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diff --git a/tz-link.html b/tz-link.html new file mode 100644 index 000000000000..13a824d7e9af --- /dev/null +++ b/tz-link.html @@ -0,0 +1,953 @@ +<!DOCTYPE html> +<html lang="en"> +<head> +<title>Sources for time zone and daylight saving time data</title> +<meta charset="UTF-8"> +</head> +<body> +<h1>Sources for time zone and daylight saving time data</h1> +<p> +<a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and +<a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a> +rules are controlled by individual +governments. They are sometimes changed with little notice, and their +histories and planned futures are often recorded only fitfully. Here +is a summary of attempts to organize and record relevant data in this +area. +</p> +<h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2> +<p> +The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a> +time zone database contains code and data +that represent the history of local time +for many representative locations around the globe. +It is updated periodically to reflect changes made by political bodies +to time zone boundaries and daylight saving rules. +This database (known as <code><abbr>tz</abbr></code>, +<code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>) +is used by several implementations, +including +<a href="https://www.gnu.org/software/libc/">the +<abbr title="GNU's Not Unix">GNU</abbr> +C Library</a> (used in +<a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>), +<a href="https://www.android.com">Android</a>, +<a href="https://developer.mozilla.org/en-US/docs/Mozilla/B2G_OS">B2G +<abbr title="Operating System">OS</abbr></a>, +<a href="https://www.freebsd.org">Free<abbr +title="Berkeley Software Distribution">BSD</abbr></a>, +<a href="https://netbsd.org">Net<abbr>BSD</abbr></a>, +<a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>, +<a href="https://www.chromium.org/chromium-os">Chromium OS</a>, +<a href="https://cygwin.com">Cygwin</a>, +<a href="http://www.delorie.com/djgpp/"><abbr +title="DJ's GNU Programming Platform">DJGPP</abbr></a>, +<a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>, +<a href="https://www.mysql.com">MySQL</a>, +<a href="https://en.wikipedia.org/wiki/WebOS"><abbr +title="Web Operating System">webOS</abbr></a>, +<a href="https://ibm.com/aix"><abbr +title="Advanced Interactive eXecutive">AIX</abbr></a>, +<a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>, +<a href="https://www.apple.com/ios/"><abbr +title="iPhone OS">iOS</abbr></a>, +<a href="https://www.apple.com/macos/">macOS</a>, +<a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>, +<a href="https://www.hpe.com/info/openvms">Open<abbr +title="Virtual Memory System">VMS</abbr></a>, +<a href="https://www.oracle.com/database/index.html">Oracle Database</a>, and +<a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p> +<p> +Each location in the database represents a region where all +clocks keeping local time have agreed since 1970. +Locations are identified by continent or ocean and then by the name of +the location, which is typically the largest city within the region. +For example, <code>America/New_York</code> +represents most of the <abbr title="United States">US</abbr> eastern time zone; +<code>America/Phoenix</code> represents most of Arizona, which +uses mountain time without daylight saving time (<abbr +title="daylight saving time">DST</abbr>); +<code>America/Detroit</code> represents most of Michigan, which uses +eastern time but with different <abbr>DST</abbr> rules in 1975; +and other entries represent smaller regions like Starke County, +Indiana, which switched from central to eastern time in 1991 +and switched back in 2006. +To use the database on an extended <a +href="https://en.wikipedia.org/wiki/POSIX"><abbr +title="Portable Operating System Interface">POSIX</abbr></a> +implementation set the <code><abbr>TZ</abbr></code> +environment variable to the location's full name, +e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p> +<p> +Associated with each region is a history of offsets from +<a href="https://en.wikipedia.org/wiki/Universal_Time">Universal +Time</a> (<abbr>UT</abbr>), which is <a +href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean +Time</a> (<abbr>GMT</abbr>) with days beginning at midnight; +for time stamps after 1960 this is more precisely <a +href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated +Universal Time</a> (<abbr>UTC</abbr>). +The database also records when daylight saving time was in use, +along with some time zone abbreviations such as <abbr>EST</abbr> +for Eastern Standard Time in the <abbr>US</abbr>.</p> +<h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2> +<p> +The following <a +href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download +the latest release's two +<a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a> +to a <abbr>GNU</abbr>/Linux or similar host.