Home | History | Annotate | Line # | Download | only in time
tz-link.html revision 1.16
      1 <!DOCTYPE html>
      2 <html lang="en">
      3 <head>
      4 <title>Time zone and daylight saving time data</title>
      5 <meta charset="UTF-8">
      6 <style>
      7 pre {margin-left: 2em; white-space: pre-wrap;}
      8 </style>
      9 </head>
     10 <body>
     11 <h1>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="#coordinating">Coordinating with governments and distributors</a></li>
     30 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     31 	</ul>
     32       </li>
     33       <li>Uses of the <code><abbr>tz</abbr></code> database
     34 	<ul>
     35 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     36 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     37 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     38 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     39 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     40 	</ul>
     41       </li>
     42       <li>Related data
     43 	<ul>
     44 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     45 	  <li><a href="#maps">Maps</a></li>
     46 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     47 	</ul>
     48       </li>
     49       <li>Timekeeping concepts
     50 	<ul>
     51 	  <li><a href="#civil">Civil time concepts and history</a></li>
     52 	  <li><a href="#national">National histories of legal time</a></li>
     53 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     54 	  <li><a href="#precision">Precision timekeeping</a></li>
     55 	  <li><a href="#notation">Time notation</a></li>
     56 	</ul>
     57       </li>
     58       <li><a href="#see-also">See also</a></li>
     59     </ul>
     60   </nav>
     61 
     62 <section>
     63 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     64 <p>
     65 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     66 time zone database contains code and data
     67 that represent the history of local time
     68 for many representative locations around the globe.
     69 It is updated periodically to reflect changes made by political bodies
     70 to time zone boundaries and daylight saving rules.
     71 This database (known as <code><abbr>tz</abbr></code>,
     72 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     73 is used by several implementations,
     74 including
     75 <a href="https://www.gnu.org/software/libc/">the
     76 <abbr title="GNU's Not Unix">GNU</abbr>
     77 C Library</a> (used in
     78 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     79 <a href="https://www.android.com">Android</a>,
     80 <a href="https://www.freebsd.org">Free<abbr
     81 title="Berkeley Software Distribution">BSD</abbr></a>,
     82 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     83 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     84 <a href="https://www.chromium.org/chromium-os/">Chromium OS</a>,
     85 <a href="https://cygwin.com">Cygwin</a>,
     86 <a href="https://mariadb.org">MariaDB</a>,
     87 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     88 <a href="https://www.mysql.com">MySQL</a>,
     89 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     90 title="Web Operating System">webOS</abbr></a>,
     91 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     92 title="Advanced Interactive eXecutive">AIX</abbr></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>.1-2017</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 2022a, each release has been distributed in
    171 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
    172 ustar interchange format</a>, compressed as described above;
    173 older releases use a nearly compatible format.
    174 Since version 2016h, each release has contained a text file named
    175 "<code>version</code>" whose first (and currently only) line is the version.
    176 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
    177 and are also available in an
    178 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    179 title="File Transfer Protocol">FTP</abbr> directory</a> via a
    180 less secure protocol.</p>
    181 <p>Alternatively, a development repository of code and data can be
    182 retrieved from <a href="https://github.com">GitHub</a> via the shell
    183 command:</p>
    184 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    185 </code></pre>
    186 <p>
    187 Since version 2012e, each release has been tagged in development repositories.
    188 Untagged commits are less well tested and probably contain
    189 more errors.</p>
    190 <p>
    191 After obtaining the code and data files, see the
    192 <code>README</code> file for what to do next.
    193 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    194 machine-readable binary files, one for each location. The binary files
    195 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
    196 specified by <a href="https://datatracker.ietf.org/doc/html/8536">Internet
    197 <abbr>RFC</abbr> 8536</a>.
    198 The code also lets
    199 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    200 location.</p>
    201 </section>
    202 
    203 <section>
    204 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    205 <p>
    206 The <code><abbr>tz</abbr></code> code and data
    207 are by no means authoritative. If you find errors, please
    208 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    209 the time zone mailing list. You can also <a
    210 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    211 and browse the <a
    212 href="https://mm.icann.org/pipermail/tz/">archive of old
    213 messages</a>.
