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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="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
    193 Java, <a href="https://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="https://metacpan.org/release/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="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    326 which is freely
    327 available under both the <abbr>GPL</abbr> and the Perl Artistic
    328 License. DateTime::TimeZone also contains a script
    329 <code>tests_from_zdump</code> that generates test cases for each clock
    330 transition in the <code><abbr>tz</abbr></code> database.</li>
    331 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    332 Database Parser</a> is a
    333 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    334 runtime library that is <a
    335 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0355r4.html">moving
    336 forward</a> for inclusion in the next iteration of <a
    337 href="https://isocpp.org/std/the-standard"><em><abbr
    338 title="International Organization for Standardization">ISO</abbr>
    339 International Standard ISO/IEC 14882:2017(E) &ndash; Programming
    340 Language C++</em></a>.
    341 It is freely available under the
    342 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    343 <li><a id="ICU" href="http://site.icu-project.org">International Components for
    344 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    345 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    346 libraries for internationalization that
    347 has a compiler from <code><abbr>tz</abbr></code> source
    348 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    349 (mentioned <a href="#CLDR">below</a>)
    350 into an <abbr>ICU</abbr>-specific format.
    351 <abbr>ICU</abbr> is freely available under a
    352 <abbr>BSD</abbr>-style license.</li>
    353 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    354 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    355 and compiles tz source and exposes <abbr
    356 title="Application Program Interface">API</abbr>s for use. It is
    357 freely available under the <abbr>MIT</abbr> license.</li>
    358 <li>Java-based compilers and libraries include:
    359 <ul>
    360 <li>The <a
    361 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
    362 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    363 Oracle Java.</li>
    364 <li>The <a
    365 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    366 SE <code>java.time</code> <abbr>API</abbr></a> in Java 8 and later
    367 can be supplemented by <a
    368 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    369 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    370 <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    371 and time <abbr>API</abbr></a> contains a class
    372 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    373 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    374 Java 8 <code>java.time</code>, which its users should migrate to once
    375 they can assume Java 8 or later. It is available under the <a
    376 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    377 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    378 time library for Android</a> and
    379 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
    380 time and interval library for Java</a> compile
    381 <code><abbr>tz</abbr></code> source into a binary format.
    382 Time4A is available under the Apache License and Time4J is
    383 available under the <a
    384 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    385 General Public License (<abbr title="Lesser General Public
    386 License">LGPL</abbr>)</a>.</li>
    387 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    388 Java-based libraries.</li>
    389 </ul>
    390 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    391 time <abbr>API</abbr> for .NET</a>
    392 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    393 It is freely available under the Apache License.</li>
    394 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
    395 compilers and libraries include:
    396 <ul>
    397 <li><a
    398 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
    399 compiles time zone data into a compact form designed for
    400 JavaScript. It is freely available under a combination of
    401 the <abbr>MIT</abbr> license and the Apache License.</li>
    402 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    403 plugin for the <a href="https://momentjs.com">Moment.js</a> date
    404 manipulation library. It is freely available under the <abbr>MIT</abbr>
    405 license.</li>
    406 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
    407 <abbr>API</abbr> is upward compatible with standard JavaScript
    408 Dates. It is freely available under the Apache License.</li>
    409 <li><a href="https://github.com/sproutsocial/walltime-js">Walltime-js</a>
    410 translates <abbr>UT</abbr> to local time. It is freely available under
    411 the <abbr>MIT</abbr> license.</li>
    412 </ul>
    413 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    414 compiler from <code><abbr>tz</abbr></code> source into
    415 <a href="https://julialang.org/">Julia</a>. It is freely available
    416 under the <abbr>MIT</abbr> license.</li>
    417 <li><a href="https://github.com/pavkam/tzdb">Delphi/<abbr
    418 title="Free Pascal Compiler">FPC</abbr> Time Zone Database</a>
    419 compiles from <code><abbr>tz</abbr></code> source into
    420 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    421 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    422 and <a
    423 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    424 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    425 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    426 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    427 <a href="https://www.python.org">Python</a>.