</p> +<pre style="margin-left: 2em"><code>mkdir tzdb +cd tzdb +<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz +wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz +<a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf - +gzip -dc tzdata-latest.tar.gz | tar -xf - +</code></pre> +<p>Alternatively, the following shell commands download the same +release in a single-tarball format containing extra data +useful for regression testing:</p> +<pre style="margin-left: 2em"><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a> +<a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf - +</code></pre> +<p>These commands use convenience links to the latest release +of the <code><abbr>tz</abbr></code> database hosted by the +<a href="https://www.iana.org/time-zones">Time Zone Database website</a> +of the <a href="https://www.iana.org">Internet Assigned Numbers +Authority (IANA)</a>. +Older releases are in files named +<code>tzcode<var>V</var>.tar.gz</code>, +<code>tzdata<var>V</var>.tar.gz</code>, and +<code>tzdb-<var>V</var>.tar.lz</code>, +where <code><var>V</var></code> is the version. +Since 1996, each version has been a four-digit year followed by +lower-case letter (<samp>a</samp> through <samp>z</samp>, +then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp> +through <samp>zzz</samp>, and so on). +Since version 2016h, each release has contained a text file named +"<samp>version</samp>" whose first (and currently only) line is the version. +The releases are also available in an +<a href="ftp://ftp.iana.org/tz/releases/"><abbr +title="File Transfer Protocol">FTP</abbr> directory</a> via a +less-secure protocol.</p> +<p>Alternatively, a development repository of code and data can be +retrieved from <a href="https://github.com">GitHub</a> via the shell +command:</p> +<pre style="margin-left: 2em"><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a> +</code></pre> +<p> +Since version 2012e, each release has been tagged in development repositories. +Untagged commits are less well tested and probably contain +more errors.</p> +<p> +After obtaining the code and data files, see the +<code>README</code> file for what to do next. +The code lets you compile the <code><abbr>tz</abbr></code> source files into +machine-readable binary files, one for each location. It also lets +you read a <code><abbr>tz</abbr></code> binary file and interpret time stamps for that +location.</p> +<h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2> +<p> +The <code><abbr>tz</abbr></code> code and data +are by no means authoritative. If you find errors, please +send changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>, +the time zone mailing list. You can also <a +href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it +and browse the <a +href="https://mm.icann.org/pipermail/tz/">archive of old +messages</a>.</p> +<p> +If your government plans to change its time zone boundaries or +daylight saving rules, inform <code>tz@iana.org</code> well in +advance, as this will coordinate updates to many cell phones, +computers, and other devices around the world. With +less than a year's notice there is a good chance that some +computer-based clocks will operate incorrectly after the change, due +to delays in propagating updates to software and data. The shorter +the notice, the more likely clock problems will arise; see "<a +href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On +the Timing of Time Zone Changes</a>" for examples. +</p> +<p> +Changes to the <code><abbr>tz</abbr></code> code and data are often +propagated to clients via operating system updates, so +client <code><abbr>tz</abbr></code> data can often be corrected by +applying these updates. With GNU/Linux and similar systems, if your +maintenance provider has not yet adopted the +latest <code><abbr>tz</abbr></code> data, you can often short-circuit +the process by tailoring the generic instructions in +the <code><abbr>tz</abbr> README</code> file and installing the latest +data yourself. System-specific instructions for installing the +latest <code><abbr>tz</abbr></code> data have also been published +for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>, +<a +href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>, +<a +href="http://userguide.icu-project.org/datetime/timezone"><abbr +title="International Components for Unicode">ICU</abbr></a>, +<a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a> +and <a +href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a> +Java, <a href="http://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a +href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>, +and <a +href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below). +</p> +<p>Sources for the <code><abbr>tz</abbr></code> database are +<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr +title="Unicode Transformation Format 8-bit">UTF-8</abbr></a> +<a href="https://en.wikipedia.org/wiki/Text_file">text files</a> +with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr +title="linefeed">LF</abbr></a>, +which can be modified by common text editors such +as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>, +<a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and +<a href="https://www.vim.org">vim</a>. +Specialized source-file editing can be done via the +<a href="https://packagecontrol.io/packages/zoneinfo">Sublime +zoneinfo</a> package for <a +href="https://www.sublimetext.com">Sublime Text</a> and the <a +href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode +zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual +Studio Code</a>. +</p> +<p> +For further information about updates, please see +<a href="https://tools.ietf.org/html/rfc6557">Procedures for +Maintaining the Time Zone Database</a> (Internet <abbr +title="Request For Comments">RFC</abbr> 6557). More detail can be +found in <a href="theory.html">Theory and pragmatics of the tz code and data</a>. +</p> +<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2> +<ul> +<li>The article +<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is +an encyclopedic summary.</li> +<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 +href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A +literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the +database's style.