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      4 <title>Sources for time zone and daylight saving time data</title>
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     11 <h1>Sources for time zone and daylight saving time data</h1>
     12 <p>
     13 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
     14 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
     15 rules are controlled by individual
     16 governments. They are sometimes changed with little notice, and their
     17 histories and planned futures are often recorded only fitfully. Here
     18 is a summary of attempts to organize and record relevant data in this
     19 area.
     20 </p>
     21 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     22 <p>
     23 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     24 time zone database contains code and data
     25 that represent the history of local time
     26 for many representative locations around the globe.
     27 It is updated periodically to reflect changes made by political bodies
     28 to time zone boundaries and daylight saving rules.
     29 This database (known as <code><abbr>tz</abbr></code>,
     30 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     31 is used by several implementations,
     32 including
     33 <a href="https://www.gnu.org/software/libc/">the
     34 <abbr title="GNU's Not Unix">GNU</abbr>
     35 C Library</a> (used in
     36 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     37 <a href="https://www.android.com">Android</a>,
     38 <a href="https://www.freebsd.org">Free<abbr
     39 title="Berkeley Software Distribution">BSD</abbr></a>,
     40 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     41 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     42 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
     43 <a href="https://cygwin.com">Cygwin</a>,
     44 <a href="http://www.delorie.com/djgpp/"><abbr
     45 title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
     46 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     47 <a href="https://www.mysql.com">MySQL</a>,
     48 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     49 title="Web Operating System">webOS</abbr></a>,
     50 <a href="https://ibm.com/aix"><abbr
     51 title="Advanced Interactive eXecutive">AIX</abbr></a>,
     52 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
     53 <a href="https://www.apple.com/ios/"><abbr
     54 title="iPhone OS">iOS</abbr></a>,
     55 <a href="https://www.apple.com/macos/">macOS</a>,
     56 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     57 <a href="https://www.hpe.com/info/openvms">Open<abbr
     58 title="Virtual Memory System">VMS</abbr></a>,
     59 <a href="https://www.oracle.com/database/index.html">Oracle Database</a>, and
     60 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
     61 <p>
     62 Each main entry in the database represents a <dfn>timezone</dfn>
     63 for a set of civil-time clocks that have all agreed since 1970.
     64 Timezones are typically identified by continent or ocean and then by the
     65 name of the largest city within the region containing the clocks.
     66 For example, <code>America/New_York</code>
     67 represents most of the <abbr title="United States">US</abbr> eastern time zone;
     68 <code>America/Phoenix</code> represents most of Arizona, which
     69 uses mountain time without daylight saving time (<abbr>DST</abbr>);
     70 <code>America/Detroit</code> represents most of Michigan, which uses
     71 eastern time but with different <abbr>DST</abbr> rules in 1975;
     72 and other entries represent smaller regions like Starke County,
     73 Indiana, which switched from central to eastern time in 1991
     74 and switched back in 2006.
     75 To use the database on an extended <a
     76 href="https://en.wikipedia.org/wiki/POSIX"><abbr
     77 title="Portable Operating System Interface">POSIX</abbr></a>
     78 implementation set the <code><abbr>TZ</abbr></code>
     79 environment variable to the location's full name,
     80 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
     81 <p>
     82 Associated with each timezone is a history of offsets from
     83 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
     84 Time</a> (<abbr>UT</abbr>), which is <a
     85 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
     86 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
     87 for timestamps after 1960 this is more precisely <a
     88 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
     89 Universal Time</a> (<abbr>UTC</abbr>).
     90 The database also records when daylight saving time was in use,
     91 along with some time zone abbreviations such as <abbr>EST</abbr>
     92 for Eastern Standard Time in the <abbr>US</abbr>.</p>
     93 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
     94 <p>
     95 The following <a
     96 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
     97 the latest release's two
     98 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
     99 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    100 <pre><code>mkdir tzdb
    101 cd tzdb
    102 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    103 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    104 <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 -
    105 gzip -dc tzdata-latest.tar.gz | tar -xf -
    106 </code></pre>
    107 <p>Alternatively, the following shell commands download the same
    108 release in a single-tarball format containing extra data
    109 useful for regression testing:</p>
    110 <pre><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>
    111 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    112 </code></pre>
    113 <p>These commands use convenience links to the latest release
    114 of the <code><abbr>tz</abbr></code> database hosted by the
    115 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    116 of the <a href="https://www.iana.org">Internet Assigned Numbers
    117 Authority (IANA)</a>.
    118 Older releases are in files named
    119 <code>tzcode<var>V</var>.tar.gz</code>,
    120 <code>tzdata<var>V</var>.tar.gz</code>, and
    121 <code>tzdb-<var>V</var>.tar.lz</code>,
    122 where <code><var>V</var></code> is the version.