    428 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    429 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    430 Ruby Timezone Library</a>
    431 compiles <code><abbr>tz</abbr></code> source into
    432 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    433 It is freely available under the <abbr>MIT</abbr> license.</li>
    434 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
    435 Library</a> is
    436 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    437 library that compiles <code><abbr>tz</abbr></code> source into a time
    438 zone repository whose format
    439 is either proprietary or an <abbr>XML</abbr>-encoded
    440 representation.</li>
    441 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    442 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    443 source into text files, along with a runtime that can read those
    444 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    445 license.</li>
    446 </ul>
    447 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    448 <ul>
    449 <li>The <a
    450 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    451 Library</a>
    452 has an independent, thread-safe implementation of
    453 a <abbr>TZif</abbr> file reader.
    454 This library is freely available under the LGPL
    455 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    456 <li><a href="https://www.gnome.org">GNOME</a>'s
    457 <a href="https://developer.gnome.org/glib/">GLib</a> has
    458 a <abbr>TZif</abbr> file reader written in C that
    459 creates a <code>GTimeZone</code> object representing sets
    460 of <abbr>UT</abbr> offsets.
    461 It is freely available under the <abbr>LGPL</abbr>.</li>
    462 <li>The
    463 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    464 <code>baltzo::TimeZoneUtil</code> component contains a C++
    465 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    466 the Apache License.</li>
    467 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    468 library that translates between <abbr>UT</abbr> and civil time and
    469 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    470 License.</li>
    471 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
    472 is a <abbr>TZif</abbr> file reader written in Java.
    473 It is freely available under the <abbr>LGPL</abbr>.</li>
    474 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    475 library that reads <abbr>TZif</abbr> files and converts
    476 timestamps from one time zone or format to another.
    477 It is used by <a href="https://secure.php.net"><abbr
    478 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    479 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    480 and <a href="https://www.mongodb.com">MongoDB</a>.
    481 It is freely available under the <abbr>MIT</abbr> license.</li>
    482 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    483 JavaScript library that supports date arithmetic that is time zone
    484 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    485 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    486 <abbr>TZif</abbr> file reader.</li>
    487 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    488 DateTime::TimeZone::Tzfile</a>
    489 is a <abbr>TZif</abbr> file reader written in Perl.
    490 It is freely available under the same terms as Perl
    491 (dual <abbr>GPL</abbr> and Artistic license).</li>
    492 <li>The
    493 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    494 library contains a Python tool that
    495 converts <abbr>TZif</abbr> data into
    496 <abbr>JSON</abbr>-format data suitable for use
    497 in its JavaScript library for time zone conversion. Dates before 1970
    498 are not supported.</li>
    499 <li>The <a
    500 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    501 package contains <a href="https://www.haskell.org">Haskell</a> code that
    502 parses and uses <abbr>TZif</abbr> data. It is freely
    503 available under a <abbr>BSD</abbr>-style license.</li>
    504 </ul>
    505 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    506 <ul>
    507 <li><a href="https://foxclocks.org">FoxClocks</a>
    508 is an extension for <a href="https://www.google.com/chrome/">Google
    509 Chrome</a> and for <a
    510 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
    511 Toolkit</a> applications like <a
    512 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    513 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    514 It displays multiple clocks in the application window, and has a mapping
    515 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    516 It is freely available under the <abbr>GPL</abbr>.</li>
    517 <li><a href="https://golang.org">Go programming language</a>
    518 implementations contain a copy of a 32-bit subset of a recent
    519 <code><abbr>tz</abbr></code> database in a
    520 Go-specific format.</li>
    521 <li><a
    522 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
    523 clock (intclock)</a> is a clock that displays multiple time zones on
    524 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
    525 under the <abbr>GPL</abbr>.</li>
    526 <li>Microsoft Windows 8.1
    527 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    528 data (mentioned <a href="#CLDR">below</a>) used by the
    529 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    530 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    531 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    532 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    533 <a id="System.TimeZoneInfo"
    534 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    535 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    536 the older, proprietary method of Microsoft Windows 2000 and later,
    537 which stores time zone data in the
    538 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    539 <a
    540 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    541 Tzid table</a> or <a
    542 href="https://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    543 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    544 to <code><abbr>tz</abbr></code> names.