</li> +</ul> +<h2 id="web">Web sites using recent versions of the +<code><abbr>tz</abbr></code> database</h2> +<p> +These are listed roughly in ascending order of complexity and fanciness. +</p> +<ul> +<li><a href="https://time.is">Time.is</a> shows locations' +time and zones.</li> +<li><a href="https://www.timejones.com">TimeJones.com</a>, +<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and +<a href="http://worldclock.com">The World Clock</a> +are time zone converters.</li> +<li><a +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://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> +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="https://www.timeanddate.com/worldclock/">The World Clock – +Worldwide</a> lets you sort zone names and convert times.</li> +<li><a href="https://24timezones.com">24TimeZones</a> has a world +time map and a time converter.</li> +<li><a href="https://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> and +<a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li> +</ul> +<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2> +<ul> +<li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s +<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data +Distribution Service (tzdist) working group</a> defined <a +href="https://tools.ietf.org/html/rfc7808">TZDIST</a> +(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service, +along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a> +(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for +transferring time zone data by reference. The draft <a +id="TZDIST-Geolocate" +href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate-01">TZDIST +Geolocate Extension</a> lets a client determine its time zone region +from its geographic location using a <a +href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li> +<li>The <a href="https://tools.ietf.org/html/rfc5545"> +Internet Calendaring and Scheduling Core Object Specification +(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445) +covers time zone +data; see its VTIMEZONE calendar component. +The iCalendar format requires specialized parsers and generators; a +variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a> +(Internet <abbr>RFC</abbr> 6321) uses +<a href="https://www.w3.org/XML/"><abbr +title="Extensible Markup Language">XML</abbr></a> format, and a variant +<a href="https://tools.ietf.org/html/rfc7265">jCal</a> +(Internet <abbr>RFC</abbr> 7265) +uses <a href="https://www.json.org"><abbr +title="JavaScript Object Notation">JSON</abbr></a> format.</li> +</ul> +<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2> +<ul> +<li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a +href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a> +program that compiles +<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files. +Vzic is freely +available under the <a +href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> +General Public License (<abbr +title="General Public License">GPL</abbr>)</a>.</li> +<li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz +database conversion utility</a> is like Vzic, except for the <a +href="https://www.microsoft.com/net">.NET framework</a> +and with a <abbr>BSD</abbr>-style license.</li> +<li><a +href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a> +contains a script <code>parse_olson</code> that compiles +<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a> +modules. It is part of the Perl <a +href="http://datetime.perl.org">DateTime Project</a>, which is freely +available under both the <abbr>GPL</abbr> and the Perl Artistic +License. DateTime::TimeZone also contains a script +<code>tests_from_zdump</code> that generates test cases for each clock +transition in the <code><abbr>tz</abbr></code> database.</li> +<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone +Database Parser</a> is a +<a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and +runtime library that is <a +href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0355r4.html">moving +forward</a> for inclusion in the next iteration of <a +href="https://isocpp.org/std/the-standard"><em><abbr +title="International Organization for Standardization">ISO</abbr> +International Standard ISO/IEC 14882:2017(E) – Programming +Language C++</em></a>. +It is freely available under the +<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li> +<li><a id="ICU" href="http://site.icu-project.org">International Components for +Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a +href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a> +libraries for internationalization that +has a compiler from <code><abbr>tz</abbr></code> source +and from <abbr title="Common Locale Data Repository">CLDR</abbr> data +(mentioned <a href="#CLDR">below</a>) +into an <abbr>ICU</abbr>-specific format. +<abbr>ICU</abbr> is freely available under a +<abbr>BSD</abbr>-style license.</li> +<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for +the <a href="https://elixir-lang.org">Elixir</a> language downloads +and compiles tz source and exposes <abbr +title="Application Program Interface">API</abbr>s for use. It is +freely available under the <abbr>MIT</abbr> license.</li> +<li>Java-based compilers and libraries include: +<ul> +<li>The <a +href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater +tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by +Oracle Java.</li> +<li>The <a +href="http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java +8 <code>java.time</code> <abbr>API</abbr></a> can be supplemented by <a +href="http://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>, +which is freely available under a <abbr>BSD</abbr>-style license.