    123 Since 1996, each version has been a four-digit year followed by
    124 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    125 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    126 through <samp>zzz</samp>, and so on).
    127 Since version 2016h, each release has contained a text file named
    128 "<samp>version</samp>" whose first (and currently only) line is the version.
    129 The releases are also available in an
    130 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    131 title="File Transfer Protocol">FTP</abbr> directory</a> via a
    132 less-secure protocol.</p>
    133 <p>Alternatively, a development repository of code and data can be
    134 retrieved from <a href="https://github.com">GitHub</a> via the shell
    135 command:</p>
    136 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    137 </code></pre>
    138 <p>
    139 Since version 2012e, each release has been tagged in development repositories.
    140 Untagged commits are less well tested and probably contain
    141 more errors.</p>
    142 <p>
    143 After obtaining the code and data files, see the
    144 <code>README</code> file for what to do next.
    145 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    146 machine-readable binary files, one for each location. The binary files
    147 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>).
    148 The code also lets
    149 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    150 location.</p>
    151 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    152 <p>
    153 The <code><abbr>tz</abbr></code> code and data
    154 are by no means authoritative. If you find errors, please
    155 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    156 the time zone mailing list. You can also <a
    157 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    158 and browse the <a
    159 href="https://mm.icann.org/pipermail/tz/">archive of old
    160 messages</a>.</p>
    161 <p>
    162 If your government plans to change its time zone boundaries or
    163 daylight saving rules, inform <code>tz (a] iana.org</code> well in
    164 advance, as this will coordinate updates to many cell phones,
    165 computers, and other devices around the world. With
    166 less than a year's notice there is a good chance that some
    167 computer-based clocks will operate incorrectly after the change, due
    168 to delays in propagating updates to software and data. The shorter
    169 the notice, the more likely clock problems will arise; see "<a
    170 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    171 the Timing of Time Zone Changes</a>" for examples.
    172 </p>
    173 <p>
    174 Changes to the <code><abbr>tz</abbr></code> code and data are often
    175 propagated to clients via operating system updates, so
    176 client <code><abbr>tz</abbr></code> data can often be corrected by
    177 applying these updates. With GNU/Linux and similar systems, if your
    178 maintenance provider has not yet adopted the
    179 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    180 the process by tailoring the generic instructions in
    181 the <code><abbr>tz</abbr> README</code> file and installing the latest
    182 data yourself. System-specific instructions for installing the
    183 latest <code><abbr>tz</abbr></code> data have also been published
    184 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
    185 <a
    186 href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
    187 <a
    188 href="http://userguide.icu-project.org/datetime/timezone"><abbr
    189 title="International Components for Unicode">ICU</abbr></a>,
    190 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
    191 and <a
    192 href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
    193 Java, <a href="http://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    194 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    195 and <a
    196 href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
    197 </p>
    198 <p>Sources for the <code><abbr>tz</abbr></code> database are
    199 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    200 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    201 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    202 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    203 title="linefeed">LF</abbr></a>,
    204 which can be modified by common text editors such
    205 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    206 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
    207 <a href="https://www.vim.org">vim</a>.
    208 Specialized source-file editing can be done via the
    209 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    210 zoneinfo</a> package for <a
    211 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    212 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    213 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    214 Studio Code</a>.
    215 </p>
    216 <p>
    217 For further information about updates, please see
    218 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
    219 Maintaining the Time Zone Database</a> (Internet <abbr
    220 title="Request For Comments">RFC</abbr> 6557). More detail can be
    221 found in <a href="theory.html">Theory and pragmatics of the tz code and data</a>.
    222 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    223 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    224 </p>
    225 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    226 <ul>
    227 <li>The article
    228 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    229 an encyclopedic summary.</li>
    230 <li><a href="tz-how-to.html">How to Read the
    231 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    232 database format.</li>
    233 <li><a
    234 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    235 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    236 database's style.</li>
    237 </ul>
    238 <h2 id="web">Web sites using recent versions of the
    239 <code><abbr>tz</abbr></code> database</h2>
    240 <p>
    241 These are listed roughly in ascending order of complexity and fanciness.
    242 </p>
    243 <ul>
    244 <li><a href="https://time.is">Time.is</a> shows locations'
    245 time and zones.</li>
    246 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    247 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    248 <a href="http://worldclock.com">The World Clock</a>
    249 are time zone converters.</li>
    250 <li><a
    251 href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    252 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    253 <li><a
    254 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    255 Time in 1000 Places</a> uses descriptions of the values.</li>
    256 <li><a href="http://www.timezoneconverter.com/cgi-bin/tzc.tzc">Time Zone
    257 Converter</a>
    258 uses a pulldown menu.</li>
    259 <li><a href="http://home.kpn.nl/vanadovv/time/TZworld.html">Complete
    260 timezone information for all countries</a> displays tables of DST rules.
