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      4 <title>Sources for time zone and daylight saving time data</title>
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      6 <style>
      7 pre {margin-left: 2em; white-space: pre-wrap;}
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     10 <body>
     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   <h3>Outline</h3>
     22   <nav>
     23     <ul>
     24       <li>The <code><abbr>tz</abbr></code> database product and process
     25 	<ul>
     26 	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
     27 	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
     28 	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
     29 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     30 	</ul>
     31       </li>
     32       <li>Uses of the <code><abbr>tz</abbr></code> database
     33 	<ul>
     34 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     35 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     36 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     37 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     38 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     39 	</ul>
     40       </li>
     41       <li>Related data
     42 	<ul>
     43 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     44 	  <li><a href="#maps">Maps</a></li>
     45 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     46 	</ul>
     47       </li>
     48       <li>Timekeeping concepts
     49 	<ul>
     50 	  <li><a href="#civil">Civil time concepts and history</a></li>
     51 	  <li><a href="#national">National histories of legal time</a></li>
     52 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     53 	  <li><a href="#precision">Precision timekeeping</a></li>
     54 	  <li><a href="#notation">Time notation</a></li>
     55 	  <li><a href="#see-also">See also</a></li>
     56 	</ul>
     57       </li>
     58     </ul>
     59   </nav>
     60 
     61 <section>
     62 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     63 <p>
     64 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     65 time zone database contains code and data
     66 that represent the history of local time
     67 for many representative locations around the globe.
     68 It is updated periodically to reflect changes made by political bodies
     69 to time zone boundaries and daylight saving rules.
     70 This database (known as <code><abbr>tz</abbr></code>,
     71 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     72 is used by several implementations,
     73 including
     74 <a href="https://www.gnu.org/software/libc/">the
     75 <abbr title="GNU's Not Unix">GNU</abbr>
     76 C Library</a> (used in
     77 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     78 <a href="https://www.android.com">Android</a>,
     79 <a href="https://www.freebsd.org">Free<abbr
     80 title="Berkeley Software Distribution">BSD</abbr></a>,
     81 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     82 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     83 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
     84 <a href="https://cygwin.com">Cygwin</a>,
     85 <a href="https://mariadb.org">MariaDB</a>,
     86 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     87 <a href="https://www.mysql.com">MySQL</a>,
     88 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     89 title="Web Operating System">webOS</abbr></a>,
     90 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     91 title="Advanced Interactive eXecutive">AIX</abbr></a>,
     92 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
     93 <a href="https://www.apple.com/ios"><abbr
     94 title="iPhone OS">iOS</abbr></a>,
     95 <a href="https://www.apple.com/macos">macOS</a>,
     96 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     97 <a href="https://www.vmssoftware.com">Open<abbr
     98 title="Virtual Memory System">VMS</abbr></a>,
     99 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
    100 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
    101 <p>
    102 Each main entry in the database represents a <dfn>timezone</dfn>
    103 for a set of civil-time clocks that have all agreed since 1970.
    104 Timezones are typically identified by continent or ocean and then by the
    105 name of the largest city within the region containing the clocks.
    106 For example, <code>America/New_York</code>
    107 represents most of the <abbr title="United States">US</abbr> eastern time zone;
    108 <code>America/Phoenix</code> represents most of Arizona, which
    109 uses mountain time without daylight saving time (<abbr>DST</abbr>);
    110 <code>America/Detroit</code> represents most of Michigan, which uses
    111 eastern time but with different <abbr>DST</abbr> rules in 1975;
    112 and other entries represent smaller regions like Starke County,
    113 Indiana, which switched from central to eastern time in 1991
    114 and switched back in 2006.
    115 To use the database on an extended <a
    116 href="https://en.wikipedia.org/wiki/POSIX"><abbr
    117 title="Portable Operating System Interface">POSIX</abbr></a>
    118 implementation set the <code><abbr>TZ</abbr></code>
    119 environment variable to the location's full name,
    120 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    121 <p>
    122 Associated with each timezone is a history of offsets from
    123 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    124 Time</a> (<abbr>UT</abbr>), which is <a
    125 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    126 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    127 for timestamps after 1960 this is more precisely <a
    128 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    129 Universal Time</a> (<abbr>UTC</abbr>).