</li> +<li><a href="http://www.joda.org/joda-time/">Joda-Time – Java date +and time <abbr>API</abbr></a> contains a class +<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles +<code><abbr>tz</abbr></code> source into a binary format. It inspired +Java 8 <code>java.time</code>, which its users should migrate to once +they can assume Java 8 or later. It is available under the <a +href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li> +<li><a href="https://github.com/MenoData/Time4J/">Time4J – +Advanced date, time and interval library for Java</a> contains a class +<code>net.time4j.tool.TimezoneRepositoryCompiler</code> that compiles +<code><abbr>tz</abbr></code> source into a binary format. Time4J is +available under the <a +href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser +General Public License (<abbr title="Lesser General Public +License">LGPL</abbr>)</a>.</li> +<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and +Java-based libraries.</li> +</ul> +<li><a href="https://nodatime.org">Noda Time – Date and +time <abbr>API</abbr> for .NET</a> +and <a href="http://www.babiej.demon.nl/Tz4Net/main.htm">TZ4Net</a> +are similar to Joda-Time and Time4J, but for the .NET framework instead of +Java. They are freely available under the +Apache License +and a <abbr>BSD</abbr>-style license, respectively.</li> +<li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based +compilers and libraries include: +<ul> +<li><a +href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a> +compiles time zone data into a compact form designed for +JavaScript. It is freely available under a combination of +the <abbr>MIT</abbr> license and the Apache License.</li> +<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a +plugin for the <a href="https://momentjs.com">Moment.js</a> date +manipulation library. It is freely available under the <abbr>MIT</abbr> +license.</li> +<li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s +<abbr>API</abbr> is upward compatible with standard JavaScript +Dates. It is freely available under the Apache License.</li> +<li><a href="https://github.com/sproutsocial/walltime-js">Walltime-js</a> +translates <abbr>UT</abbr> to local time. It is freely available under +the <abbr>MIT</abbr> license.</li> +</ul> +<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a +compiler from <code><abbr>tz</abbr></code> source into +<a href="https://julialang.org/">Julia</a>. It is freely available +under the <abbr>MIT</abbr> license.</li> +<li><a href="http://pytz.sourceforge.net">pytz – World Timezone +Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into +<a href="https://www.python.org">Python</a>. +It is freely available under a <abbr>BSD</abbr>-style license.</li> +<li><a href="https://tzinfo.github.io">TZInfo – +Ruby Timezone Library</a> +compiles <code><abbr>tz</abbr></code> source into +<a href="https://www.ruby-lang.org/en/">Ruby</a>. +It is freely available under the <abbr>MIT</abbr> license.</li> +<li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time +Library</a> is +a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class +library that compiles <code><abbr>tz</abbr></code> source into a time +zone repository whose format +is either proprietary or an <abbr>XML</abbr>-encoded +representation.</li> +<li><a id="Tcl" href="https://tcl.tk">Tcl</a> +contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code> +source into text files, along with a runtime that can read those +files. Tcl is freely available under a <abbr>BSD</abbr>-style +license.</li> +</ul> +<h2 id="binary">Other <code><abbr>tz</abbr></code> binary file readers</h2> +<ul> +<li>The <a +href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C +Library</a> +has an independent, thread-safe implementation of +a <code><abbr>tz</abbr></code> binary file reader. +This library is freely available under the LGPL +and is widely used in <abbr>GNU</abbr>/Linux systems.</li> +<li><a href="https://www.gnome.org">GNOME</a>'s +<a href="https://developer.gnome.org/glib/">GLib</a> has +a <code><abbr>tz</abbr></code> binary file reader written in C that +creates a <code>GTimeZone</code> object representing sets +of <abbr>UT</abbr> offsets. +It is freely available under the <abbr>LGPL</abbr>.</li> +<li>The +<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s +<code>baltzo::TimeZoneUtil</code> component contains a C++ +implementation of a binary file reader. It is freely available under +the Apache License.</li> +<li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++ +library that translates between <abbr>UT</abbr> and civil time and +can read binary files. It is freely available under the Apache +License.</li> +<li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a> +is a <code><abbr>tz</abbr></code> binary file reader written in Java. +It is freely available under the <abbr>LGPL</abbr>.</li> +<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C +library that reads tz binary files and converts +time stamps from one time zone or format to another. +It is used by <a href="https://secure.php.net"><abbr +title="PHP: Hypertext Preprocessor">PHP</abbr></a>, +<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>, +and <a href="https://www.mongodb.com">MongoDB</a>. +It is freely available under the <abbr>MIT</abbr> license.</li> +<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a +JavaScript library that supports date arithmetic that is time zone +aware. It is freely available under the <abbr>MIT</abbr> license.</li> +<li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a +<code><abbr>tz</abbr></code> binary file reader.</li> +<li><a href="http://search.cpan.org/perldoc?DateTime::TimeZone::Tzfile"> +DateTime::TimeZone::Tzfile</a> +is a <code><abbr>tz</abbr></code> binary file reader written in Perl. +It is freely available under the same terms as Perl +(dual <abbr>GPL</abbr> and Artistic license).