    261 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    262 Worldwide</a> lets you sort zone names and convert times.</li>
    263 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    264 time map and a time converter.</li>
    265 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    266 calculates the current time difference between locations.</li>
    267 <li><a href="http://www.wx-now.com">Weather Now</a> and
    268 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    269 </ul>
    270 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    271 <ul>
    272 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    273 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    274 Distribution Service (tzdist) working group</a> defined <a
    275 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
    276 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    277 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
    278 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    279 transferring time zone data by reference.
    280 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    281 mailing list</a> discusses two Internet drafts: <a
    282 id="TZDIST-Geolocate"
    283 href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
    284 Geolocate Extension</a> lets a client determine its timezone
    285 from its geographic location using a <a
    286 href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>, and
    287 <a href="https://tools.ietf.org/html/draft-murchison-tzdist-tzif">The
    288 Time Zone Information Format (<abbr>TZif</abbr>)</a> specifies the format of
    289 <abbr>TZif</abbr> data.</li>
    290 <li>The <a href="https://tools.ietf.org/html/rfc5545">
    291 Internet Calendaring and Scheduling Core Object Specification
    292 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    293 covers time zone
    294 data; see its VTIMEZONE calendar component.
    295 The iCalendar format requires specialized parsers and generators; a
    296 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
    297 (Internet <abbr>RFC</abbr> 6321) uses
    298 <a href="https://www.w3.org/XML/"><abbr
    299 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    300 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
    301 (Internet <abbr>RFC</abbr> 7265)
    302 uses <a href="https://www.json.org"><abbr
    303 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    304 </ul>
    305 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    306 <ul>
    307 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    308 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
    309 program that compiles
    310 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    311 Vzic is freely
    312 available under the <a
    313 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    314 General Public License (<abbr
    315 title="General Public License">GPL</abbr>)</a>.</li>
    316 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    317 database conversion utility</a> is like Vzic, except for the <a
    318 href="https://www.microsoft.com/net">.NET framework</a>
    319 and with a <abbr>BSD</abbr>-style license.</li>
    320 <li><a
    321 href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
    322 contains a script <code>parse_olson</code> that compiles
    323 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    324 modules. It is part of the Perl <a
    325 href="http://datetime.perl.org">DateTime Project</a>, which is freely
    326 available under both the <abbr>GPL</abbr> and the Perl Artistic
    327 License. DateTime::TimeZone also contains a script
    328 <code>tests_from_zdump</code> that generates test cases for each clock
    329 transition in the <code><abbr>tz</abbr></code> database.</li>
    330 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    331 Database Parser</a> is a
    332 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    333 runtime library that is <a
    334 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0355r4.html">moving
    335 forward</a> for inclusion in the next iteration of <a
    336 href="https://isocpp.org/std/the-standard"><em><abbr
    337 title="International Organization for Standardization">ISO</abbr>
    338 International Standard ISO/IEC 14882:2017(E) &ndash; Programming
    339 Language C++</em></a>.
    340 It is freely available under the
    341 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    342 <li><a id="ICU" href="http://site.icu-project.org">International Components for
    343 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    344 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    345 libraries for internationalization that
    346 has a compiler from <code><abbr>tz</abbr></code> source
    347 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    348 (mentioned <a href="#CLDR">below</a>)
    349 into an <abbr>ICU</abbr>-specific format.