    214 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
    215 discussions</a> and corresponding data changes can be
    216 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    217 </p>
    218 <p>
    219 Changes to the <code><abbr>tz</abbr></code> code and data are often
    220 propagated to clients via operating system updates, so
    221 client <code><abbr>tz</abbr></code> data can often be corrected by
    222 applying these updates. With GNU/Linux and similar systems, if your
    223 maintenance provider has not yet adopted the
    224 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    225 the process by tailoring the generic instructions in
    226 the <code><abbr>tz</abbr> README</code> file and installing the latest
    227 data yourself. System-specific instructions for installing the
    228 latest <code><abbr>tz</abbr></code> data have also been published
    229 for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
    230 <a
    231 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
    232 <a
    233 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
    234 title="International Components for Unicode">ICU</abbr></a>,
    235 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
    236 JDK</a>,
    237 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    238 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    239 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    240 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
    241 </p>
    242 <p>Since version 2013a,
    243 sources for the <code><abbr>tz</abbr></code> database have been
    244 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    245 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    246 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    247 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    248 title="linefeed">LF</abbr></a>,
    249 which can be modified by common text editors such
    250 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    251 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
    252 <a href="https://www.vim.org">vim</a>.
    253 Specialized source-file editing can be done via the
    254 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    255 zoneinfo</a> package for <a
    256 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    257 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    258 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    259 Studio Code</a>.
    260 </p>
    261 <p>
    262 For further information about updates, please see
    263 <a href="https://datatracker.ietf.org/doc/html/rfc6557">Procedures for
    264 Maintaining the Time Zone Database</a> (Internet <abbr
    265 title="Request For Comments">RFC</abbr> 6557). More detail can be
    266 found in <a href="theory.html">Theory and pragmatics of the
    267 <code><abbr>tz</abbr></code> code and data</a>.
    268 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    269 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    270 </p>
    271 </section>
    272 
    273 <section>
    274 <h2 id="coordinating">Coordinating with governments and distributors</h2>
    275 <p>
    276 As discussed in
    277 "<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
    278 Time Zones Are Coordinated</a>", the time zone database relies on
    279 collaboration among governments, the time zone database volunteer
    280 community, and data distributors downstream.
    281 <p>
    282 If your government plans to change its time zone boundaries or
    283 daylight saving rules, please send email to <a
    284 href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a> well in advance,
    285 as this will lessen confusion and will coordinate updates to many cell phones,
    286 computers, and other devices around the world.
    287 In your email, please cite the legislation or regulation that specifies
    288 the change, so that it can be checked for details such as the exact times
    289 when clock transitions occur.
    290 It is OK if a rule change is planned to affect clocks
    291 far into the future, as a long-planned change can easily be reverted
    292 or otherwise altered with a year's notice before the change would have
    293 affected clocks.</p>
    294 <p>
    295 There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
    296 However, typically a release occurs every few months.
    297 Many downstream timezone data distributors wait for
    298 a <code><abbr>tzdb</abbr></code> release before they produce an update
    299 to time zone behavior in consumer devices and software products.
    300 After a release, various parties must integrate, test,
    301 and roll out an update before <a
    302 href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
    303 These updates can be expensive, for both the <a
    304 href="https://en.wikipedia.org/wiki/Quality_assurance">quality
    305 assurance</a> process and the overall cost of shipping and installing
    306 updates to each device's copy of <code><abbr>tzdb</abbr></code>.
    307 Updates may be batched with other updates and may take substantial
    308 time to reach end users after a release.
    309 Older devices may no longer be supported and thus may never be updated,
    310 which means they will continue to use out-of-date rules.</p>
    311 <p>
    312 For these reasons any rule change should be promulgated at least a
    313 year before it affects how clocks operate; otherwise, there is a good
    314 chance that many clocks will be wrong due to delays in propagating updates,
    315 and that residents will be confused or even actively resist the change.
    316 The shorter the notice, the more likely clock problems will arise; see "<a
    317 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    318 the Timing of Time Zone Changes</a>" for examples.
