Home | History | Annotate | Line # | Download | only in time
tz-link.html revision 1.1
      1 <!DOCTYPE html>
      2 <html lang="en">
      3 <head>
      4 <title>Sources for time zone and daylight saving time data</title>
      5 <meta charset="UTF-8">
      6 </head>
      7 <body>
      8 <h1>Sources for time zone and daylight saving time data</h1>
      9 <p>
     10 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
     11 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
     12 rules are controlled by individual
     13 governments. They are sometimes changed with little notice, and their
     14 histories and planned futures are often recorded only fitfully. Here
     15 is a summary of attempts to organize and record relevant data in this
     16 area.
     17 </p>
     18 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     19 <p>
     20 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     21 time zone database contains code and data
     22 that represent the history of local time
     23 for many representative locations around the globe.
     24 It is updated periodically to reflect changes made by political bodies
     25 to time zone boundaries and daylight saving rules.
     26 This database (known as <code><abbr>tz</abbr></code>,
     27 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     28 is used by several implementations,
     29 including
     30 <a href="https://www.gnu.org/software/libc/">the
     31 <abbr title="GNU's Not Unix">GNU</abbr>
     32 C Library</a> (used in
     33 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     34 <a href="https://www.android.com">Android</a>,
     35 <a href="https://developer.mozilla.org/en-US/docs/Mozilla/B2G_OS">B2G
     36 <abbr title="Operating System">OS</abbr></a>,
     37 <a href="https://www.freebsd.org">Free<abbr
     38 title="Berkeley Software Distribution">BSD</abbr></a>,
     39 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     40 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     41 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
     42 <a href="https://cygwin.com">Cygwin</a>,
     43 <a href="http://www.delorie.com/djgpp/"><abbr
     44 title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
     45 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     46 <a href="https://www.mysql.com">MySQL</a>,
     47 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     48 title="Web Operating System">webOS</abbr></a>,
     49 <a href="https://ibm.com/aix"><abbr
     50 title="Advanced Interactive eXecutive">AIX</abbr></a>,
     51 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
     52 <a href="https://www.apple.com/ios/"><abbr
     53 title="iPhone OS">iOS</abbr></a>,
     54 <a href="https://www.apple.com/macos/">macOS</a>,
     55 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     56 <a href="https://www.hpe.com/info/openvms">Open<abbr
     57 title="Virtual Memory System">VMS</abbr></a>,
     58 <a href="https://www.oracle.com/database/index.html">Oracle Database</a>, and
     59 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
     60 <p>
     61 Each location in the database represents a region where all
     62 clocks keeping local time have agreed since 1970.
     63 Locations are identified by continent or ocean and then by the name of
     64 the location, which is typically the largest city within the region.
     65 For example, <code>America/New_York</code>
     66 represents most of the <abbr title="United States">US</abbr> eastern time zone;
     67 <code>America/Phoenix</code> represents most of Arizona, which
     68 uses mountain time without daylight saving time (<abbr
     69 title="daylight saving time">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 region 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 time stamps 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 style="margin-left: 2em"><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 style="margin-left: 2em"><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
    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 style="margin-left: 2em"><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    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. It also lets
    147 you read a <code><abbr>tz</abbr></code> binary file and interpret time stamps for that
    148 location.</p>
    149 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    150 <p>
    151 The <code><abbr>tz</abbr></code> code and data
    152 are by no means authoritative. If you find errors, please
    153 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    154 the time zone mailing list. You can also <a
    155 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    156 and browse the <a
    157 href="https://mm.icann.org/pipermail/tz/">archive of old
    158 messages</a>.</p>
    159 <p>
    160 If your government plans to change its time zone boundaries or
    161 daylight saving rules, inform <code>tz (a] iana.org</code> well in
    162 advance, as this will coordinate updates to many cell phones,
    163 computers, and other devices around the world. With
    164 less than a year's notice there is a good chance that some
    165 computer-based clocks will operate incorrectly after the change, due
    166 to delays in propagating updates to software and data. The shorter
    167 the notice, the more likely clock problems will arise; see "<a
    168 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    169 the Timing of Time Zone Changes</a>" for examples.