    350 <abbr>ICU</abbr> is freely available under a
    351 <abbr>BSD</abbr>-style license.</li>
    352 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    353 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    354 and compiles tz source and exposes <abbr
    355 title="Application Program Interface">API</abbr>s for use. It is
    356 freely available under the <abbr>MIT</abbr> license.</li>
    357 <li>Java-based compilers and libraries include:
    358 <ul>
    359 <li>The <a
    360 href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
    361 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    362 Oracle Java.</li>
    363 <li>The <a
    364 href="http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    365 8 <code>java.time</code> <abbr>API</abbr></a> can be supplemented by <a
    366 href="http://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    367 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    368 <li><a href="http://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    369 and time <abbr>API</abbr></a> contains a class
    370 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    371 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    372 Java 8 <code>java.time</code>, which its users should migrate to once
    373 they can assume Java 8 or later. It is available under the <a
    374 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    375 <li><a href="https://github.com/MenoData/Time4J/">Time4J &ndash;
    376 Advanced date, time and interval library for Java</a> contains a class
    377 <code>net.time4j.tool.TimezoneRepositoryCompiler</code> that compiles
    378 <code><abbr>tz</abbr></code> source into a binary format. Time4J is
    379 available under the <a
    380 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    381 General Public License (<abbr title="Lesser General Public
    382 License">LGPL</abbr>)</a>.</li>
    383 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    384 Java-based libraries.</li>
    385 </ul>
    386 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    387 time <abbr>API</abbr> for .NET</a>
    388 and <a href="http://www.babiej.demon.nl/Tz4Net/main.htm">TZ4Net</a>
    389 are similar to Joda-Time and Time4J, but for the .NET framework instead of
    390 Java. They are freely available under the
    391 Apache License
    392 and a <abbr>BSD</abbr>-style license, respectively.</li>
    393 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
    394 compilers and libraries include:
    395 <ul>
    396 <li><a
    397 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
    398 compiles time zone data into a compact form designed for
    399 JavaScript. It is freely available under a combination of
    400 the <abbr>MIT</abbr> license and the Apache License.</li>
    401 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    402 plugin for the <a href="https://momentjs.com">Moment.js</a> date
    403 manipulation library. It is freely available under the <abbr>MIT</abbr>
    404 license.</li>
    405 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
    406 <abbr>API</abbr> is upward compatible with standard JavaScript
    407 Dates. It is freely available under the Apache License.</li>
    408 <li><a href="https://github.com/sproutsocial/walltime-js">Walltime-js</a>
    409 translates <abbr>UT</abbr> to local time. It is freely available under
    410 the <abbr>MIT</abbr> license.</li>
    411 </ul>
    412 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    413 compiler from <code><abbr>tz</abbr></code> source into
    414 <a href="https://julialang.org/">Julia</a>. It is freely available
    415 under the <abbr>MIT</abbr> license.</li>
    416 <li><a href="https://github.com/pavkam/tzdb">Delphi/<abbr
    417 title="Free Pascal Compiler">FPC</abbr> Time Zone Database</a>
    418 compiles from <code><abbr>tz</abbr></code> source into
    419 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    420 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    421 and <a
    422 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    423 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    424 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    425 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    426 <a href="https://www.python.org">Python</a>.
    427 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    428 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    429 Ruby Timezone Library</a>
    430 compiles <code><abbr>tz</abbr></code> source into
    431 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    432 It is freely available under the <abbr>MIT</abbr> license.</li>
    433 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
    434 Library</a> is
    435 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    436 library that compiles <code><abbr>tz</abbr></code> source into a time
    437 zone repository whose format
    438 is either proprietary or an <abbr>XML</abbr>-encoded
    439 representation.</li>
    440 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    441 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    442 source into text files, along with a runtime that can read those
    443 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    444 license.</li>
    445 </ul>
    446 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    447 <ul>
    448 <li>The <a
    449 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    450 Library</a>
    451 has an independent, thread-safe implementation of
    452 a <abbr>TZif</abbr> file reader.
    453 This library is freely available under the LGPL
    454 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    455 <li><a href="https://www.gnome.org">GNOME</a>'s
    456 <a href="https://developer.gnome.org/glib/">GLib</a> has
    457 a <abbr>TZif</abbr> file reader written in C that
    458 creates a <code>GTimeZone</code> object representing sets
    459 of <abbr>UT</abbr> offsets.
    460 It is freely available under the <abbr>LGPL</abbr>.</li>
    461 <li>The
    462 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    463 <code>baltzo::TimeZoneUtil</code> component contains a C++
    464 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    465 the Apache License.</li>
    466 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    467 library that translates between <abbr>UT</abbr> and civil time and
    468 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    469 License.</li>
    470 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
    471 is a <abbr>TZif</abbr> file reader written in Java.
    472 It is freely available under the <abbr>LGPL</abbr>.</li>
    473 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    474 library that reads <abbr>TZif</abbr> files and converts
    475 timestamps from one time zone or format to another.
    476 It is used by <a href="https://secure.php.net"><abbr
    477 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    478 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    479 and <a href="https://www.mongodb.com">MongoDB</a>.
    480 It is freely available under the <abbr>MIT</abbr> license.</li>
    481 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    482 JavaScript library that supports date arithmetic that is time zone
    483 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    484 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    485 <abbr>TZif</abbr> file reader.</li>
    486 <li><a href="http://search.cpan.org/perldoc?DateTime::TimeZone::Tzfile">
    487 DateTime::TimeZone::Tzfile</a>
    488 is a <abbr>TZif</abbr> file reader written in Perl.