    130 The database also records when daylight saving time was in use,
    131 along with some time zone abbreviations such as <abbr>EST</abbr>
    132 for Eastern Standard Time in the <abbr>US</abbr>.</p>
    133 </section>
    134 
    135 <section>
    136 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    137 <p>
    138 The following <a
    139 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    140 the latest release's two
    141 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    142 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    143 <pre><code>mkdir tzdb
    144 cd tzdb
    145 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    146 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    147 <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 -
    148 gzip -dc tzdata-latest.tar.gz | tar -xf -
    149 </code></pre>
    150 <p>Alternatively, the following shell commands download the same
    151 release in a single-tarball format containing extra data
    152 useful for regression testing:</p>
    153 <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>
    154 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    155 </code></pre>
    156 <p>These commands use convenience links to the latest release
    157 of the <code><abbr>tz</abbr></code> database hosted by the
    158 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    159 of the <a href="https://www.iana.org">Internet Assigned Numbers
    160 Authority (IANA)</a>.
    161 Older releases are in files named
    162 <code>tzcode<var>V</var>.tar.gz</code>,
    163 <code>tzdata<var>V</var>.tar.gz</code>, and
    164 <code>tzdb-<var>V</var>.tar.lz</code>,
    165 where <code><var>V</var></code> is the version.
    166 Since 1996, each version has been a four-digit year followed by
    167 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    168 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    169 through <samp>zzz</samp>, and so on).
    170 Since version 2016h, each release has contained a text file named
    171 "<samp>version</samp>" whose first (and currently only) line is the version.
    172 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
    173 and are also available in an
    174 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    175 title="File Transfer Protocol">FTP</abbr> directory</a> via a
    176 less-secure protocol.</p>
    177 <p>Alternatively, a development repository of code and data can be
    178 retrieved from <a href="https://github.com">GitHub</a> via the shell
    179 command:</p>
    180 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    181 </code></pre>
    182 <p>
    183 Since version 2012e, each release has been tagged in development repositories.
    184 Untagged commits are less well tested and probably contain
    185 more errors.</p>
    186 <p>
    187 After obtaining the code and data files, see the
    188 <code>README</code> file for what to do next.
    189 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    190 machine-readable binary files, one for each location. The binary files
    191 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
    192 specified by <a href="https://tools.ietf.org/html/8536">Internet
    193 <abbr>RFC</abbr> 8536</a>.
    194 The code also lets
    195 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    196 location.</p>
    197 </section>
    198 
    199 <section>
    200 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    201 <p>
    202 The <code><abbr>tz</abbr></code> code and data
    203 are by no means authoritative. If you find errors, please
    204 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    205 the time zone mailing list. You can also <a
    206 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    207 and browse the <a
    208 href="https://mm.icann.org/pipermail/tz/">archive of old
    209 messages</a>.
    210 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
    211 discussions</a> and corresponding data changes can be
    212 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    213 </p>
    214 <p>
    215 If your government plans to change its time zone boundaries or
    216 daylight saving rules, inform <code>tz (a] iana.org</code> well in
    217 advance, as this will coordinate updates to many cell phones,
    218 computers, and other devices around the world. With
    219 less than a year's notice there is a good chance that some
    220 computer-based clocks will operate incorrectly after the change, due
    221 to delays in propagating updates to software and data. The shorter
    222 the notice, the more likely clock problems will arise; see "<a
    223 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    224 the Timing of Time Zone Changes</a>" for examples.
    225 </p>
    226 <p>
    227 Changes to the <code><abbr>tz</abbr></code> code and data are often
    228 propagated to clients via operating system updates, so
    229 client <code><abbr>tz</abbr></code> data can often be corrected by
    230 applying these updates. With GNU/Linux and similar systems, if your
    231 maintenance provider has not yet adopted the
    232 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    233 the process by tailoring the generic instructions in
    234 the <code><abbr>tz</abbr> README</code> file and installing the latest
    235 data yourself. System-specific instructions for installing the
    236 latest <code><abbr>tz</abbr></code> data have also been published
    237 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
    238 <a
    239 href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
    240 <a
    241 href="http://userguide.icu-project.org/datetime/timezone"><abbr
    242 title="International Components for Unicode">ICU</abbr></a>,
    243 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
    244 and <a
    245 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
    246 Java, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    247 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    248 and <a
    249 href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
    250 </p>
    251 <p>Sources for the <code><abbr>tz</abbr></code> database are
    252 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    253 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    254 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    255 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    256 title="linefeed">LF</abbr></a>,
    257 which can be modified by common text editors such
    258 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    259 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
    260 <a href="https://www.vim.org">vim</a>.
    261 Specialized source-file editing can be done via the
    262 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    263 zoneinfo</a> package for <a
    264 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    265 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    266 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    267 Studio Code</a>.
    268 </p>
    269 <p>
    270 For further information about updates, please see
    271 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
    272 Maintaining the Time Zone Database</a> (Internet <abbr
    273 title="Request For Comments">RFC</abbr> 6557). More detail can be
    274 found in <a href="theory.html">Theory and pragmatics of the
    275 <code><abbr>tz</abbr></code> code and data</a>.