    170 </p>
    171 <p>
    172 Changes to the <code><abbr>tz</abbr></code> code and data are often
    173 propagated to clients via operating system updates, so
    174 client <code><abbr>tz</abbr></code> data can often be corrected by
    175 applying these updates. With GNU/Linux and similar systems, if your
    176 maintenance provider has not yet adopted the
    177 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    178 the process by tailoring the generic instructions in
    179 the <code><abbr>tz</abbr> README</code> file and installing the latest
    180 data yourself. System-specific instructions for installing the
    181 latest <code><abbr>tz</abbr></code> data have also been published
    182 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
    183 <a
    184 href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
    185 <a
    186 href="http://userguide.icu-project.org/datetime/timezone"><abbr
    187 title="International Components for Unicode">ICU</abbr></a>,
    188 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
    189 and <a
    190 href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
    191 Java, <a href="http://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    192 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    193 and <a
    194 href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
    195 </p>
    196 <p>Sources for the <code><abbr>tz</abbr></code> database are
    197 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    198 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    199 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    200 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    201 title="linefeed">LF</abbr></a>,
    202 which can be modified by common text editors such
    203 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    204 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
    205 <a href="https://www.vim.org">vim</a>.
    206 Specialized source-file editing can be done via the
    207 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    208 zoneinfo</a> package for <a
    209 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    210 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    211 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    212 Studio Code</a>.
    213 </p>
    214 <p>
    215 For further information about updates, please see
    216 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
    217 Maintaining the Time Zone Database</a> (Internet <abbr
    218 title="Request For Comments">RFC</abbr> 6557). More detail can be
    219 found in <a href="theory.html">Theory and pragmatics of the tz code and data</a>.
    220 </p>
    221 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    222 <ul>
    223 <li>The article
    224 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    225 an encyclopedic summary.</li>
    226 <li><a href="tz-how-to.html">How to Read the
    227 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    228 database format.</li>
    229 <li><a
    230 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    231 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    232 database's style.</li>
    233 </ul>
    234 <h2 id="web">Web sites using recent versions of the
    235 <code><abbr>tz</abbr></code> database</h2>
    236 <p>
    237 These are listed roughly in ascending order of complexity and fanciness.
    238 </p>
    239 <ul>
    240 <li><a href="https://time.is">Time.is</a> shows locations'
    241 time and zones.</li>
    242 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    243 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    244 <a href="http://worldclock.com">The World Clock</a>
    245 are time zone converters.</li>
    246 <li><a
    247 href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    248 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    249 <li><a
    250 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    251 Time in 1000 Places</a> uses descriptions of the values.</li>
    252 <li><a href="http://www.timezoneconverter.com/cgi-bin/tzc.tzc">Time Zone
    253 Converter</a>
    254 uses a pulldown menu.</li>
    255 <li><a href="http://home.kpn.nl/vanadovv/time/TZworld.html">Complete
    256 timezone information for all countries</a> displays tables of DST rules.
    257 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    258 Worldwide</a> lets you sort zone names and convert times.</li>
    259 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    260 time map and a time converter.</li>
    261 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    262 calculates the current time difference between locations.</li>
    263 <li><a href="http://www.wx-now.com">Weather Now</a> and
    264 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    265 </ul>
    266 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    267 <ul>
    268 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    269 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    270 Distribution Service (tzdist) working group</a> defined <a
    271 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
    272 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    273 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
    274 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    275 transferring time zone data by reference. The draft <a
    276 id="TZDIST-Geolocate"
    277 href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate-01">TZDIST
    278 Geolocate Extension</a> lets a client determine its time zone region
    279 from its geographic location using a <a
    280 href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li>
    281 <li>The <a href="https://tools.ietf.org/html/rfc5545">
    282 Internet Calendaring and Scheduling Core Object Specification
    283 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    284 covers time zone
    285 data; see its VTIMEZONE calendar component.