</li> +<li>The +public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a> +library contains a Python tool that +converts <code><abbr>tz</abbr></code> binary data into +<abbr>JSON</abbr>-format data suitable for use +in its JavaScript library for time zone conversion. Dates before 1970 +are not supported.</li> +<li>The <a +href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a> +package contains <a href="https://www.haskell.org">Haskell</a> code that +parses and uses <code><abbr>tz</abbr></code> binary data. It is freely +available under a <abbr>BSD</abbr>-style license.</li> +</ul> +<h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2> +<ul> +<li><a href="https://foxclocks.org">FoxClocks</a> +is an extension for <a href="https://www.google.com/chrome/">Google +Chrome</a> and for <a +href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla +Toolkit</a> applications like <a +href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a +href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>. +It displays multiple clocks in the application window, and has a mapping +interface to <a href="https://www.google.com/earth/">Google Earth</a>. +It is freely available under the <abbr>GPL</abbr>.</li> +<li><a href="https://golang.org">Go programming language</a> +implementations contain a copy of a 32-bit subset of a recent +<code><abbr>tz</abbr></code> database in a +Go-specific format.</li> +<li><a +href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International +clock (intclock)</a> is a clock that displays multiple time zones on +<abbr>GNU</abbr>/Linux and similar systems. It is freely available +under the <abbr>GPL</abbr>.</li> +<li>Microsoft Windows 8.1 +and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr> +data (mentioned <a href="#CLDR">below</a>) used by +<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> +classes such as <a +href="https://msdn.microsoft.com/en-us/library/windows/apps/windows.globalization.datetimeformatting.datetimeformatter.aspx"><code>DateTimeFormatter</code></a>. +<a id="System.TimeZoneInfo" +href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring +Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes +the older, proprietary method of Microsoft Windows 2000 and later, +which stores time zone data in the +<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The +<a +href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone → +Tzid table</a> or <a +href="https://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml"><abbr>XML</abbr> +file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs +to <code><abbr>tz</abbr></code> names. +<li><a +href="https://www.oracle.com/java/index.html">Oracle +Java</a> contains a copy of a subset of a recent +<code><abbr>tz</abbr></code> database in a +Java-specific format.</li> +<li><a href="https://www.relativedata.com/time-zone-master">Time Zone +Master</a> is a Microsoft Windows clock program that can automatically +download, compile and use <code>tz</code> releases. The Basic version +is free.</li> +<li><a +href="http://veladg.com/velaterra.html">VelaTerra</a> is +a macOS program. Its developers +<a href="http://veladg.com/tzoffer.html">offer free +licenses</a> to <code><abbr>tz</abbr></code> contributors.</li> +</ul> +<h2 id="other-dbs">Other time zone databases</h2> +<ul> +<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a> +is Astrodienst's Web version of Shanks and Pottenger's +time zone history atlases also published in <a +href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a> +form by <a href="https://astrocom.com">ACS-Starcrafts</a>. +These atlases are extensive but unreliable, as Shanks appears to have +guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no +sources and do not indicate which entries are guesswork.</li> +<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in +its own <code>tztab</code>(4) format.</li> +<li>Microsoft Windows has proprietary data mentioned +<a href="#System.TimeZoneInfo">above</a>.</li> +<li><a href="https://www.worldtimeserver.com">World Time Server</a> +is another time zone database.</li> +<li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a> +contains data from the Time Service Department of the +<abbr>US</abbr> Naval Observatory.</li> +<li>The <a +href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard +Schedules Information Manual</a> of the +International Air Transport Association +gives current time zone rules for airports served by commercial aviation.</li> +</ul> +<h2 id="maps">Maps</h2> +<ul> +<li>The <a href="https://www.cia.gov/index.html">United States Central +Intelligence Agency (<abbr +title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a +href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time +zone map</a>; the +<a +href="https://www.lib.utexas.edu/maps/world.html">Perry–Castañeda +Library Map Collection</a> +of the University of Texas at Austin has copies of +recent editions. +The pictorial quality is good, +but the maps do not indicate daylight saving time, +and parts of the data are a few years out of date.</li> +<li><a href="https://www.worldtimezone.com">Current time around the world +and standard time zones map of the world</a> +has several fancy time zone maps; it covers Russia particularly well. +The maps' pictorial quality is not quite as good as the +<abbr>CIA</abbr>'s +but the maps are more up to date.</li> +<li><a +href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How +much is time wrong around the world?</a> maps the difference between +mean solar and standard time, highlighting areas such as western China +where the two differ greatly. It's a bit out of date, unfortunately.