    319 </p>
    320 </section>
    321 
    322 <section>
    323 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    324 <ul>
    325 <li>The article
    326 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    327 an encyclopedic summary.</li>
    328 <li><a href="tz-how-to.html">How to Read the
    329 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    330 database format.</li>
    331 <li><a
    332 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    333 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    334 database's style.</li>
    335 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
    336 managing time in the internet</a> analyzes the database longitudinally.</li>
    337 </ul>
    338 </section>
    339 
    340 <section>
    341 <h2 id="web">Web sites using recent versions of the
    342 <code><abbr>tz</abbr></code> database</h2>
    343 <p>
    344 These are listed roughly in ascending order of complexity and fanciness.
    345 </p>
    346 <ul>
    347 <li><a href="https://time.is">Time.is</a> shows locations'
    348 time and zones.</li>
    349 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    350 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    351 <a href="https://www.worldclock.com">The World Clock</a>
    352 are time zone converters.</li>
    353 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
    354 publishes <code><abbr>tzdb</abbr></code>-derived data in
    355 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
    356 title="comma-separated values">CSV</abbr></a> and
    357 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
    358 title="Structured Query Language">SQL</abbr></a> form.</li>
    359 <li><a
    360 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    361 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    362 <li><a
    363 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    364 Time in 1000 Places</a> uses descriptions of the values.</li>
    365 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    366 Worldwide</a> lets you sort zone names and convert times.</li>
    367 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    368 time map and a time converter.</li>
    369 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    370 calculates the current time difference between locations.</li>
    371 <li><a href="https://www.wx-now.com">Weather Now</a> and
    372 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    373 </ul>
    374 </section>
    375 
    376 <section>
    377 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    378 <ul>
    379 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    380 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    381 Distribution Service (tzdist) working group</a> defined <a
    382 href="https://datatracker.ietf.org/doc/html/rfc7808">TZDIST</a>
    383 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    384 along with <a href="https://datatracker.ietf.org/doc/html/rfc7809">CalDAV</a>
    385 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    386 transferring time zone data by reference.
    387 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
    388 implementations</a> are available.
    389 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    390 mailing list</a> discusses possible extensions.</li>
    391 <li>The <a href="https://datatracker.ietf.org/doc/html/rfc5545">
    392 Internet Calendaring and Scheduling Core Object Specification
    393 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    394 covers time zone
    395 data; see its VTIMEZONE calendar component.
    396 The iCalendar format requires specialized parsers and generators; a
    397 variant <a href="https://datatracker.ietf.org/doc/html/rfc6321">xCal</a>
    398 (Internet <abbr>RFC</abbr> 6321) uses
    399 <a href="https://www.w3.org/XML/"><abbr
    400 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    401 <a href="https://datatracker.ietf.org/doc/html/rfc7265">jCal</a>
    402 (Internet <abbr>RFC</abbr> 7265)
    403 uses <a href="https://www.json.org"><abbr
    404 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    405 </ul>
    406 </section>
    407 
    408 <section>
    409 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    410 <p>Although some of these do not fully support
    411 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    412 distributions you can generally work around compatibility problems by
    413 running the command <code>make rearguard_tarballs</code> and compiling
    414 from the resulting tarballs instead.</p>
    415 <ul>
    416 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    417 href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
    418 program that compiles
    419 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    420 Vzic is freely
    421 available under the <a
    422 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    423 General Public License (<abbr
    424 title="General Public License">GPL</abbr>)</a>.</li>
    425 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    426 database conversion utility</a> is like Vzic, except for the <a
    427 href="https://dotnet.microsoft.com">.NET framework</a>
    428 and with a <abbr>BSD</abbr>-style license.</li>
    429 <li><a
    430 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    431 contains a script <code>parse_olson</code> that compiles
    432 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    433 modules. It is part of the Perl <a
    434 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    435 which is freely
    436 available under both the <abbr>GPL</abbr> and the Perl Artistic
    437 License. DateTime::TimeZone also contains a script
    438 <code>tests_from_zdump</code> that generates test cases for each clock
    439 transition in the <code><abbr>tz</abbr></code> database.</li>
    440 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    441 Database Parser</a> is a
    442 <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
    443 runtime library with <a
    444 href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
    445 adopted by
    446 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
    447 the current iteration of the C++ standard.
    448 It is freely available under the
    449 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    450 <li><a id="ICU" href="https://icu.unicode.org">International Components for
    451 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    452 href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
    453 libraries for internationalization that
    454 has a compiler from <code><abbr>tz</abbr></code> source
    455 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    456 (mentioned <a href="#CLDR">below</a>)
    457 into an <abbr>ICU</abbr>-specific format.
    458 <abbr>ICU</abbr> is freely available under a
    459 <abbr>BSD</abbr>-style license.</li>
    460 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    461 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    462 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    463 title="Application Program Interface">API</abbr>s for use. It is
    464 freely available under the <abbr>MIT</abbr> license.</li>
    465 <li>Java-based compilers and libraries include:
    466 <ul>
    467 <li>The <a
    468 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
    469 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    470 <a href="https://openjdk.java.net/">OpenJDK</a> and
    471 <a href="https://jdk.java.net/">Oracle JDK</a>.