    286 The iCalendar format requires specialized parsers and generators; a
    287 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
    288 (Internet <abbr>RFC</abbr> 6321) uses
    289 <a href="https://www.w3.org/XML/"><abbr
    290 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    291 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
    292 (Internet <abbr>RFC</abbr> 7265)
    293 uses <a href="https://www.json.org"><abbr
    294 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    295 </ul>
    296 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    297 <ul>
    298 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    299 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
    300 program that compiles
    301 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    302 Vzic is freely
    303 available under the <a
    304 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    305 General Public License (<abbr
    306 title="General Public License">GPL</abbr>)</a>.</li>
    307 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    308 database conversion utility</a> is like Vzic, except for the <a
    309 href="https://www.microsoft.com/net">.NET framework</a>
    310 and with a <abbr>BSD</abbr>-style license.</li>
    311 <li><a
    312 href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
    313 contains a script <code>parse_olson</code> that compiles
    314 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    315 modules. It is part of the Perl <a
    316 href="http://datetime.perl.org">DateTime Project</a>, which is freely
    317 available under both the <abbr>GPL</abbr> and the Perl Artistic
    318 License. DateTime::TimeZone also contains a script
    319 <code>tests_from_zdump</code> that generates test cases for each clock
    320 transition in the <code><abbr>tz</abbr></code> database.</li>
    321 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    322 Database Parser</a> is a
    323 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    324 runtime library that is <a
    325 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0355r4.html">moving
    326 forward</a> for inclusion in the next iteration of <a
    327 href="https://isocpp.org/std/the-standard"><em><abbr
    328 title="International Organization for Standardization">ISO</abbr>
    329 International Standard ISO/IEC 14882:2017(E) &ndash; Programming
    330 Language C++</em></a>.
    331 It is freely available under the
    332 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    333 <li><a id="ICU" href="http://site.icu-project.org">International Components for
    334 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    335 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    336 libraries for internationalization that
    337 has a compiler from <code><abbr>tz</abbr></code> source
    338 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    339 (mentioned <a href="#CLDR">below</a>)
    340 into an <abbr>ICU</abbr>-specific format.
    341 <abbr>ICU</abbr> is freely available under a
    342 <abbr>BSD</abbr>-style license.</li>
    343 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    344 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    345 and compiles tz source and exposes <abbr
    346 title="Application Program Interface">API</abbr>s for use. It is
    347 freely available under the <abbr>MIT</abbr> license.</li>
    348 <li>Java-based compilers and libraries include:
    349 <ul>
    350 <li>The <a
    351 href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
    352 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    353 Oracle Java.</li>
    354 <li>The <a
    355 href="http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    356 8 <code>java.time</code> <abbr>API</abbr></a> can be supplemented by <a
    357 href="http://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    358 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    359 <li><a href="http://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    360 and time <abbr>API</abbr></a> contains a class
    361 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    362 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    363 Java 8 <code>java.time</code>, which its users should migrate to once
    364 they can assume Java 8 or later. It is available under the <a
    365 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    366 <li><a href="https://github.com/MenoData/Time4J/">Time4J &ndash;
    367 Advanced date, time and interval library for Java</a> contains a class
    368 <code>net.time4j.tool.TimezoneRepositoryCompiler</code> that compiles
    369 <code><abbr>tz</abbr></code> source into a binary format. Time4J is
    370 available under the <a
    371 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    372 General Public License (<abbr title="Lesser General Public
    373 License">LGPL</abbr>)</a>.</li>
    374 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    375 Java-based libraries.</li>
    376 </ul>
    377 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    378 time <abbr>API</abbr> for .NET</a>
    379 and <a href="http://www.babiej.demon.nl/Tz4Net/main.htm">TZ4Net</a>
    380 are similar to Joda-Time and Time4J, but for the .NET framework instead of
    381 Java. They are freely available under the
    382 Apache License
    383 and a <abbr>BSD</abbr>-style license, respectively.</li>
    384 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
    385 compilers and libraries include:
    386 <ul>
    387 <li><a
    388 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
    389 compiles time zone data into a compact form designed for
    390 JavaScript. It is freely available under a combination of
    391 the <abbr>MIT</abbr> license and the Apache License.</li>
    392 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    393 plugin for the <a href="https://momentjs.com">Moment.js</a> date
    394 manipulation library. It is freely available under the <abbr>MIT</abbr>
    395 license.</li>
    396 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
    397 <abbr>API</abbr> is upward compatible with standard JavaScript
    398 Dates. It is freely available under the Apache License.</li>
    399 <li><a href="https://github.com/sproutsocial/walltime-js">Walltime-js</a>
    400 translates <abbr>UT</abbr> to local time. It is freely available under
    401 the <abbr>MIT</abbr> license.</li>
    402 </ul>
    403 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    404 compiler from <code><abbr>tz</abbr></code> source into
    405 <a href="https://julialang.org/">Julia</a>. It is freely available
    406 under the <abbr>MIT</abbr> license.</li>
    407 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    408 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    409 <a href="https://www.python.org">Python</a>.