    489 It is freely available under the same terms as Perl
    490 (dual <abbr>GPL</abbr> and Artistic license).</li>
    491 <li>The
    492 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    493 library contains a Python tool that
    494 converts <abbr>TZif</abbr> data into
    495 <abbr>JSON</abbr>-format data suitable for use
    496 in its JavaScript library for time zone conversion. Dates before 1970
    497 are not supported.</li>
    498 <li>The <a
    499 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    500 package contains <a href="https://www.haskell.org">Haskell</a> code that
    501 parses and uses <abbr>TZif</abbr> data. It is freely
    502 available under a <abbr>BSD</abbr>-style license.</li>
    503 </ul>
    504 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    505 <ul>
    506 <li><a href="https://foxclocks.org">FoxClocks</a>
    507 is an extension for <a href="https://www.google.com/chrome/">Google
    508 Chrome</a> and for <a
    509 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
    510 Toolkit</a> applications like <a
    511 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    512 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    513 It displays multiple clocks in the application window, and has a mapping
    514 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    515 It is freely available under the <abbr>GPL</abbr>.</li>
    516 <li><a href="https://golang.org">Go programming language</a>
    517 implementations contain a copy of a 32-bit subset of a recent
    518 <code><abbr>tz</abbr></code> database in a
    519 Go-specific format.</li>
    520 <li><a
    521 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
    522 clock (intclock)</a> is a clock that displays multiple time zones on
    523 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
    524 under the <abbr>GPL</abbr>.</li>
    525 <li>Microsoft Windows 8.1
    526 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    527 data (mentioned <a href="#CLDR">below</a>) used by the
    528 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    529 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    530 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    531 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    532 <a id="System.TimeZoneInfo"
    533 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    534 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    535 the older, proprietary method of Microsoft Windows 2000 and later,
    536 which stores time zone data in the
    537 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    538 <a
    539 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    540 Tzid table</a> or <a
    541 href="https://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    542 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    543 to <code><abbr>tz</abbr></code> names.
    544 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    545 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    546 <li><a
    547 href="https://www.oracle.com/java/index.html">Oracle
    548 Java</a> contains a copy of a subset of a recent
    549 <code><abbr>tz</abbr></code> database in a
    550 Java-specific format.</li>
    551 <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
    552 Master</a> is a Microsoft Windows clock program that can automatically
    553 download, compile and use <code>tz</code> releases. The Basic version
    554 is free.</li>
    555 <li><a
    556 href="http://veladg.com/velaterra.html">VelaTerra</a> is
    557 a macOS program. Its developers
    558 <a href="http://veladg.com/tzoffer.html">offer free
    559 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
    560 </ul>
    561 <h2 id="other-dbs">Other time zone databases</h2>
    562 <ul>
    563 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    564 is Astrodienst's Web version of Shanks and Pottenger's
    565 time zone history atlases also published in <a
    566 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
    567 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
    568 These atlases are extensive but unreliable, as Shanks appears to have
    569 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    570 sources and do not indicate which entries are guesswork.</li>
    571 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    572 its own <code>tztab</code>(4) format.</li>
    573 <li>Microsoft Windows has proprietary data mentioned
    574 <a href="#System.TimeZoneInfo">above</a>.</li>
    575 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    576 is another time zone database.</li>
    577 <li>The <a
    578 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    579 Schedules Information Manual</a> of the
    580 International Air Transport Association
    581 gives current time zone rules for airports served by commercial aviation.</li>
    582 </ul>
    583 <h2 id="maps">Maps</h2>
    584 <ul>
    585 <li>The <a href="https://www.cia.gov/index.html">United States Central
    586 Intelligence Agency (<abbr
    587 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
    588 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
    589 zone map</a>; the
    590 <a
    591 href="https://www.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    592 Library Map Collection</a>
    593 of the University of Texas at Austin has copies of
    594 recent editions.
    595 The pictorial quality is good,
    596 but the maps do not indicate daylight saving time,
    597 and parts of the data are a few years out of date.</li>
    598 <li><a href="https://www.worldtimezone.com">Current time around the world
    599 and standard time zones map of the world</a>
    600 has several fancy time zone maps; it covers Russia particularly well.