    472 Although its source code is proprietary, its executable is available under the
    473 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
    474 Timezone Updater License Agreement</a>.</li>
    475 <li>The <a
    476 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    477 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    478 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    479 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    480 <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    481 and time <abbr>API</abbr></a> contains a class
    482 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    483 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    484 Java 8 <code>java.time</code>, which its users should migrate to once
    485 they can assume Java 8 or later. It is available under the <a
    486 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    487 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
    488 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
    489 are alternatives to TZUpdater. IANA Updater's license is unclear;
    490 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
    491 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    492 time library for Android</a> and
    493 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
    494 time and interval library for Java</a> compile
    495 <code><abbr>tz</abbr></code> source into a binary format.
    496 Time4A is available under the Apache License and Time4J is
    497 available under the <a
    498 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    499 General Public License (<abbr title="Lesser General Public
    500 License">LGPL</abbr>)</a>.</li>
    501 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    502 Java-based libraries.</li>
    503 </ul>
    504 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    505 time <abbr>API</abbr> for .NET</a>
    506 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    507 It is freely available under the Apache License.</li>
    508 <li>Many modern
    509 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
    510 runtimes support <code><abbr>tz</abbr></code> natively via the
    511 <code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>.
    512 This can be used as-is or with most of the following libraries,
    513 many of which also support runtimes lacking the <code>timeZone</code> option.
    514 <ul>
    515 <li>The <a
    516 href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
    517 timezone polyfill</a>
    518 is freely available under a <abbr>BSD</abbr>-style license.</li>
    519 <li>The <a href="https://date-fns.org/">date-fns</a>
    520 library manipulates timezone-aware timestamps in browsers and
    521 in <a href="https://nodejs.org/en/">Node.js</a>.
    522 It is freely available under the <abbr>MIT</abbr> license.</li>
    523 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
    524 minimalist replacement for the date and time API of
    525 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
    526 manipulation library.
    527 It is freely available under the <abbr>MIT</abbr> license.</li>
    528 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
    529 timezone support for the <code>Intl</code> API.
    530 It is freely available under the <abbr>MIT</abbr> license.</li>
    531 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    532 Moment.js plugin.
    533 It is freely available under the <abbr>MIT</abbr> license.</li>
    534 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    535 JavaScript library that supports date arithmetic that is time zone
    536 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    537 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
    538 supports live <code><abbr>tzdb</abbr></code> updates,
    539 astronomical and atomic time, a command-line interface,
    540 and full <a
    541 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
    542 Its companion <a
    543 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
    544 can generate <abbr>TZif</abbr> and other files, and a companion website
    545 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
    546 convert timestamps, view transition histories, and download code and data.
    547 It is freely available under the <abbr>MIT</abbr> license.</li>
    548 </ul>
    549 The proposed <a
    550 href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
    551 objects</a> let programs access an abstract view of
    552 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
    553 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
    554 problematic <code>Date</code> objects</a> when working with dates and times.
    555 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    556 compiler from <code><abbr>tz</abbr></code> source into
    557 <a href="https://julialang.org/">Julia</a>. It is freely available
    558 under the <abbr>MIT</abbr> license.</li>
    559 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
    560 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    561 title="Free Pascal Compiler">FPC</abbr></a>
    562 compiles from <code><abbr>tz</abbr></code> source into
    563 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    564 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    565 and <a
    566 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    567 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    568 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    569 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    570 <a href="https://www.python.org">Python</a>.
    571 It is freely available under a <abbr>BSD</abbr>-style license.
    572 In code that can assume Python 3.6 or later it is largely superseded; see <a
    573 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
    574 The Fastest Footgun in the West</a>.</li>
    575 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    576 Ruby Timezone Library</a>
    577 compiles <code><abbr>tz</abbr></code> source into
    578 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    579 It is freely available under the <abbr>MIT</abbr> license.</li>
    580 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
    581 Library</a> is
    582 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    583 library that compiles <code><abbr>tz</abbr></code> source into a time
    584 zone repository whose format
    585 is either proprietary or an <abbr>XML</abbr>-encoded
    586 representation.</li>
    587 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    588 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    589 source into text files, along with a runtime that can read those
    590 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    591 license.</li>
    592 </ul>
    593 </section>
    594 
    595 <section>
    596 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    597 <ul>
    598 <li>The <a
    599 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    600 Library</a>
    601 has an independent, thread-safe implementation of
    602 a <abbr>TZif</abbr> file reader.