    276 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    277 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    278 </p>
    279 </section>
    280 
    281 <section>
    282 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    283 <ul>
    284 <li>The article
    285 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    286 an encyclopedic summary.</li>
    287 <li><a href="tz-how-to.html">How to Read the
    288 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    289 database format.</li>
    290 <li><a
    291 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    292 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    293 database's style.</li>
    294 </ul>
    295 </section>
    296 
    297 <section>
    298 <h2 id="web">Web sites using recent versions of the
    299 <code><abbr>tz</abbr></code> database</h2>
    300 <p>
    301 These are listed roughly in ascending order of complexity and fanciness.
    302 </p>
    303 <ul>
    304 <li><a href="https://time.is">Time.is</a> shows locations'
    305 time and zones.</li>
    306 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    307 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    308 <a href="https://www.worldclock.com">The World Clock</a>
    309 are time zone converters.</li>
    310 <li><a
    311 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    312 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    313 <li><a
    314 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    315 Time in 1000 Places</a> uses descriptions of the values.</li>
    316 <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
    317 uses a pulldown menu.</li>
    318 <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
    319 timezone information for all countries</a> displays tables of DST rules.
    320 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    321 Worldwide</a> lets you sort zone names and convert times.</li>
    322 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    323 time map and a time converter.</li>
    324 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    325 calculates the current time difference between locations.</li>
    326 <li><a href="http://www.wx-now.com">Weather Now</a> and
    327 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    328 </ul>
    329 </section>
    330 
    331 <section>
    332 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    333 <ul>
    334 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    335 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    336 Distribution Service (tzdist) working group</a> defined <a
    337 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
    338 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    339 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
    340 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    341 transferring time zone data by reference.
    342 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    343 mailing list</a> discussed the Internet draft <a
    344 id="TZDIST-Geolocate"
    345 href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
    346 Geolocate Extension</a> (now expired), which let a client determine its timezone
    347 from its geographic location using a <a
    348 href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li>
    349 <li>The <a href="https://tools.ietf.org/html/rfc5545">
    350 Internet Calendaring and Scheduling Core Object Specification
    351 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    352 covers time zone
    353 data; see its VTIMEZONE calendar component.
    354 The iCalendar format requires specialized parsers and generators; a
    355 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
    356 (Internet <abbr>RFC</abbr> 6321) uses
    357 <a href="https://www.w3.org/XML/"><abbr
    358 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    359 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
    360 (Internet <abbr>RFC</abbr> 7265)
    361 uses <a href="https://www.json.org"><abbr
    362 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    363 </ul>
    364 </section>
    365 
    366 <section>
    367 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    368 <p>Although some of these do not fully support
    369 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    370 distributions you can generally work around compatibility problems by
    371 running the command <code>make rearguard_tarballs</code> and compiling
    372 from the resulting tarballs instead.</p>
    373 <ul>
    374 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    375 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
    376 program that compiles
    377 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    378 Vzic is freely
    379 available under the <a
    380 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    381 General Public License (<abbr
    382 title="General Public License">GPL</abbr>)</a>.</li>
    383 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    384 database conversion utility</a> is like Vzic, except for the <a
    385 href="https://dotnet.microsoft.com">.NET framework</a>
    386 and with a <abbr>BSD</abbr>-style license.</li>
    387 <li><a
    388 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    389 contains a script <code>parse_olson</code> that compiles
    390 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    391 modules. It is part of the Perl <a
    392 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    393 which is freely
    394 available under both the <abbr>GPL</abbr> and the Perl Artistic
    395 License. DateTime::TimeZone also contains a script
    396 <code>tests_from_zdump</code> that generates test cases for each clock
    397 transition in the <code><abbr>tz</abbr></code> database.</li>
    398 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    399 Database Parser</a> is a
    400 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    401 runtime library with API <a
    402 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">adopted
    403 into the draft standard</a> for
    404 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
    405 the next iteration of the C++ standard.
    406 It is freely available under the
    407 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    408 <li><a id="ICU" href="http://site.icu-project.org">International Components for
    409 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    410 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    411 libraries for internationalization that
    412 has a compiler from <code><abbr>tz</abbr></code> source
    413 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    414 (mentioned <a href="#CLDR">below</a>)
    415 into an <abbr>ICU</abbr>-specific format.
    416 <abbr>ICU</abbr> is freely available under a
    417 <abbr>BSD</abbr>-style license.</li>
    418 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    419 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    420 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    421 title="Application Program Interface">API</abbr>s for use. It is
    422 freely available under the <abbr>MIT</abbr> license.</li>
    423 <li>Java-based compilers and libraries include:
    424 <ul>
    425 <li>The <a
    426 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
    427 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    428 Oracle Java.</li>
    429 <li>The <a
    430 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    431 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    432 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    433 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    434 <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    435 and time <abbr>API</abbr></a> contains a class
    436 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    437 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    438 Java 8 <code>java.time</code>, which its users should migrate to once
    439 they can assume Java 8 or later. It is available under the <a
    440 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    441 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a>,
    442 <a href="https://github.com/akashche/tzdbgen">tzdbgen</a>, and <a
    443 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
    444 are other alternatives to TZUpdater. IANA Updater's license is unclear;
    445 the others are licensed under the <abbr>GPL</abbr>.</li>
    446 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    447 time library for Android</a> and
    448 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
    449 time and interval library for Java</a> compile
    450 <code><abbr>tz</abbr></code> source into a binary format.