    545 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    546 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    547 <li><a
    548 href="https://www.oracle.com/java/index.html">Oracle
    549 Java</a> contains a copy of a subset of a recent
    550 <code><abbr>tz</abbr></code> database in a
    551 Java-specific format.</li>
    552 <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
    553 Master</a> is a Microsoft Windows clock program that can automatically
    554 download, compile and use <code><abbr>tz</abbr></code> releases.
    555 The Basic version is free.</li>
    556 <li><a
    557 href="http://veladg.com/velaterra.html">VelaTerra</a> is
    558 a macOS program. Its developers
    559 <a href="http://veladg.com/tzoffer.html">offer free
    560 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
    561 </ul>
    562 <h2 id="other-dbs">Other time zone databases</h2>
    563 <ul>
    564 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    565 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    566 time zone history atlases
    567 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    568 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>,
    569 now published in <a
    570 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
    571 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
    572 Although these extensive atlases
    573 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    574 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    575 they are unreliable as Shanks appears to have
    576 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    577 sources and do not indicate which entries are guesswork.</li>
    578 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    579 its own <code>tztab</code>(4) format.</li>
    580 <li>Microsoft Windows has proprietary data mentioned
    581 <a href="#System.TimeZoneInfo">above</a>.</li>
    582 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    583 is another time zone database.</li>
    584 <li>The <a
    585 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    586 Schedules Information Manual</a> of the
    587 International Air Transport Association
    588 gives current time zone rules for airports served by commercial aviation.</li>
    589 </ul>
    590 <h2 id="maps">Maps</h2>
    591 <ul>
    592 <li>The <a href="https://www.cia.gov/index.html">United States Central
    593 Intelligence Agency (<abbr
    594 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
    595 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
    596 zone map</a>; the
    597 <a
    598 href="https://www.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    599 Library Map Collection</a>
    600 of the University of Texas at Austin has copies of
    601 recent editions.
    602 The pictorial quality is good,
    603 but the maps do not indicate daylight saving time,
    604 and parts of the data are a few years out of date.</li>
    605 <li><a href="https://www.worldtimezone.com">Current time around the world
    606 and standard time zones map of the world</a>
    607 has several fancy time zone maps; it covers Russia particularly well.
    608 The maps' pictorial quality is not quite as good as the
    609 <abbr>CIA</abbr>'s
    610 but the maps are more up to date.</li>
    611 <li><a
    612 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    613 much is time wrong around the world?</a> maps the difference between
    614 mean solar and standard time, highlighting areas such as western China
    615 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    616 </ul>
    617 <h2 id="boundaries">Time zone boundaries</h2>
    618 <p>Geographical boundaries between timezones are available
    619 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
    620 services and other sources.</p>
    621 <ul>
    622 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    623 Boundary Builder</a> extracts
    624 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    625 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    626 Its code is freely available under the <abbr>MIT</abbr> license, and
    627 its data entries are freely available under the
    628 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    629 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    630 <li>Programmatic interfaces that map geographical coordinates via tz_world to
    631 <code><abbr>tzdb</abbr></code> timezones include:
    632 <ul>
    633 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    634 written in <a
    635 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    636 and is freely available under the <abbr>MIT</abbr> license.</li>
    637 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    638 is written in Go and is freely available under the Apache License.</li>
    639 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    640 in both Java and
    641 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    642 form, is freely available under the MIT license.</li>
    643 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
    644 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    645 is freely available under the MIT license, and
    646 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    647 is in the public domain.</li>
    648 <li>The <a
    649 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    650 library for Python is freely available under the MIT license.
    651 <li>The <a
    652 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    653 library for Ruby is freely available under the MIT license.</li>
    654 </ul></li>
    655 <li>Free access via a network API, if you register a key, is provided by
    656 the <a
    657 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    658 Timezone web service</a>, the <a
    659 href="https://developers.google.com/maps/documentation/timezone/intro">Google
    660 Maps Time Zone API</a>, and
    661 the <a href="https://timezonedb.com">Time Zone Database &amp; API</a>.