    410 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    411 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    412 Ruby Timezone Library</a>
    413 compiles <code><abbr>tz</abbr></code> source into
    414 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    415 It is freely available under the <abbr>MIT</abbr> license.</li>
    416 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
    417 Library</a> is
    418 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    419 library that compiles <code><abbr>tz</abbr></code> source into a time
    420 zone repository whose format
    421 is either proprietary or an <abbr>XML</abbr>-encoded
    422 representation.</li>
    423 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    424 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    425 source into text files, along with a runtime that can read those
    426 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    427 license.</li>
    428 </ul>
    429 <h2 id="binary">Other <code><abbr>tz</abbr></code> binary file readers</h2>
    430 <ul>
    431 <li>The <a
    432 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    433 Library</a>
    434 has an independent, thread-safe implementation of
    435 a <code><abbr>tz</abbr></code> binary file reader.
    436 This library is freely available under the LGPL
    437 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    438 <li><a href="https://www.gnome.org">GNOME</a>'s
    439 <a href="https://developer.gnome.org/glib/">GLib</a> has
    440 a <code><abbr>tz</abbr></code> binary file reader written in C that
    441 creates a <code>GTimeZone</code> object representing sets
    442 of <abbr>UT</abbr> offsets.
    443 It is freely available under the <abbr>LGPL</abbr>.</li>
    444 <li>The
    445 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    446 <code>baltzo::TimeZoneUtil</code> component contains a C++
    447 implementation of a binary file reader. It is freely available under
    448 the Apache License.</li>
    449 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    450 library that translates between <abbr>UT</abbr> and civil time and
    451 can read binary files. It is freely available under the Apache
    452 License.</li>
    453 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
    454 is a <code><abbr>tz</abbr></code> binary file reader written in Java.
    455 It is freely available under the <abbr>LGPL</abbr>.</li>
    456 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    457 library that reads tz binary files and converts
    458 time stamps from one time zone or format to another.
    459 It is used by <a href="https://secure.php.net"><abbr
    460 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    461 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    462 and <a href="https://www.mongodb.com">MongoDB</a>.
    463 It is freely available under the <abbr>MIT</abbr> license.</li>
    464 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    465 JavaScript library that supports date arithmetic that is time zone
    466 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    467 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    468 <code><abbr>tz</abbr></code> binary file reader.</li>
    469 <li><a href="http://search.cpan.org/perldoc?DateTime::TimeZone::Tzfile">
    470 DateTime::TimeZone::Tzfile</a>
    471 is a <code><abbr>tz</abbr></code> binary file reader written in Perl.
    472 It is freely available under the same terms as Perl
    473 (dual <abbr>GPL</abbr> and Artistic license).</li>
    474 <li>The
    475 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    476 library contains a Python tool that
    477 converts <code><abbr>tz</abbr></code> binary data into
    478 <abbr>JSON</abbr>-format data suitable for use
    479 in its JavaScript library for time zone conversion. Dates before 1970
    480 are not supported.</li>
    481 <li>The <a
    482 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    483 package contains <a href="https://www.haskell.org">Haskell</a> code that
    484 parses and uses <code><abbr>tz</abbr></code> binary data. It is freely
    485 available under a <abbr>BSD</abbr>-style license.</li>
    486 </ul>
    487 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    488 <ul>
    489 <li><a href="https://foxclocks.org">FoxClocks</a>
    490 is an extension for <a href="https://www.google.com/chrome/">Google
    491 Chrome</a> and for <a
    492 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
    493 Toolkit</a> applications like <a
    494 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    495 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    496 It displays multiple clocks in the application window, and has a mapping
    497 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    498 It is freely available under the <abbr>GPL</abbr>.</li>
    499 <li><a href="https://golang.org">Go programming language</a>
    500 implementations contain a copy of a 32-bit subset of a recent
    501 <code><abbr>tz</abbr></code> database in a
    502 Go-specific format.</li>
    503 <li><a
    504 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
    505 clock (intclock)</a> is a clock that displays multiple time zones on
    506 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
    507 under the <abbr>GPL</abbr>.</li>
    508 <li>Microsoft Windows 8.1
    509 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    510 data (mentioned <a href="#CLDR">below</a>) used by
    511 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a>
    512 classes such as <a
    513 href="https://msdn.microsoft.com/en-us/library/windows/apps/windows.globalization.datetimeformatting.datetimeformatter.aspx"><code>DateTimeFormatter</code></a>.