    451 Time4A is available under the Apache License and Time4J is
    452 available under the <a
    453 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    454 General Public License (<abbr title="Lesser General Public
    455 License">LGPL</abbr>)</a>.</li>
    456 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    457 Java-based libraries.</li>
    458 </ul>
    459 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    460 time <abbr>API</abbr> for .NET</a>
    461 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    462 It is freely available under the Apache License.</li>
    463 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
    464 compilers and libraries include:
    465 <ul>
    466 <li><a
    467 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
    468 compiles time zone data into a compact form designed for
    469 JavaScript. It is freely available under a combination of
    470 the <abbr>MIT</abbr> license and the Apache License.</li>
    471 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    472 plugin for the <a href="https://momentjs.com">Moment.js</a> date
    473 manipulation library. It is freely available under the <abbr>MIT</abbr>
    474 license.</li>
    475 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
    476 <abbr>API</abbr> is upward compatible with standard JavaScript
    477 Dates. It is freely available under the Apache License.</li>
    478 </ul>
    479 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    480 compiler from <code><abbr>tz</abbr></code> source into
    481 <a href="https://julialang.org/">Julia</a>. It is freely available
    482 under the <abbr>MIT</abbr> license.</li>
    483 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
    484 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    485 title="Free Pascal Compiler">FPC</abbr></a>
    486 compiles from <code><abbr>tz</abbr></code> source into
    487 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    488 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    489 and <a
    490 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    491 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    492 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    493 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    494 <a href="https://www.python.org">Python</a>.
    495 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    496 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    497 Ruby Timezone Library</a>
    498 compiles <code><abbr>tz</abbr></code> source into
    499 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    500 It is freely available under the <abbr>MIT</abbr> license.</li>
    501 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
    502 Library</a> is
    503 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    504 library that compiles <code><abbr>tz</abbr></code> source into a time
    505 zone repository whose format
    506 is either proprietary or an <abbr>XML</abbr>-encoded
    507 representation.</li>
    508 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    509 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    510 source into text files, along with a runtime that can read those
    511 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    512 license.</li>
    513 </ul>
    514 </section>
    515 
    516 <section>
    517 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    518 <ul>
    519 <li>The <a
    520 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    521 Library</a>
    522 has an independent, thread-safe implementation of
    523 a <abbr>TZif</abbr> file reader.
    524 This library is freely available under the LGPL
    525 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    526 <li><a href="https://www.gnome.org">GNOME</a>'s
    527 <a href="https://developer.gnome.org/glib/">GLib</a> has
    528 a <abbr>TZif</abbr> file reader written in C that
    529 creates a <code>GTimeZone</code> object representing sets
    530 of <abbr>UT</abbr> offsets.
    531 It is freely available under the <abbr>LGPL</abbr>.</li>
    532 <li>The
    533 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    534 <code>baltzo::TimeZoneUtil</code> component contains a C++
    535 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    536 the Apache License.</li>
    537 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    538 library that translates between <abbr>UT</abbr> and civil time and
    539 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    540 License.</li>
    541 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
    542 is a <abbr>TZif</abbr> file reader written in Java.
    543 It is freely available under the <abbr>LGPL</abbr>.</li>
    544 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    545 library that reads <abbr>TZif</abbr> files and converts
    546 timestamps from one time zone or format to another.
    547 It is used by <a href="https://secure.php.net"><abbr
    548 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    549 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    550 and <a href="https://www.mongodb.com">MongoDB</a>.
    551 It is freely available under the <abbr>MIT</abbr> license.</li>
    552 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    553 JavaScript library that supports date arithmetic that is time zone
    554 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    555 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    556 <abbr>TZif</abbr> file reader.</li>
    557 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    558 DateTime::TimeZone::Tzfile</a>
    559 is a <abbr>TZif</abbr> file reader written in Perl.