</li> +</ul> +<h2 id="boundaries">Time zone boundaries</h2> +<p>Geographical boundaries between time zone regions are available +from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a> +services and other sources.</p> +<ul> +<li>Databases of time zone boundaries include: +<ul> +<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone +Boundary Builder</a> extracts +<a href="https://www.openstreetmap.org">Open Street Map</a> data to build +boundaries of <code><abbr>tz</abbr></code> regions. +Its code is freely available under the <abbr>MIT</abbr> license, and +its data entries are freely available under the +<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons +Open Database License</a>. The maps' borders appear to be quite accurate.</li> +<li><a href="http://efele.net/maps/tz/"><abbr>TZ</abbr> timezones +maps</a> contains <a +href="https://en.wikipedia.org/wiki/Shapefile">shapefiles</a> of +sets of <code><abbr>tz</abbr></code> regions. This includes +<a href="http://efele.net/maps/tz/world/">tz_world</a>, a shapefile +for all the world's regions. These maps are no longer maintained and +are superseded by the Timezone Boundary Builder.</li> +<li><a +href="https://github.com/straup/whereonearth-timezone">Whereonearth-timezone</a> +is in <a href="https://tools.ietf.org/html/rfc7946">GeoJSON</a> format +(Internet <abbr>RFC</abbr> 7946), and combines the +the tz_world shapefiles with the +<a href="https://developer.yahoo.com/geo/geoplanet/">GeoPlanet</a> +dataset.</li> +</ul></li> +<li>Programmatic interfaces that map geographical coordinates via tz_world to +<code><abbr>tz</abbr></code> regions include: +<ul> +<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is +written in <a +href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a> +and is freely available under the <abbr>MIT</abbr> license.</li> +<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a> +is written in Go and is freely available under the Apache License.</li> +<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>, +in both Java and +<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a> +form, is freely available under the MIT license.</li> +<li>For <a href="https://nodejs.org/en/">Node.js</a>, +the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a> +is freely available under the MIT license, and +the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a> +is in the public domain.</li> +<li>The <a +href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a> +library for Python is freely available under the MIT license. +<li>The <a +href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a> +library for Ruby is freely available under the MIT license.</li> +<li><a href="https://derickrethans.nl/what-time-is-it.html">What Time +is It Here?</a> applies MongoDB +geospatial query operators to shapefiles' data.</li> +</ul></li> +<li>Free access via a network API, if you register a key, is provided by +the <a href="http://www.geonames.org/export/web-services.html#timezone">GeoNames Timezone web service</a>, +the <a href="https://developers.google.com/maps/documentation/timezone/intro">Google Maps Time Zone API</a>, and +the <a href="https://timezonedb.com">Time Zone Database & API</a>. +Commercial network API access is provided +by <a href="https://askgeo.com">AskGeo</a> +and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>. +</li> +<li>"<a +href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How +to get a time zone from a location using latitude and longitude +coordinates?</a>" discusses other geolocation possibilities.</li> +<li><a href="http://statoids.com/statoids.html">Administrative +Divisions of Countries ("Statoids")</a> lists +political subdivision data related to time zones.</li> +<li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time +zone boundaries for multizone countries</a> summarizes legal +boundaries between time zones within countries.</li> +<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software +– GIS and Database Tools</a> includes a Manifold-format map of +world time zone boundaries distributed under the +<abbr>GPL</abbr>.</li> +<li>A ship within the <a +href="https://en.wikipedia.org/wiki/Territorial_waters">territorial +waters</a> of any nation uses that nation's time. In international +waters, time zone boundaries are meridians 15° apart, except that +<abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5° +wide and are separated by +the 180° meridian (not by the International Date Line, which is +for land and territorial waters only). A captain can change ship's +clocks any time after entering a new time zone; midnight changes are +common.</li> +</ul> +<h2 id="civil">Civil time concepts and history</h2> +<ul> +<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A +Walk through Time</a> +surveys the evolution of timekeeping.</li> +<li><a href="http://www.webexhibits.org/daylightsaving/">About Daylight +Saving Time – History, rationale, laws & dates</a> +is an overall history of <abbr>DST</abbr>.</li> +<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a> +contains guidelines and best practices for software applications that +deal with civil time.</li> +<li><a href="http://seizethedaylight.com/dst/">A Brief +History of Daylight Saving Time</a> summarizes some of the contentious +history of <abbr>DST</abbr>.</li> +<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of +the International Date Line</a> tells the story of the most important +time zone boundary.</li> +<li><a href="http://statoids.com/tconcept.html">Basic Time +Zone Concepts</a> discusses terminological issues behind time zones.