    601 The maps' pictorial quality is not quite as good as the
    602 <abbr>CIA</abbr>'s
    603 but the maps are more up to date.</li>
    604 <li><a
    605 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    606 much is time wrong around the world?</a> maps the difference between
    607 mean solar and standard time, highlighting areas such as western China
    608 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    609 </ul>
    610 <h2 id="boundaries">Time zone boundaries</h2>
    611 <p>Geographical boundaries between timezones are available
    612 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
    613 services and other sources.</p>
    614 <ul>
    615 <li>Databases of time zone boundaries include:
    616 <ul>
    617 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    618 Boundary Builder</a> extracts
    619 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    620 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    621 Its code is freely available under the <abbr>MIT</abbr> license, and
    622 its data entries are freely available under the
    623 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    624 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    625 <li><a href="http://efele.net/maps/tz/"><abbr>TZ</abbr> timezones
    626 maps</a> contains <a
    627 href="https://en.wikipedia.org/wiki/Shapefile">shapefiles</a> of
    628 sets of <code><abbr>tzdb</abbr></code> timezones. This includes
    629 <a href="http://efele.net/maps/tz/world/">tz_world</a>, a shapefile
    630 for all the world's timezones. These maps are no longer maintained and
    631 are superseded by the Timezone Boundary Builder.</li>
    632 <li><a
    633 href="https://github.com/straup/whereonearth-timezone">Whereonearth-timezone</a>
    634 is in <a href="https://tools.ietf.org/html/rfc7946">GeoJSON</a> format
    635 (Internet <abbr>RFC</abbr> 7946), and combines the
    636 the tz_world shapefiles with the
    637 <a href="https://developer.yahoo.com/geo/geoplanet/">GeoPlanet</a>
    638 dataset.</li>
    639 </ul></li>
    640 <li>Programmatic interfaces that map geographical coordinates via tz_world to
    641 <code><abbr>tzdb</abbr></code> timezones include:
    642 <ul>
    643 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    644 written in <a
    645 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    646 and is freely available under the <abbr>MIT</abbr> license.</li>
    647 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    648 is written in Go and is freely available under the Apache License.</li>
    649 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    650 in both Java and
    651 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    652 form, is freely available under the MIT license.</li>
    653 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
    654 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    655 is freely available under the MIT license, and
    656 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    657 is in the public domain.</li>
    658 <li>The <a
    659 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    660 library for Python is freely available under the MIT license.
    661 <li>The <a
    662 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    663 library for Ruby is freely available under the MIT license.</li>
    664 <li><a href="https://derickrethans.nl/what-time-is-it.html">What Time
    665 is It Here?</a> applies MongoDB
    666 geospatial query operators to shapefiles' data.</li>
    667 </ul></li>
    668 <li>Free access via a network API, if you register a key, is provided by
    669 the <a href="http://www.geonames.org/export/web-services.html#timezone">GeoNames Timezone web service</a>,
    670 the <a href="https://developers.google.com/maps/documentation/timezone/intro">Google Maps Time Zone API</a>, and
    671 the <a href="https://timezonedb.com">Time Zone Database &amp; API</a>.
    672 Commercial network API access is provided
    673 by <a href="https://askgeo.com">AskGeo</a>
    674 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    675 </li>
    676 <li>"<a
    677 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    678 to get a time zone from a location using latitude and longitude
    679 coordinates?</a>" discusses other geolocation possibilities.</li>
    680 <li><a href="http://statoids.com/statoids.html">Administrative
    681 Divisions of Countries ("Statoids")</a> lists
    682 political subdivision data related to time zones.</li>
    683 <li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time
    684 zone boundaries for multizone countries</a> summarizes legal
    685 boundaries between time zones within countries.</li>
    686 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
    687 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    688 world time zone boundaries distributed under the
    689 <abbr>GPL</abbr>.</li>
    690 <li>A ship within the <a
    691 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    692 waters</a> of any nation uses that nation's time. In international
    693 waters, time zone boundaries are meridians 15&deg; apart, except that
    694 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    695 wide and are separated by
    696 the 180&deg; meridian (not by the International Date Line, which is
    697 for land and territorial waters only). A captain can change ship's
    698 clocks any time after entering a new time zone; midnight changes are
    699 common.</li>
    700 </ul>
    701 <h2 id="civil">Civil time concepts and history</h2>
    702 <ul>
    703 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    704 Walk through Time</a>
    705 surveys the evolution of timekeeping.</li>
    706 <li><a href="http://www.webexhibits.org/daylightsaving/">About Daylight
    707 Saving Time &ndash; History, rationale, laws &amp; dates</a>
    708 is an overall history of <abbr>DST</abbr>.</li>
    709 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    710 contains guidelines and best practices for software applications that
    711 deal with civil time.</li>
    712 <li><a href="http://seizethedaylight.com/dst/">A Brief
    713 History of Daylight Saving Time</a> summarizes some of the contentious
    714 history of <abbr>DST</abbr>.</li>
    715 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    716 the International Date Line</a> tells the story of the most important
    717 time zone boundary.</li>
    718 <li><a href="http://statoids.com/tconcept.html">Basic Time
    719 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    720 </ul>
    721 <h2 id="national">National histories of legal time</h2>
    722 <dl>
    723 <dt>Australia</dt>
    724 <dd>The Parliamentary Library has commissioned a <a
    725 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    726 paper on daylight saving time in Australia</a>.