    560 It is freely available under the same terms as Perl
    561 (dual <abbr>GPL</abbr> and Artistic license).</li>
    562 <li>The
    563 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    564 library contains a Python tool that
    565 converts <abbr>TZif</abbr> data into
    566 <abbr>JSON</abbr>-format data suitable for use
    567 in its JavaScript library for time zone conversion. Dates before 1970
    568 are not supported.</li>
    569 <li>The <a
    570 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    571 package contains <a href="https://www.haskell.org">Haskell</a> code that
    572 parses and uses <abbr>TZif</abbr> data. It is freely
    573 available under a <abbr>BSD</abbr>-style license.</li>
    574 </ul>
    575 </section>
    576 
    577 <section>
    578 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    579 <ul>
    580 <li><a href="https://foxclocks.org">FoxClocks</a>
    581 is an extension for <a href="https://www.google.com/chrome/">Google
    582 Chrome</a> and for <a
    583 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
    584 Toolkit</a> applications like <a
    585 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    586 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    587 It displays multiple clocks in the application window, and has a mapping
    588 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    589 It is freely available under the <abbr>GPL</abbr>.</li>
    590 <li><a href="https://golang.org">Go programming language</a>
    591 implementations contain a copy of a 32-bit subset of a recent
    592 <code><abbr>tz</abbr></code> database in a
    593 Go-specific format.</li>
    594 <li><a
    595 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
    596 clock (intclock)</a> is a clock that displays multiple time zones on
    597 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
    598 under the <abbr>GPL</abbr>.</li>
    599 <li>Microsoft Windows 8.1
    600 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    601 data (mentioned <a href="#CLDR">below</a>) used by the
    602 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    603 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    604 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    605 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    606 <a id="System.TimeZoneInfo"
    607 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    608 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    609 the older, proprietary method of Microsoft Windows 2000 and later,
    610 which stores time zone data in the
    611 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    612 <a
    613 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    614 Tzid table</a> or <a
    615 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    616 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    617 to <code><abbr>tz</abbr></code> names.
    618 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    619 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    620 <li><a
    621 href="https://www.oracle.com/java/index.html">Oracle
    622 Java</a> contains a copy of a subset of a recent
    623 <code><abbr>tz</abbr></code> database in a
    624 Java-specific format.</li>
    625 <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
    626 Master</a> is a Microsoft Windows clock program that can automatically
    627 download, compile and use <code><abbr>tz</abbr></code> releases.
    628 The Basic version is free.</li>
    629 <li><a
    630 href="http://veladg.com/velaterra.html">VelaTerra</a> is
    631 a macOS program. Its developers
    632 <a href="http://veladg.com/tzoffer.html">offer free
    633 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
    634 </ul>
    635 </section>
    636 
    637 <section>
    638 <h2 id="other-dbs">Other time zone databases</h2>
    639 <ul>
    640 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    641 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    642 time zone history atlases
    643 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    644 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>,
    645 now published in <a
    646 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
    647 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
    648 Although these extensive atlases
    649 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    650 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    651 they are unreliable as Shanks appears to have
    652 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    653 sources and do not indicate which entries are guesswork.</li>
    654 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    655 its own <code>tztab</code>(4) format.</li>
    656 <li>Microsoft Windows has proprietary data mentioned
    657 <a href="#System.TimeZoneInfo">above</a>.</li>
    658 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    659 is another time zone database.</li>
    660 <li>The <a
    661 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    662 Schedules Information Manual</a> of the
    663 International Air Transport Association
    664 gives current time zone rules for airports served by commercial aviation.</li>
    665 </ul>
    666 </section>
    667 
    668 <section>
    669 <h2 id="maps">Maps</h2>
    670 <ul>
    671 <li>The <a href="https://www.cia.gov/index.html">United States Central
    672 Intelligence Agency (<abbr
    673 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
    674 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
    675 zone map</a>; the
    676 <a
    677 href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    678 Library Map Collection</a>
    679 of the University of Texas at Austin has copies of
    680 recent editions.
    681 The pictorial quality is good,
    682 but the maps do not indicate daylight saving time,
    683 and parts of the data are a few years out of date.</li>
    684 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    685 with current time</a>
    686 has several fancy time zone maps; it covers Russia particularly well.
    687 The maps' pictorial quality is not quite as good as the
    688 <abbr>CIA</abbr>'s
    689 but the maps are more up to date.</li>
    690 <li><a
    691 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    692 much is time wrong around the world?</a> maps the difference between
    693 mean solar and standard time, highlighting areas such as western China
    694 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    695 </ul>
    696 </section>
    697 
    698 <section>
    699 <h2 id="boundaries">Time zone boundaries</h2>
    700 <p>Geographical boundaries between timezones are available
    701 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
    702 services and other sources.</p>
    703 <ul>
    704 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    705 Boundary Builder</a> extracts
    706 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    707 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    708 Its code is freely available under the <abbr>MIT</abbr> license, and
    709 its data entries are freely available under the
    710 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    711 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    712 <li>Programmatic interfaces that map geographical coordinates via tz_world to
    713 <code><abbr>tzdb</abbr></code> timezones include:
    714 <ul>
    715 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    716 written in <a
    717 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    718 and is freely available under the <abbr>MIT</abbr> license.</li>
    719 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    720 is written in Go and is freely available under the Apache License.</li>
    721 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    722 in both Java and
    723 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    724 form, is freely available under the MIT license.</li>
    725 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
    726 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    727 is freely available under the MIT license, and
    728 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    729 is in the public domain.</li>
    730 <li>The <a
    731 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    732 library for Python is freely available under the MIT license.
