TCL</abbr>). +A <a href="https://www.bipm.org/documents/20126/284836054/CGPM-2026-Draft-Resolutions.pdf">draft +resolution of the 13–15 October 2026 General Conference on Weights +and Measures</a> recommends that the <a href="https://www.bipm.org/en/">Bureau +International des Poids et Mesures</a> collaborate with space agencies +and other organizations on this matter. </p> <p> @@ -1583,7 +1576,15 @@ Sources for time on other planets: (2016-01-20). </li> </ul> -</section> +</main> + +<aside> +<h2 id="see-also">See also</h2> +<ul> +<li><a href="tz-link.html">Time zone and daylight saving time data</a></li> +<li><a href="tz-art.html">Time and the arts</a></li> +</ul> +</aside> <footer> <hr> |