</li> +</ul> +<h2 id="national">National histories of legal time</h2> +<dl> +<dt>Australia</dt> +<dd>The Parliamentary Library has commissioned a <a +href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research +paper on daylight saving time in Australia</a>. +The Bureau of Meteorology publishes a list of <a +href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation +Dates of Daylight Savings Time within Australia</a>.</dd> +<dt>Belgium</dt> +<dd>The Royal Observatory of Belgium maintains a table of <a +href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html" +hreflang="nl">time in Belgium (in Dutch)</a>.</dd> +<dt>Brazil</dt> +<dd>The Time Service Department of the National Observatory +records <a href="http://pcdsh01.on.br/DecHV.html" +hreflang="pt-BR">Brazil's daylight saving time decrees (in +Portuguese)</a>.</dd> +<dt>Canada</dt> +<dd>National Research Council Canada publishes current +and some older information about <a +href="https://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time +zones & daylight saving time</a>.</dd> +<dt>Chile</dt> +<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a +<a href="http://www.horaoficial.cl/historia_hora.html" hreflang="es">history of +Chile's official time (in Spanish)</a>.</dd> +<dt>Czech Republic</dt> +<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas" +hreflang="cs">When daylight saving time starts and ends (in Czech)</a> +summarizes and cites historical DST regulations.</dd> +<dt>Germany</dt> +<dd>The National Institute for Science and Technology maintains the <a +href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation +of Legal Time in Germany</a>.</dd> +<dt>Israel</dt> +<dd>The Interior Ministry periodically issues <a +href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements" +hreflang="he">announcements (in Hebrew)</a>.</dd> +<dt>Italy</dt> +<dd>The National Institute of Metrological Research maintains a +<a href="http://www.nanospin.eu/res/tf/ora_legale_i.shtml">table of civil time +(in Italian)</a>.</dd> +<dt>Mexico</dt> +<dd>The Investigation and Analysis Service of the Mexican Library of +Congress has published a <a +href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm" +hreflang="es">history of Mexican local time (in Spanish)</a>.</dd> +<dt>Malaysia</dt> +<dd>See Singapore <a href="#Singapore">below</a>.</dd> +<dt>Netherlands</dt> +<dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm" +hreflang="nl">Legal time in the Netherlands (in Dutch)</a> +covers the history of local time in the Netherlands from ancient times.</dd> +<dt>New Zealand</dt> +<dd>The Department of Internal Affairs maintains a brief <a +href="https://www.dia.govt.nz/Daylight-Saving-History">History of +Daylight Saving</a>. The privately-maintained <a +href="http://astrologyschool.com/nztime.html">History of New Zealand +time</a> has more details.</dd> +<dt>Singapore</dt> +<dd><a id="Singapore" +href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why +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="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 +href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates">Archive +of Summer time dates</a>.</dd> +<dt>United States</dt> +<dd>The Department of Transportation's <a +href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent +Time Zone Proceedings</a> lists changes to time zone boundaries.</dd> +<dt>Uruguay</dt> +<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan +Navy (SOHMA) publishes an annual <a +href="http://www.armada.mil.uy/Pagina/institucion/dimat/sohma/almanaque.html" hreflang="es">almanac +(in Spanish)</a>.</dd> +</dl> +<h2 id="precision">Precision timekeeping</h2> +<ul> +<li><a +href="http://leapsecond.com/hpan/an1289.pdf">The +Science of Timekeeping</a> is a thorough introduction +to the theory and practice of precision timekeeping.</li> +<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of +Time 2016</a> contains several freely-readable papers.</li> +<li><a href="http://www.ntp.org"><abbr +title="Network Time Protocol">NTP</abbr>: The Network +Time Protocol</a> (Internet <abbr>RFC</abbr> 5905) +discusses how to synchronize clocks of +Internet hosts.</li> +<li>The <a +href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision +Time Protocol</a> (<abbr +title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588) +can achieve submicrosecond clock accuracy on a local area network.</li> +<li><a +href="https://tools.ietf.org/html/rfc4833">Timezone +Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> +(Internet <abbr>RFC</abbr> 4833) +specifies a <a +href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a> +option for a server to configure +a client's time zone and daylight saving settings automatically.</li> +<li><a +href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical +Times</a> explains more abstruse astronomical time scales like +<abbr title="Terrestrial Dynamic Time">TDT</abbr>, +<abbr title="Geocentric Coordinate Time">TCG</abbr>, and +<abbr title="Barycentric Dynamic Time">TDB</abbr>. +<a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time +Scales</a> goes into more detail, particularly for historical variants.</li> +<li>The <a href="https://www.iau.org"><abbr +title="International Astronomical Union">IAU</abbr></a>'s <a +href="http://www.iausofa.org"><abbr +title="Standards Of Fundamental Astronomy">SOFA</abbr></a> +collection contains C and <a +href="https://en.wikipedia.org/wiki/Fortran">Fortran</a> +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="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock +– Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the +diverse local time +scales used by each landed mission on Mars.