    733 <li>The <a
    734 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    735 library for Ruby is freely available under the MIT license.</li>
    736 </ul></li>
    737 <li>Free access via a network API, if you register a key, is provided by
    738 the <a
    739 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    740 Timezone web service</a>, the <a
    741 href="https://developers.google.com/maps/documentation/timezone/intro">Google
    742 Maps Time Zone API</a>, and
    743 the <a href="https://timezonedb.com">Time Zone Database &amp; API</a>.
    744 Commercial network API access is provided
    745 by <a href="https://askgeo.com">AskGeo</a>
    746 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    747 </li>
    748 <li>"<a
    749 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    750 to get a time zone from a location using latitude and longitude
    751 coordinates?</a>" discusses other geolocation possibilities.</li>
    752 <li><a href="http://statoids.com/statoids.html">Administrative
    753 Divisions of Countries ("Statoids")</a> lists
    754 political subdivision data related to time zones.</li>
    755 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
    756 zone boundaries for multizone countries</a> summarizes legal
    757 boundaries between time zones within countries.</li>
    758 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
    759 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    760 world time zone boundaries distributed under the
    761 <abbr>GPL</abbr>.</li>
    762 <li>A ship within the <a
    763 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    764 waters</a> of any nation uses that nation's time. In international
    765 waters, time zone boundaries are meridians 15&deg; apart, except that
    766 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    767 wide and are separated by
    768 the 180&deg; meridian (not by the International Date Line, which is
    769 for land and territorial waters only). A captain can change ship's
    770 clocks any time after entering a new time zone; midnight changes are
    771 common.</li>
    772 </ul>
    773 </section>
    774 
    775 <section>
    776 <h2 id="civil">Civil time concepts and history</h2>
    777 <ul>
    778 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    779 Walk through Time</a>
    780 surveys the evolution of timekeeping.</li>
    781 <li>The history of daylight saving time is surveyed in <a
    782 href="http://www.webexhibits.org/daylightsaving/">About Daylight
    783 Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    784 <a href="http://seizethedaylight.com/dst/">A Brief
    785 History of Daylight Saving Time</a>.</li>
    786 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    787 Lords</a> discusses how authoritarians manipulate civil time.</li>
    788 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    789 contains guidelines and best practices for software applications that
    790 deal with civil time.</li>
    791 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    792 the International Date Line</a> tells the story of the most important
    793 time zone boundary.</li>
    794 <li><a href="http://statoids.com/tconcept.html">Basic Time
    795 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    796 </ul>
    797 </section>
    798 
    799 <section>
    800 <h2 id="national">National histories of legal time</h2>
    801 <dl>
    802 <dt>Australia</dt>
    803 <dd>The Parliamentary Library commissioned a <a
    804 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    805 paper on daylight saving time in Australia</a>.
    806 The Bureau of Meteorology publishes a list of <a
    807 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    808 Dates of Daylight Savings Time within Australia</a>.</dd>
    809 <dt>Belgium</dt>
    810 <dd>The Royal Observatory of Belgium maintains a table of time in
    811 Belgium (in
    812 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    813 hreflang="nl">Dutch</a> and <a
    814 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
    815 hreflang="fr">French</a>).</dd>
    816 <dt>Brazil</dt>
    817 <dd>The Time Service Department of the National Observatory
    818 records <a href="http://pcdsh01.on.br/DecHV.html"
    819 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    820 Portuguese)</a>.</dd>
    821 <dt>Canada</dt>
    822 <dd>National Research Council Canada publishes current
    823 and some older information about <a
    824 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    825 zones and daylight saving time</a>.</dd>
    826 <dt>Chile</dt>
    827 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    828 <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
    829 Chile's official time (in Spanish)</a>.</dd>
    830 <dt>China</dt>
    831 <dd>The Hong Kong Observatory maintains a
    832 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    833  summer time in Hong Kong</a>,
    834 and Macau's Meteorological and Geophysical Bureau maintains a <a
    835 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    836 history for Macau</a>.