    514 <a id="System.TimeZoneInfo"
    515 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    516 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    517 the older, proprietary method of Microsoft Windows 2000 and later,
    518 which stores time zone data in the
    519 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    520 <a
    521 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    522 Tzid table</a> or <a
    523 href="https://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    524 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    525 to <code><abbr>tz</abbr></code> names.
    526 <li><a
    527 href="https://www.oracle.com/java/index.html">Oracle
    528 Java</a> contains a copy of a subset of a recent
    529 <code><abbr>tz</abbr></code> database in a
    530 Java-specific format.</li>
    531 <li><a href="https://www.relativedata.com/time-zone-master">Time Zone
    532 Master</a> is a Microsoft Windows clock program that can automatically
    533 download, compile and use <code>tz</code> releases. The Basic version
    534 is free.</li>
    535 <li><a
    536 href="http://veladg.com/velaterra.html">VelaTerra</a> is
    537 a macOS program. Its developers
    538 <a href="http://veladg.com/tzoffer.html">offer free
    539 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
    540 </ul>
    541 <h2 id="other-dbs">Other time zone databases</h2>
    542 <ul>
    543 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    544 is Astrodienst's Web version of Shanks and Pottenger's
    545 time zone history atlases also published in <a
    546 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
    547 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
    548 These atlases are extensive but unreliable, as Shanks appears to have
    549 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    550 sources and do not indicate which entries are guesswork.</li>
    551 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    552 its own <code>tztab</code>(4) format.</li>
    553 <li>Microsoft Windows has proprietary data mentioned
    554 <a href="#System.TimeZoneInfo">above</a>.</li>
    555 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    556 is another time zone database.</li>
    557 <li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a>
    558 contains data from the Time Service Department of the
    559 <abbr>US</abbr> Naval Observatory.</li>
    560 <li>The <a
    561 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    562 Schedules Information Manual</a> of the
    563 International Air Transport Association
    564 gives current time zone rules for airports served by commercial aviation.</li>
    565 </ul>
    566 <h2 id="maps">Maps</h2>
    567 <ul>
    568 <li>The <a href="https://www.cia.gov/index.html">United States Central
    569 Intelligence Agency (<abbr
    570 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
    571 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
    572 zone map</a>; the
    573 <a
    574 href="https://www.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    575 Library Map Collection</a>
    576 of the University of Texas at Austin has copies of
    577 recent editions.
    578 The pictorial quality is good,
    579 but the maps do not indicate daylight saving time,
    580 and parts of the data are a few years out of date.</li>
    581 <li><a href="https://www.worldtimezone.com">Current time around the world
    582 and standard time zones map of the world</a>
    583 has several fancy time zone maps; it covers Russia particularly well.
    584 The maps' pictorial quality is not quite as good as the
    585 <abbr>CIA</abbr>'s
    586 but the maps are more up to date.</li>
    587 <li><a
    588 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    589 much is time wrong around the world?</a> maps the difference between
    590 mean solar and standard time, highlighting areas such as western China
    591 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    592 </ul>
    593 <h2 id="boundaries">Time zone boundaries</h2>
    594 <p>Geographical boundaries between time zone regions are available
    595 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
    596 services and other sources.</p>
    597 <ul>
    598 <li>Databases of time zone boundaries include:
    599 <ul>
    600 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    601 Boundary Builder</a> extracts
    602 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    603 boundaries of <code><abbr>tz</abbr></code> regions.
    604 Its code is freely available under the <abbr>MIT</abbr> license, and
    605 its data entries are freely available under the
    606 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    607 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    608 <li><a href="http://efele.net/maps/tz/"><abbr>TZ</abbr> timezones
    609 maps</a> contains <a
    610 href="https://en.wikipedia.org/wiki/Shapefile">shapefiles</a> of
    611 sets of <code><abbr>tz</abbr></code> regions. This includes
    612 <a href="http://efele.net/maps/tz/world/">tz_world</a>, a shapefile
    613 for all the world's regions. These maps are no longer maintained and
    614 are superseded by the Timezone Boundary Builder.</li>
    615 <li><a
    616 href="https://github.com/straup/whereonearth-timezone">Whereonearth-timezone</a>
    617 is in <a href="https://tools.ietf.org/html/rfc7946">GeoJSON</a> format
    618 (Internet <abbr>RFC</abbr> 7946), and combines the
    619 the tz_world shapefiles with the
    620 <a href="https://developer.yahoo.com/geo/geoplanet/">GeoPlanet</a>
    621 dataset.</li>
    622 </ul></li>
    623 <li>Programmatic interfaces that map geographical coordinates via tz_world to
    624 <code><abbr>tz</abbr></code> regions include:
    625 <ul>
    626 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    627 written in <a
    628 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    629 and is freely available under the <abbr>MIT</abbr> license.</li>
    630 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    631 is written in Go and is freely available under the Apache License.</li>
    632 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    633 in both Java and
    634 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    635 form, is freely available under the MIT license.</li>
    636 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
    637 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    638 is freely available under the MIT license, and
    639 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    640 is in the public domain.</li>
    641 <li>The <a
    642 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    643 library for Python is freely available under the MIT license.