    727 The Bureau of Meteorology publishes a list of <a
    728 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    729 Dates of Daylight Savings Time within Australia</a>.</dd>
    730 <dt>Belgium</dt>
    731 <dd>The Royal Observatory of Belgium maintains a table of <a
    732 href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    733 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
    734 <dt>Brazil</dt>
    735 <dd>The Time Service Department of the National Observatory
    736 records <a href="http://pcdsh01.on.br/DecHV.html"
    737 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    738 Portuguese)</a>.</dd>
    739 <dt>Canada</dt>
    740 <dd>National Research Council Canada publishes current
    741 and some older information about <a
    742 href="https://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time
    743 zones &amp; daylight saving time</a>.</dd>
    744 <dt>Chile</dt>
    745 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    746 <a href="http://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
    747 Chile's official time (in Spanish)</a>.</dd>
    748 <dt>China</dt>
    749 <dd>The Hong Kong Observatory maintains a
    750 <a href="https://www.hko.gov.hk/gts/time/Summertime.htm">history of
    751  summer time in Hong Kong</a>,
    752 and Macau's Meteorological and Geophysical Bureau maintains a <a
    753 href="http://www.smg.gov.mo/smg/geophysics/e_t_Summer%20Time.htm">similar
    754 history for Macau</a>.
    755 Unfortunately the latter is incomplete and has errors.</dd>
    756 <dt>Czech Republic</dt>
    757 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    758 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    759 summarizes and cites historical DST regulations.</dd>
    760 <dt>Germany</dt>
    761 <dd>The National Institute for Science and Technology maintains the <a
    762 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    763 of Legal Time in Germany</a>.</dd>
    764 <dt>Israel</dt>
    765 <dd>The Interior Ministry periodically issues <a
    766 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    767 hreflang="he">announcements (in Hebrew)</a>.</dd>
    768 <dt>Italy</dt>
    769 <dd>The National Institute of Metrological Research maintains a
    770 <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml">table of civil time
    771 (in Italian)</a>.</dd>
    772 <dt>Malaysia</dt>
    773 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    774 <dt>Mexico</dt>
    775 <dd>The Investigation and Analysis Service of the Mexican Library of
    776 Congress has published a <a
    777 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    778 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    779 <dt>Netherlands</dt>
    780 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    781 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    782 covers the history of local time in the Netherlands from ancient times.</dd>
    783 <dt>New Zealand</dt>
    784 <dd>The Department of Internal Affairs maintains a brief <a
    785 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    786 Daylight Saving</a>. The privately-maintained <a
    787 href="http://astrologyschool.com/nztime.html">History of New Zealand
    788 time</a> has more details.</dd>
    789 <dt>Singapore</dt>
    790 <dd><a id="Singapore"
    791 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
    792 is Singapore in the "Wrong" Time Zone?</a> details the
    793 history of legal time in Singapore and Malaysia.</dd>
    794 <dt>United Kingdom</dt>
    795 <dd><a
    796 href="https://www.polyomino.org.uk/british-time/">History of
    797 legal time in Britain</a> discusses in detail the country
    798 with perhaps the best-documented history of clock adjustments.
    799 The National Physical Laboratory also maintains an <a
    800 href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates">Archive
    801 of Summer time dates</a>.</dd>
    802 <dt>United States</dt>
    803 <dd>The Department of Transportation's <a
    804 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    805 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    806 <dt>Uruguay</dt>
    807 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    808 Navy (SOHMA) publishes an annual <a
    809 href="http://www.armada.mil.uy/Pagina/institucion/dimat/sohma/almanaque.html" hreflang="es">almanac
    810 (in Spanish)</a>.</dd>
    811 </dl>
    812 <h2 id="precision">Precision timekeeping</h2>
    813 <ul>
    814 <li><a
    815 href="http://leapsecond.com/hpan/an1289.pdf">The
    816 Science of Timekeeping</a> is a thorough introduction
    817 to the theory and practice of precision timekeeping.</li>
    818 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    819 Time 2016</a> contains several freely-readable papers.</li>
    820 <li><a href="http://www.ntp.org"><abbr
    821 title="Network Time Protocol">NTP</abbr>: The Network
    822 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    823 discusses how to synchronize clocks of
    824 Internet hosts.</li>
    825 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
    826 family of software algorithms can achieve accuracy to a few tens of
    827 nanoseconds in scalable server farms without special hardware.</li>
    828 <li>The <a
    829 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
    830 Time Protocol</a> (<abbr
    831 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
    832 can achieve submicrosecond clock accuracy on a local area network
    833 with special-purpose hardware.</li>
    834 <li><a
    835 href="https://tools.ietf.org/html/rfc4833">Timezone
    836 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
    837 (Internet <abbr>RFC</abbr> 4833)
    838 specifies a <a
    839 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
    840 option for a server to configure
    841 a client's time zone and daylight saving settings automatically.</li>
    842 <li><a
    843 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
    844 Times</a> explains more abstruse astronomical time scales like
    845 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
    846 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
    847 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
    848 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
    849 Scales</a> goes into more detail, particularly for historical variants.</li>
    850 <li>The <a href="https://www.iau.org"><abbr
    851 title="International Astronomical Union">IAU</abbr></a>'s <a
    852 href="http://www.iausofa.org"><abbr
    853 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
    854 collection contains C and <a
    855 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