    837 Unfortunately the latter is incomplete and has errors.</dd>
    838 <dt>Czech Republic</dt>
    839 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    840 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    841 summarizes and cites historical DST regulations.</dd>
    842 <dt>Germany</dt>
    843 <dd>The National Institute for Science and Technology maintains the <a
    844 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    845 of Legal Time in Germany</a>.</dd>
    846 <dt>Israel</dt>
    847 <dd>The Interior Ministry periodically issues <a
    848 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    849 hreflang="he">announcements (in Hebrew)</a>.</dd>
    850 <dt>Italy</dt>
    851 <dd>The National Institute of Metrological Research publishes a
    852 <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
    853 of civil time (in Italian)</a>.</dd>
    854 <dt>Malaysia</dt>
    855 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    856 <dt>Mexico</dt>
    857 <dd>The Investigation and Analysis Service of the Mexican Library of
    858 Congress has published a <a
    859 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    860 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    861 <dt>Netherlands</dt>
    862 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    863 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    864 covers the history of local time in the Netherlands from ancient times.</dd>
    865 <dt>New Zealand</dt>
    866 <dd>The Department of Internal Affairs maintains a brief <a
    867 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    868 Daylight Saving</a>.</dd>
    869 <dt>Singapore</dt>
    870 <dd><a id="Singapore"
    871 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    872 is Singapore in the "Wrong" Time Zone?</a> details the
    873 history of legal time in Singapore and Malaysia.</dd>
    874 <dt>United Kingdom</dt>
    875 <dd><a
    876 href="https://www.polyomino.org.uk/british-time/">History of
    877 legal time in Britain</a> discusses in detail the country
    878 with perhaps the best-documented history of clock adjustments.</dd>
    879 <dt>United States</dt>
    880 <dd>The Department of Transportation's <a
    881 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    882 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    883 <dt>Uruguay</dt>
    884 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    885 Navy (SOHMA) publishes an annual <a
    886 href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    887 (in Spanish)</a>.</dd>
    888 </dl>
    889 </section>
    890 
    891 <section>
    892 <h2 id="costs">Costs and benefits of time shifts</h2>
    893 <p>Various sources argue for and against daylight saving time and time
    894 zone shifts, and many scientific studies have been conducted. This
    895 section summarizes reviews of scientific literature in the area.</p>
    896 <ul>
    897 <li>Carey RN, Sarma KM.
    898 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    899 daylight saving time on road traffic collision risk: a systematic
    900 review</a>.
    901 <em>BMJ Open.</em> 2017;7(6):e014319. doi:<a href="https://doi.org/10.1136/bmjopen-2016-014319">10.1136/bmjopen-2016-014319</a>.
    902 This reviews research literature and concludes that the evidence
    903 neither supports nor refutes road safety benefits from
    904 shifts in time zones.</li>
    905 <li>Havranek T, Herman D, Irsova D.
    906 <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
    907 saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
    908 2018;39(2).
    909 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    910 This analyzes research literature and concludes, "Electricity savings
    911 are larger for countries farther away from the equator, while
    912 subtropical regions consume more electricity because of DST."</li>
    913 <li>Roenneberg T, Winnebeck EC, Klerman EB.
    914 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
    915 saving time and artificial time zones &ndash; a battle between
    916 biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
    917 doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
    918 This reviews evidence about the health effects of DST and concludes,
    919 "In summary, the scientific literature strongly argues against the
    920 switching between DST and Standard Time and even more so against
    921 adopting DST permanently."</li>
    922 </ul>
    923 </section>
    924 
    925 <section>
    926 <h2 id="precision">Precision timekeeping</h2>
    927 <ul>
    928 <li><a
    929 href="http://leapsecond.com/hpan/an1289.pdf">The
    930 Science of Timekeeping</a> is a thorough introduction
    931 to the theory and practice of precision timekeeping.</li>
    932 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    933 Time 2016</a> contains several freely-readable papers.</li>
    934 <li><a href="http://www.ntp.org"><abbr
    935 title="Network Time Protocol">NTP</abbr>: The Network
    936 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    937 discusses how to synchronize clocks of
    938 Internet hosts.</li>
    939 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
    940 family of software algorithms can achieve accuracy to a few tens of
    941 nanoseconds in scalable server farms without special hardware.</li>
    942 <li>The <a
    943 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
    944 Time Protocol</a> (<abbr
    945 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
    946 can achieve submicrosecond clock accuracy on a local area network
    947 with special-purpose hardware.</li>
    948 <li><a
    949 href="https://tools.ietf.org/html/rfc4833">Timezone
    950 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
    951 (Internet <abbr>RFC</abbr> 4833)
    952 specifies a <a
    953 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
    954 option for a server to configure
    955 a client's time zone and daylight saving settings automatically.</li>
    956 <li><a
    957 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
    958 Times</a> explains more abstruse astronomical time scales like
    959 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
    960 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
    961 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
    962 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
    963 Scales</a> goes into more detail, particularly for historical variants.</li>
    964 <li>The <a href="https://www.iau.org"><abbr
    965 title="International Astronomical Union">IAU</abbr></a>'s <a
    966 href="http://www.iausofa.org"><abbr
    967 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
    968 collection contains C and <a
    969 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
    970 code for converting among time scales like
    971 <abbr title="International Atomic Time">TAI</abbr>,
    972 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
    973 <abbr>UTC</abbr>.</li>
    974 <li><a
    975 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