    644 <li>The <a
    645 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    646 library for Ruby is freely available under the MIT license.</li>
    647 <li><a href="https://derickrethans.nl/what-time-is-it.html">What Time
    648 is It Here?</a> applies MongoDB
    649 geospatial query operators to shapefiles' data.</li>
    650 </ul></li>
    651 <li>Free access via a network API, if you register a key, is provided by
    652 the <a href="http://www.geonames.org/export/web-services.html#timezone">GeoNames Timezone web service</a>,
    653 the <a href="https://developers.google.com/maps/documentation/timezone/intro">Google Maps Time Zone API</a>, and
    654 the <a href="https://timezonedb.com">Time Zone Database &amp; API</a>.
    655 Commercial network API access is provided
    656 by <a href="https://askgeo.com">AskGeo</a>
    657 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    658 </li>
    659 <li>"<a
    660 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    661 to get a time zone from a location using latitude and longitude
    662 coordinates?</a>" discusses other geolocation possibilities.</li>
    663 <li><a href="http://statoids.com/statoids.html">Administrative
    664 Divisions of Countries ("Statoids")</a> lists
    665 political subdivision data related to time zones.</li>
    666 <li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time
    667 zone boundaries for multizone countries</a> summarizes legal
    668 boundaries between time zones within countries.</li>
    669 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
    670 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    671 world time zone boundaries distributed under the
    672 <abbr>GPL</abbr>.</li>
    673 <li>A ship within the <a
    674 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    675 waters</a> of any nation uses that nation's time. In international
    676 waters, time zone boundaries are meridians 15&deg; apart, except that
    677 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    678 wide and are separated by
    679 the 180&deg; meridian (not by the International Date Line, which is
    680 for land and territorial waters only). A captain can change ship's
    681 clocks any time after entering a new time zone; midnight changes are
    682 common.</li>
    683 </ul>
    684 <h2 id="civil">Civil time concepts and history</h2>
    685 <ul>
    686 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    687 Walk through Time</a>
    688 surveys the evolution of timekeeping.</li>
    689 <li><a href="http://www.webexhibits.org/daylightsaving/">About Daylight
    690 Saving Time &ndash; History, rationale, laws &amp; dates</a>
    691 is an overall history of <abbr>DST</abbr>.</li>
    692 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    693 contains guidelines and best practices for software applications that
    694 deal with civil time.</li>
    695 <li><a href="http://seizethedaylight.com/dst/">A Brief
    696 History of Daylight Saving Time</a> summarizes some of the contentious
    697 history of <abbr>DST</abbr>.</li>
    698 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    699 the International Date Line</a> tells the story of the most important
    700 time zone boundary.</li>
    701 <li><a href="http://statoids.com/tconcept.html">Basic Time
    702 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    703 </ul>
    704 <h2 id="national">National histories of legal time</h2>
    705 <dl>
    706 <dt>Australia</dt>
    707 <dd>The Parliamentary Library has commissioned a <a
    708 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    709 paper on daylight saving time in Australia</a>.