    856 code for converting among time scales like
    857 <abbr title="International Atomic Time">TAI</abbr>,
    858 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
    859 <abbr>UTC</abbr>.</li>
    860 <li><a
    861 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
    862 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
    863 diverse local time
    864 scales used by each landed mission on Mars.</li>
    865 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
    866 dedicated not only to leap seconds but to precise time and frequency
    867 in general. It covers the state of the art in amateur timekeeping, and
    868 how the art has progressed over the past few decades.</li>
    869 <li><a
    870 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
    871 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
    872 Bulletins</a> contains official publications of the International
    873 Earth Rotation and Reference Systems Service, which decides when leap
    874 seconds occur. The <code>tz</code> code and data support leap seconds
    875 via an optional "<code>right</code>" configuration, as opposed to the
    876 default "<code>posix</code>" configuration.</li>
    877 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
    878 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
    879 leap second so that they disagree with <abbr>UTC</abbr> by at most a
    880 half second, even though every <abbr>POSIX</abbr> minute has exactly
    881 sixty seconds. This approach works with the default <code>tz</code>
    882 "<code>posix</code>" configuration, is <a
    883 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
    884 the <abbr>NTP</abbr> reference implementation, and is used by major
    885 cloud service providers.</li>
    886 <li>The <a
    887 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
    888 Second Discussion List</a> covers <a
    889 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
    890 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
    891 discussed further in
    892 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
    893 leap second: its history and possible future</a>.
    894 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
    895 might be redefined
    896 without Leap Seconds</a> gives pointers on this
    897 contentious issue, which was active until 2015 and could become active
    898 again.</li>
    899 </ul>
    900 <h2 id="notation">Time notation</h2>
    901 <ul>
    902 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
    903 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
    904 zone names, abbreviations, identifiers, and formats. For example, it
    905 contains French translations for "Eastern European Summer Time",
    906 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
    907 "Bucharest". Its
    908 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
    909 charts</a> show these values for many locales. Data values are available in
    910 both <abbr title="Locale Data Markup Language">LDML</abbr>
    911 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
    912 <li>
    913 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
    914 the international standard date and time notation</a> is a good
    915 summary of
    916 <a
    917 href="https://www.iso.org/standard/40874.html"><em><abbr>ISO</abbr>
    918 8601:2004 &ndash; Data elements and interchange formats &ndash; Information
    919 interchange &ndash; Representation of dates and times</em></a>.</li>
    920 <li>
    921 <a href="https://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
    922 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
    923 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
    924 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
    925 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
    926 specifies the time notation used in email and <a
    927 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
    928 headers.</li>
    929 <li>
    930 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
    931 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
    932 specifies an <abbr>ISO</abbr> 8601
    933 profile for use in new Internet
    934 protocols.</li>
    935 <li>
    936 <a href="https://www.hackcraft.net/web/datetime/">Date &amp; Time
    937 Formats on the Web</a> surveys web- and Internet-oriented date and time
    938 formats.</li>
    939 <li>Alphabetic time zone abbreviations should not be used as unique
    940 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
    941 practice. For example, in English-speaking North America
    942 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
    943 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
    944 and French-speaking North Americans prefer
    945 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
    946 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
    947 database contains English abbreviations for many timestamps;
    948 unfortunately some of these abbreviations were merely the database maintainers'
    949 inventions, and these have been removed when possible.</li>
    950 <li>Numeric time zone abbreviations typically count hours east of
    951 <abbr>UT</abbr>, e.g., +09 for Japan and
    952 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
    953 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
    954 For example, one might use <code><abbr>TZ</abbr>="<abbr
    955 title="Japan Standard Time">JST</abbr>-9"</code> and
    956 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
    957 for Japan and Hawaii, respectively. If the
    958 <code><abbr>tz</abbr></code> database is available, it is usually better to use
    959 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
    960 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
    961 confusion, handle old timestamps better, and insulate you better from
    962 any future changes to the rules. One should never set
    963 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
    964 <code>"GMT-9"</code>, though, since this would incorrectly imply that
    965 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
    966 is called "<abbr>GMT</abbr>".</li>
    967 </ul>
    968 <h2 id="see-also">See also</h2>
    969 <ul>
    970 <li><a href="theory.html">Theory and pragmatics of the tz code and data</a></li>
    971 <li><a href="tz-art.html">Time and the Arts</a></li>
    972 </ul>
    973 <hr>
    974 <address>
    975 This web page is in the public domain, so clarified as of
    976 2009-05-17 by Arthur David Olson.
    977 <br>
    978 Please send corrections to this web page to the
    979 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
    980 </address>
    981 </body>
    982 </html>
    983