    976 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
    977 diverse local time
    978 scales used by each landed mission on Mars.</li>
    979 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
    980 dedicated not only to leap seconds but to precise time and frequency
    981 in general. It covers the state of the art in amateur timekeeping, and
    982 how the art has progressed over the past few decades.</li>
    983 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
    984 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
    985 and time-signal emissions</a>, International Telecommunication Union &ndash;
    986 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
    987 <li><a
    988 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
    989 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
    990 Bulletins</a> contains official publications of the International
    991 Earth Rotation and Reference Systems Service, which decides when leap
    992 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
    993 via an optional "<code>right</code>" configuration, as opposed to the
    994 default "<code>posix</code>" configuration.</li>
    995 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
    996 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
    997 leap second so that they disagree with <abbr>UTC</abbr> by at most a
    998 half second, even though every <abbr>POSIX</abbr> minute has exactly
    999 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1000 "<code>posix</code>" configuration, is <a
   1001 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
   1002 the <abbr>NTP</abbr> reference implementation, and is used by major
   1003 cloud service providers. However, according to
   1004 <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">&sect;3.7.1 of
   1005 Network Time Protocol Best Current Practices</a>
   1006 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1007 applications requiring accurate UTC or civil time,
   1008 and is intended for use only in single, well-controlled environments.</li>
   1009 <li>The <a
   1010 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1011 Second Discussion List</a> covers <a
   1012 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
   1013 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
   1014 discussed further in
   1015 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1016 leap second: its history and possible future</a>.
   1017 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1018 might be redefined
   1019 without Leap Seconds</a> gives pointers on this
   1020 contentious issue, which was active until 2015 and could become active
   1021 again.</li>
   1022 </ul>
   1023 </section>
   1024 
   1025 <section>
   1026 <h2 id="notation">Time notation</h2>
   1027 <ul>
   1028 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
   1029 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1030 zone names, abbreviations, identifiers, and formats. For example, it
   1031 contains French translations for "Eastern European Summer Time",
   1032 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
   1033 "Bucharest". Its
   1034 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1035 charts</a> show these values for many locales. Data values are available in
   1036 both <abbr title="Locale Data Markup Language">LDML</abbr>
   1037 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1038 <li>
   1039 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1040 the international standard date and time notation</a> covers
   1041 <a
   1042 href="https://www.iso.org/standard/70907.html"><em><abbr
   1043 title="International Organization for Standardization">ISO</abbr>
   1044 8601-1:2019 &ndash; Date and time &ndash; Representations for information
   1045 interchange &ndash; Part 1: Basic rules</em></a>.</li>
   1046 <li>
   1047 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1048 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
   1049 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1050 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
   1051 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1052 specifies the time notation used in email and <a
   1053 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1054 headers.</li>
   1055 <li>
   1056 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
   1057 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1058 specifies an <abbr>ISO</abbr> 8601
   1059 profile for use in new Internet
   1060 protocols.</li>
   1061 <li>
   1062 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1063 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1064 formats.</li>
   1065 <li>Alphabetic time zone abbreviations should not be used as unique
   1066 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1067 practice. For example, in English-speaking North America
   1068 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
   1069 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1070 and French-speaking North Americans prefer
   1071 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
   1072 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
   1073 database contains English abbreviations for many timestamps;
   1074 unfortunately some of these abbreviations were merely the database maintainers'
   1075 inventions, and these have been removed when possible.</li>
   1076 <li>Numeric time zone abbreviations typically count hours east of
   1077 <abbr>UT</abbr>, e.g., +09 for Japan and
   1078 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
   1079 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
   1080 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1081 title="Japan Standard Time">JST</abbr>-9"</code> and
   1082 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1083 for Japan and Hawaii, respectively. If the
   1084 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1085 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1086 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1087 confusion, handle old timestamps better, and insulate you better from
   1088 any future changes to the rules. One should never set
   1089 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1090 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1091 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1092 is called "<abbr>GMT</abbr>".</li>
   1093 </ul>
   1094 </section>
   1095 
   1096 <section>
   1097 <h2 id="see-also">See also</h2>
   1098 <ul>
   1099 <li><a href="theory.html">Theory and pragmatics of the
   1100 <code><abbr>tz</abbr></code> code and data</a></li>
   1101 <li><a href="tz-art.html">Time and the Arts</a></li>
   1102 </ul>
   1103 </section>
   1104 
   1105 <footer>
   1106 <hr>
   1107 This web page is in the public domain, so clarified as of
   1108 2009-05-17 by Arthur David Olson.
   1109 <br>
   1110 Please send corrections to this web page to the
   1111 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1112 </footer>
   1113 </body>
   1114 </html>
   1115