    710 The Bureau of Meteorology publishes a list of <a
    711 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    712 Dates of Daylight Savings Time within Australia</a>.</dd>
    713 <dt>Belgium</dt>
    714 <dd>The Royal Observatory of Belgium maintains a table of <a
    715 href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    716 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
    717 <dt>Brazil</dt>
    718 <dd>The Time Service Department of the National Observatory
    719 records <a href="http://pcdsh01.on.br/DecHV.html"
    720 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    721 Portuguese)</a>.</dd>
    722 <dt>Canada</dt>
    723 <dd>National Research Council Canada publishes current
    724 and some older information about <a
    725 href="https://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time
    726 zones &amp; daylight saving time</a>.</dd>
    727 <dt>Chile</dt>
    728 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    729 <a href="http://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
    730 Chile's official time (in Spanish)</a>.</dd>
    731 <dt>Czech Republic</dt>
    732 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    733 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    734 summarizes and cites historical DST regulations.</dd>
    735 <dt>Germany</dt>
    736 <dd>The National Institute for Science and Technology maintains the <a
    737 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    738 of Legal Time in Germany</a>.</dd>
    739 <dt>Israel</dt>
    740 <dd>The Interior Ministry periodically issues <a
    741 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    742 hreflang="he">announcements (in Hebrew)</a>.</dd>
    743 <dt>Italy</dt>
    744 <dd>The National Institute of Metrological Research maintains a
    745 <a href="http://www.nanospin.eu/res/tf/ora_legale_i.shtml">table of civil time
    746 (in Italian)</a>.</dd>
    747 <dt>Mexico</dt>
    748 <dd>The Investigation and Analysis Service of the Mexican Library of
    749 Congress has published a <a
    750 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    751 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    752 <dt>Malaysia</dt>
    753 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    754 <dt>Netherlands</dt>
    755 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    756 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    757 covers the history of local time in the Netherlands from ancient times.</dd>
    758 <dt>New Zealand</dt>
    759 <dd>The Department of Internal Affairs maintains a brief <a
    760 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    761 Daylight Saving</a>. The privately-maintained <a
    762 href="http://astrologyschool.com/nztime.html">History of New Zealand
    763 time</a> has more details.</dd>
    764 <dt>Singapore</dt>
    765 <dd><a id="Singapore"
    766 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
    767 is Singapore in the "Wrong" Time Zone?</a> details the
    768 history of legal time in Singapore and Malaysia.</dd>
    769 <dt>United Kingdom</dt>
    770 <dd><a
    771 href="https://www.polyomino.org.uk/british-time/">History of
    772 legal time in Britain</a> discusses in detail the country
    773 with perhaps the best-documented history of clock adjustments.
    774 The National Physical Laboratory also maintains an <a
    775 href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates">Archive
    776 of Summer time dates</a>.</dd>
    777 <dt>United States</dt>
    778 <dd>The Department of Transportation's <a
    779 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    780 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    781 <dt>Uruguay</dt>
    782 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    783 Navy (SOHMA) publishes an annual <a
    784 href="http://www.armada.mil.uy/Pagina/institucion/dimat/sohma/almanaque.html" hreflang="es">almanac
    785 (in Spanish)</a>.</dd>
    786 </dl>
    787 <h2 id="precision">Precision timekeeping</h2>
    788 <ul>
    789 <li><a
    790 href="http://leapsecond.com/hpan/an1289.pdf">The
    791 Science of Timekeeping</a> is a thorough introduction
    792 to the theory and practice of precision timekeeping.</li>
    793 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    794 Time 2016</a> contains several freely-readable papers.</li>
    795 <li><a href="http://www.ntp.org"><abbr
    796 title="Network Time Protocol">NTP</abbr>: The Network
    797 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    798 discusses how to synchronize clocks of
    799 Internet hosts.</li>
    800 <li>The <a
    801 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
    802 Time Protocol</a> (<abbr
    803 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
    804 can achieve submicrosecond clock accuracy on a local area network.</li>
    805 <li><a
    806 href="https://tools.ietf.org/html/rfc4833">Timezone
    807 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
    808 (Internet <abbr>RFC</abbr> 4833)
    809 specifies a <a
    810 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
    811 option for a server to configure
    812 a client's time zone and daylight saving settings automatically.</li>
    813 <li><a
    814 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
    815 Times</a> explains more abstruse astronomical time scales like
    816 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
    817 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
    818 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
    819 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
    820 Scales</a> goes into more detail, particularly for historical variants.</li>
    821 <li>The <a href="https://www.iau.org"><abbr
    822 title="International Astronomical Union">IAU</abbr></a>'s <a
    823 href="http://www.iausofa.org"><abbr
    824 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
    825 collection contains C and <a
    826 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
    827 code for converting among time scales like
    828 <abbr title="International Atomic Time">TAI</abbr>,
    829 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
    830 <abbr>UTC</abbr>.</li>
    831 <li><a
    832 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
    833 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
    834 diverse local time
    835 scales used by each landed mission on Mars.</li>
    836 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
    837 dedicated not only to leap seconds but to precise time and frequency
    838 in general. It covers the state of the art in amateur timekeeping, and
    839 how the art has progressed over the past few decades.</li>
    840 <li><a
    841 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
    842 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
    843 Bulletins</a> contains official publications of the International
    844 Earth Rotation and Reference Systems Service, which decides when leap
    845 seconds occur. The <code>tz</code> code and data support leap seconds
    846 via an optional "<code>right</code>" configuration, as opposed to the
    847 default "<code>posix</code>" configuration.</li>
    848 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
    849 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
    850 leap second so that they disagree with <abbr>UTC</abbr> by at most a
    851 half second, even though every <abbr>POSIX</abbr> minute has exactly
    852 sixty seconds. This approach works with the default <code>tz</code>
    853 "<code>posix</code>" configuration, is <a
    854 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
    855 the <abbr>NTP</abbr> reference implementation, and is used by major
    856 cloud service providers.</li>
    857 <li>The <a
    858 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
    859 Second Discussion List</a> covers <a
    860 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
    861 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
    862 discussed further in
    863 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
    864 leap second: its history and possible future</a>.