    662 Commercial network API access is provided
    663 by <a href="https://askgeo.com">AskGeo</a>
    664 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    665 </li>
    666 <li>"<a
    667 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    668 to get a time zone from a location using latitude and longitude
    669 coordinates?</a>" discusses other geolocation possibilities.</li>
    670 <li><a href="http://statoids.com/statoids.html">Administrative
    671 Divisions of Countries ("Statoids")</a> lists
    672 political subdivision data related to time zones.</li>
    673 <li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time
    674 zone boundaries for multizone countries</a> summarizes legal
    675 boundaries between time zones within countries.</li>
    676 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
    677 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    678 world time zone boundaries distributed under the
    679 <abbr>GPL</abbr>.</li>
    680 <li>A ship within the <a
    681 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    682 waters</a> of any nation uses that nation's time. In international
    683 waters, time zone boundaries are meridians 15&deg; apart, except that
    684 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    685 wide and are separated by
    686 the 180&deg; meridian (not by the International Date Line, which is
    687 for land and territorial waters only). A captain can change ship's
    688 clocks any time after entering a new time zone; midnight changes are
    689 common.</li>
    690 </ul>
    691 <h2 id="civil">Civil time concepts and history</h2>
    692 <ul>
    693 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    694 Walk through Time</a>
    695 surveys the evolution of timekeeping.</li>
    696 <li>The history of daylight saving time is surveyed in <a
    697 href="http://www.webexhibits.org/daylightsaving/">About Daylight
    698 Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    699 <a href="http://seizethedaylight.com/dst/">A Brief
    700 History of Daylight Saving Time</a>.</li>
    701 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    702 Lords</a> discusses how authoritarians manipulate civil time.</li>
    703 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    704 contains guidelines and best practices for software applications that
    705 deal with civil time.</li>
    706 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    707 the International Date Line</a> tells the story of the most important
    708 time zone boundary.</li>
    709 <li><a href="http://statoids.com/tconcept.html">Basic Time
    710 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    711 </ul>
    712 <h2 id="national">National histories of legal time</h2>
    713 <dl>
    714 <dt>Australia</dt>
    715 <dd>The Parliamentary Library has commissioned a <a
    716 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    717 paper on daylight saving time in Australia</a>.
    718 The Bureau of Meteorology publishes a list of <a
    719 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    720 Dates of Daylight Savings Time within Australia</a>.</dd>
    721 <dt>Belgium</dt>
    722 <dd>The Royal Observatory of Belgium maintains a table of <a
    723 href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    724 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
    725 <dt>Brazil</dt>
    726 <dd>The Time Service Department of the National Observatory
    727 records <a href="http://pcdsh01.on.br/DecHV.html"
    728 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    729 Portuguese)</a>.</dd>
    730 <dt>Canada</dt>
    731 <dd>National Research Council Canada publishes current
    732 and some older information about <a
    733 href="https://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time
    734 zones &amp; daylight saving time</a>.</dd>
    735 <dt>Chile</dt>
    736 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    737 <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
    738 Chile's official time (in Spanish)</a>.</dd>
    739 <dt>China</dt>
    740 <dd>The Hong Kong Observatory maintains a
    741 <a href="https://www.hko.gov.hk/gts/time/Summertime.htm">history of
    742  summer time in Hong Kong</a>,
    743 and Macau's Meteorological and Geophysical Bureau maintains a <a
    744 href="http://www.smg.gov.mo/smg/geophysics/e_t_Summer%20Time.htm">similar
    745 history for Macau</a>.