</li> +<li><a href="http://leapsecond.com">LeapSecond.com</a> is +dedicated not only to leap seconds but to precise time and frequency +in general. It covers the state of the art in amateur timekeeping, and +how the art has progressed over the past few decades.</li> +<li><a +href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr +title="International Earth Rotation and Reference Systems Service">IERS</abbr> +Bulletins</a> contains official publications of the International +Earth Rotation and Reference Systems Service, which decides when leap +seconds occur. The <code>tz</code> code and data support leap seconds +via an optional "<code>right</code>" configuration, as opposed to the +default "<code>posix</code>" configuration.</li> +<li><a href="https://developers.google.com/time/smear">Leap Smear</a> +discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a +leap second so that they disagree with <abbr>UTC</abbr> by at most a +half second, even though every <abbr>POSIX</abbr> minute has exactly +sixty seconds. This approach works with the default <code>tz</code> +"<code>posix</code>" configuration, is <a +href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by +the <abbr>NTP</abbr> reference implementation, and is used by major +cloud service providers.</li> +<li>The <a +href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap +Second Discussion List</a> covers <a +href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy +and Klepczynski's 1999 proposal to discontinue leap seconds</a>, +discussed further in +<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The +leap second: its history and possible future</a>. +<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr> +might be redefined +without Leap Seconds</a> gives pointers on this +contentious issue, which was active until 2015 and could become active +again.</li> +</ul> +<h2 id="notation">Time notation</h2> +<ul> +<li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data +Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time +zone names, abbreviations, identifiers, and formats. For example, it +contains French translations for "Eastern European Summer Time", +"<abbr title="Eastern European Summer Time">EEST</abbr>", and +"Bucharest". Its +<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type +charts</a> show these values for many locales. Data values are available in +both <abbr title="Locale Data Markup Language">LDML</abbr> +(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>. +<li> +<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of +the international standard date and time notation</a> is a good +summary of +<a +href="https://www.iso.org/standard/40874.html"><em><abbr>ISO</abbr> +8601:2004 – Data elements and interchange formats – Information +interchange – Representation of dates and times</em></a>.</li> +<li> +<a href="https://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr> +Schema: Datatypes – dateTime</a> specifies a format inspired by +<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li> +<li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of +Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322) +specifies the time notation used in email and <a +href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a> +headers.</li> +<li> +<a href="https://tools.ietf.org/html/rfc3339">Date and Time +on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) +specifies an <abbr>ISO</abbr> 8601 +profile for use in new Internet +protocols.</li> +<li> +<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>Alphabetic time zone abbreviations should not be used as unique +identifiers for <abbr>UT</abbr> offsets as they are ambiguous in +practice. For example, in English-speaking North America +"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>, +but in China it denotes 8 hours ahead of <abbr>UT</abbr>, +and French-speaking North Americans prefer +"<abbr title="Heure Normale du Centre">HNC</abbr>" to +"<abbr>CST</abbr>". The <code><abbr>tz</abbr></code> +database contains English abbreviations for many time stamps; +unfortunately some of these abbreviations were merely the database maintainers' +inventions, and these have been removed when possible.</li> +<li>Numeric time zone abbreviations typically count hours east of +<abbr>UT</abbr>, e.g., +09 for Japan and +−10 for Hawaii. However, the <abbr>POSIX</abbr> +<code><abbr>TZ</abbr></code> environment variable uses the opposite convention. +For example, one might use <code><abbr>TZ</abbr>="<abbr +title="Japan Standard Time">JST</abbr>-9"</code> and +<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code> +for Japan and Hawaii, respectively. If the +<code><abbr>tz</abbr></code> database is available, it is usually better to use +settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and +<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid +confusion, handle old time stamps better, and insulate you better from +any future changes to the rules. One should never set +<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like +<code>"GMT-9"</code>, though, since this would incorrectly imply that +local time is nine hours ahead of <abbr>UT</abbr> and the time zone +is called "<abbr>GMT</abbr>".</li> +</ul> +<h2 id="see-also">See also</h2> +<ul> +<li><a href="theory.html">Theory and pragmatics of the tz code and data</a></li> +<li><a href="tz-art.html">Time and the Arts</a></li> +</ul> +<hr> +<address> +This web page is in the public domain, so clarified as of +2009-05-17 by Arthur David Olson. +<br> +Please send corrections to this web page to the +<a href="mailto:tz@iana.org">time zone mailing list</a>. +</address> +</body> +</html> |