    865 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
    866 might be redefined
    867 without Leap Seconds</a> gives pointers on this
    868 contentious issue, which was active until 2015 and could become active
    869 again.</li>
    870 </ul>
    871 <h2 id="notation">Time notation</h2>
    872 <ul>
    873 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
    874 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
    875 zone names, abbreviations, identifiers, and formats. For example, it
    876 contains French translations for "Eastern European Summer Time",
    877 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
    878 "Bucharest". Its
    879 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
    880 charts</a> show these values for many locales. Data values are available in
    881 both <abbr title="Locale Data Markup Language">LDML</abbr>
    882 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
    883 <li>
    884 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
    885 the international standard date and time notation</a> is a good
    886 summary of
    887 <a
    888 href="https://www.iso.org/standard/40874.html"><em><abbr>ISO</abbr>
    889 8601:2004 &ndash; Data elements and interchange formats &ndash; Information
    890 interchange &ndash; Representation of dates and times</em></a>.</li>
    891 <li>
    892 <a href="https://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
    893 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
    894 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
    895 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
    896 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
    897 specifies the time notation used in email and <a
    898 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
    899 headers.</li>
    900 <li>
    901 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
    902 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
    903 specifies an <abbr>ISO</abbr> 8601
    904 profile for use in new Internet
    905 protocols.</li>
    906 <li>
    907 <a href="https://www.hackcraft.net/web/datetime/">Date &amp; Time
    908 Formats on the Web</a> surveys web- and Internet-oriented date and time
    909 formats.</li>
    910 <li>Alphabetic time zone abbreviations should not be used as unique
    911 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
    912 practice. For example, in English-speaking North America
    913 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
    914 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
    915 and French-speaking North Americans prefer
    916 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
    917 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
    918 database contains English abbreviations for many time stamps;
    919 unfortunately some of these abbreviations were merely the database maintainers'
    920 inventions, and these have been removed when possible.</li>
    921 <li>Numeric time zone abbreviations typically count hours east of
    922 <abbr>UT</abbr>, e.g., +09 for Japan and
    923 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
    924 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
    925 For example, one might use <code><abbr>TZ</abbr>="<abbr
    926 title="Japan Standard Time">JST</abbr>-9"</code> and
    927 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
    928 for Japan and Hawaii, respectively. If the
    929 <code><abbr>tz</abbr></code> database is available, it is usually better to use
    930 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
    931 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
    932 confusion, handle old time stamps better, and insulate you better from
    933 any future changes to the rules. One should never set
    934 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
    935 <code>"GMT-9"</code>, though, since this would incorrectly imply that
    936 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
    937 is called "<abbr>GMT</abbr>".</li>
    938 </ul>
    939 <h2 id="see-also">See also</h2>
    940 <ul>
    941 <li><a href="theory.html">Theory and pragmatics of the tz code and data</a></li>
    942 <li><a href="tz-art.html">Time and the Arts</a></li>
    943 </ul>
    944 <hr>
    945 <address>
    946 This web page is in the public domain, so clarified as of
    947 2009-05-17 by Arthur David Olson.
    948 <br>
    949 Please send corrections to this web page to the
    950 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
    951 </address>
    952 </body>
    953 </html>
    954