    746 Unfortunately the latter is incomplete and has errors.</dd>
    747 <dt>Czech Republic</dt>
    748 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    749 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    750 summarizes and cites historical DST regulations.</dd>
    751 <dt>Germany</dt>
    752 <dd>The National Institute for Science and Technology maintains the <a
    753 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    754 of Legal Time in Germany</a>.</dd>
    755 <dt>Israel</dt>
    756 <dd>The Interior Ministry periodically issues <a
    757 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    758 hreflang="he">announcements (in Hebrew)</a>.</dd>
    759 <dt>Italy</dt>
    760 <dd>The National Institute of Metrological Research maintains a
    761 <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
    762 of civil time (in Italian)</a>.</dd>
    763 <dt>Malaysia</dt>
    764 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    765 <dt>Mexico</dt>
    766 <dd>The Investigation and Analysis Service of the Mexican Library of
    767 Congress has published a <a
    768 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    769 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    770 <dt>Netherlands</dt>
    771 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    772 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    773 covers the history of local time in the Netherlands from ancient times.</dd>
    774 <dt>New Zealand</dt>
    775 <dd>The Department of Internal Affairs maintains a brief <a
    776 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    777 Daylight Saving</a>. The privately-maintained <a
    778 href="http://astrologyschool.com/nztime.html">History of New Zealand
    779 time</a> has more details.</dd>
    780 <dt>Singapore</dt>
    781 <dd><a id="Singapore"
    782 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
    783 is Singapore in the "Wrong" Time Zone?</a> details the
    784 history of legal time in Singapore and Malaysia.</dd>
    785 <dt>United Kingdom</dt>
    786 <dd><a
    787 href="https://www.polyomino.org.uk/british-time/">History of
    788 legal time in Britain</a> discusses in detail the country
    789 with perhaps the best-documented history of clock adjustments.
    790 The National Physical Laboratory also maintains an <a
    791 href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates">Archive
    792 of Summer time dates</a>.</dd>
    793 <dt>United States</dt>
    794 <dd>The Department of Transportation's <a
    795 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    796 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    797 <dt>Uruguay</dt>
    798 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    799 Navy (SOHMA) publishes an annual <a
    800 href="http://www.armada.mil.uy/Pagina/institucion/dimat/sohma/almanaque.html" hreflang="es">almanac
    801 (in Spanish)</a>.</dd>
    802 </dl>
    803 <h2 id="precision">Precision timekeeping</h2>
    804 <ul>
    805 <li><a
    806 href="http://leapsecond.com/hpan/an1289.pdf">The
    807 Science of Timekeeping</a> is a thorough introduction
    808 to the theory and practice of precision timekeeping.</li>
    809 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    810 Time 2016</a> contains several freely-readable papers.</li>
    811 <li><a href="http://www.ntp.org"><abbr
    812 title="Network Time Protocol">NTP</abbr>: The Network
    813 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    814 discusses how to synchronize clocks of
    815 Internet hosts.</li>
    816 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
    817 family of software algorithms can achieve accuracy to a few tens of
    818 nanoseconds in scalable server farms without special hardware.</li>
    819 <li>The <a
    820 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
    821 Time Protocol</a> (<abbr
    822 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
    823 can achieve submicrosecond clock accuracy on a local area network
    824 with special-purpose hardware.</li>
    825 <li><a
    826 href="https://tools.ietf.org/html/rfc4833">Timezone
    827 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
    828 (Internet <abbr>RFC</abbr> 4833)
    829 specifies a <a
    830 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
    831 option for a server to configure
    832 a client's time zone and daylight saving settings automatically.</li>
    833 <li><a
    834 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
    835 Times</a> explains more abstruse astronomical time scales like
    836 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
    837 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
    838 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
    839 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
    840 Scales</a> goes into more detail, particularly for historical variants.</li>
    841 <li>The <a href="https://www.iau.org"><abbr
    842 title="International Astronomical Union">IAU</abbr></a>'s <a
    843 href="http://www.iausofa.org"><abbr
    844 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
    845 collection contains C and <a
    846 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
    847 code for converting among time scales like
    848 <abbr title="International Atomic Time">TAI</abbr>,
    849 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
    850 <abbr>UTC</abbr>.</li>
    851 <li><a
    852 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
    853 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
    854 diverse local time
    855 scales used by each landed mission on Mars.</li>
    856 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
    857 dedicated not only to leap seconds but to precise time and frequency
    858 in general. It covers the state of the art in amateur timekeeping, and
    859 how the art has progressed over the past few decades.</li>
    860 <li><a
    861 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
    862 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
    863 Bulletins</a> contains official publications of the International
    864 Earth Rotation and Reference Systems Service, which decides when leap
    865 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
    866 via an optional "<code>right</code>" configuration, as opposed to the
    867 default "<code>posix</code>" configuration.</li>
    868 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
    869 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
    870 leap second so that they disagree with <abbr>UTC</abbr> by at most a
    871 half second, even though every <abbr>POSIX</abbr> minute has exactly
    872 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
    873 "<code>posix</code>" configuration, is <a
    874 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
    875 the <abbr>NTP</abbr> reference implementation, and is used by major
    876 cloud service providers.</li>
    877 <li>The <a
    878 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
    879 Second Discussion List</a> covers <a
    880 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
    881 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
    882 discussed further in
    883 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
    884 leap second: its history and possible future</a>.
    885 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
    886 might be redefined
    887 without Leap Seconds</a> gives pointers on this
    888 contentious issue, which was active until 2015 and could become active
    889 again.</li>
    890 </ul>
    891 <h2 id="notation">Time notation</h2>
    892 <ul>
    893 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
    894 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
    895 zone names, abbreviations, identifiers, and formats. For example, it
    896 contains French translations for "Eastern European Summer Time",
    897 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
    898 "Bucharest". Its
    899 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
    900 charts</a> show these values for many locales. Data values are available in
    901 both <abbr title="Locale Data Markup Language">LDML</abbr>
    902 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
    903 <li>
    904 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
    905 the international standard date and time notation</a> is a good
    906 summary of
    907 <a
    908 href="https://www.iso.org/standard/40874.html"><em><abbr>ISO</abbr>
    909 8601:2004 &ndash; Data elements and interchange formats &ndash; Information
    910 interchange &ndash; Representation of dates and times</em></a>.</li>
    911 <li>
    912 <a href="https://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
    913 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
    914 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
    915 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
    916 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
    917 specifies the time notation used in email and <a
    918 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
    919 headers.</li>
    920 <li>
    921 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
    922 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
    923 specifies an <abbr>ISO</abbr> 8601
    924 profile for use in new Internet
    925 protocols.</li>
    926 <li>
    927 <a href="https://www.hackcraft.net/web/datetime/">Date &amp; Time
    928 Formats on the Web</a> surveys web- and Internet-oriented date and time
    929 formats.</li>
    930 <li>Alphabetic time zone abbreviations should not be used as unique
    931 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
    932 practice. For example, in English-speaking North America
    933 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
    934 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
    935 and French-speaking North Americans prefer
    936 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
    937 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
    938 database contains English abbreviations for many timestamps;
    939 unfortunately some of these abbreviations were merely the database maintainers'
    940 inventions, and these have been removed when possible.</li>
    941 <li>Numeric time zone abbreviations typically count hours east of
    942 <abbr>UT</abbr>, e.g., +09 for Japan and
    943 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
    944 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
    945 For example, one might use <code><abbr>TZ</abbr>="<abbr
    946 title="Japan Standard Time">JST</abbr>-9"</code> and
    947 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
    948 for Japan and Hawaii, respectively. If the
    949 <code><abbr>tz</abbr></code> database is available, it is usually better to use
    950 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
    951 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
    952 confusion, handle old timestamps better, and insulate you better from
    953 any future changes to the rules. One should never set
    954 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
    955 <code>"GMT-9"</code>, though, since this would incorrectly imply that
    956 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
    957 is called "<abbr>GMT</abbr>".</li>
    958 </ul>
    959 <h2 id="see-also">See also</h2>
    960 <ul>
    961 <li><a href="theory.html">Theory and pragmatics of the tz code and data</a></li>
    962 <li><a href="tz-art.html">Time and the Arts</a></li>
    963 </ul>
    964 <hr>
    965 <address>
    966 This web page is in the public domain, so clarified as of
    967 2009-05-17 by Arthur David Olson.
    968 <br>
    969 Please send corrections to this web page to the
    970 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
    971 </address>
    972 </body>
    973 </html>
    974