    603 This library is freely available under the LGPL
    604 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    605 <li><a href="https://www.gnome.org">GNOME</a>'s
    606 <a href="https://developer.gnome.org/glib/">GLib</a> has
    607 a <abbr>TZif</abbr> file reader written in C that
    608 creates a <code>GTimeZone</code> object representing sets
    609 of <abbr>UT</abbr> offsets.
    610 It is freely available under the <abbr>LGPL</abbr>.</li>
    611 <li>The
    612 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    613 <code>baltzo::TimeZoneUtil</code> component contains a C++
    614 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    615 the Apache License.</li>
    616 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    617 library that translates between <abbr>UT</abbr> and civil time and
    618 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    619 License.</li>
    620 <li>The
    621 <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
    622 package</a> contains Python code
    623 to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
    624 <code><abbr>tz</abbr></code> settings to POSIX.1-2017-like approximations.
    625 For example, it maps <code>"Africa/Cairo"</code>
    626 to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
    627 an approximation valid for Cairo timestamps from 2023 on.
    628 This can help porting to platforms that support only POSIX.1-2017.
    629 The package is freely available under the MIT license.</li>
    630 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    631 library that reads <abbr>TZif</abbr> files and converts
    632 timestamps from one time zone or format to another.
    633 It is used by <a href="https://secure.php.net"><abbr
    634 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    635 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    636 and <a href="https://www.mongodb.com">MongoDB</a>.
    637 It is freely available under the <abbr>MIT</abbr> license.</li>
    638 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    639 <abbr>TZif</abbr> file reader.</li>
    640 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    641 DateTime::TimeZone::Tzfile</a>
    642 is a <abbr>TZif</abbr> file reader written in Perl.
    643 It is freely available under the same terms as Perl
    644 (dual <abbr>GPL</abbr> and Artistic license).</li>
    645 <li>Python has a <a id="python-zoneinfo"
    646 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    647 class</a> that reads <abbr>TZif</abbr> data and creates objects
    648 that represent <code><abbr>tzdb</abbr></code> timezones.
    649 Python is freely available under the
    650 <a href="https://docs.python.org/3/license.html">Python Software Foundation
    651 License</a>.
    652 A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
    653 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    654 supplies TZif data if the underlying system data cannot be found;
    655 it is freely available under the Apache License.</li>
    656 <li>The
    657 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    658 library contains a Python tool that
    659 converts <abbr>TZif</abbr> data into
    660 <abbr>JSON</abbr>-format data suitable for use
    661 in its JavaScript library for time zone conversion. Dates before 1970
    662 are not supported.</li>
    663 <li>The <a
    664 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    665 package contains <a href="https://www.haskell.org">Haskell</a> code that
    666 parses and uses <abbr>TZif</abbr> data. It is freely
    667 available under a <abbr>BSD</abbr>-style license.</li>
    668 </ul>
    669 </section>
    670 
    671 <section>
    672 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    673 <ul>
    674 <li><a href="https://foxclocks.org">FoxClocks</a>
    675 is an extension for <a href="https://www.google.com/chrome/">Google
    676 Chrome</a>, <a
    677 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    678 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    679 It displays multiple clocks in the application window, and has a mapping
    680 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    681 It is freely available under the <abbr>GPL</abbr>.</li>
    682 <li><a href="https://golang.org">Go programming language</a>
    683 implementations contain a copy of a 32-bit subset of a recent
    684 <code><abbr>tz</abbr></code> database in a
    685 Go-specific format.</li>
    686 <li>Microsoft Windows 8.1
    687 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    688 data (mentioned <a href="#CLDR">below</a>) used by the
    689 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    690 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    691 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    692 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    693 <a id="System.TimeZoneInfo"
    694 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    695 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    696 the older, proprietary method of Microsoft Windows 2000 and later,
    697 which stores time zone data in the
    698 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    699 <a
    700 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    701 Tzid table</a> or <a
    702 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    703 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    704 to <code><abbr>tz</abbr></code> names.
    705 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    706 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    707 <li><a
    708 href="https://www.oracle.com/java/index.html">Oracle
    709 Java</a> contains a copy of a subset of a recent
    710 <code><abbr>tz</abbr></code> database in a
    711 Java-specific format.</li>
    712 </ul>
    713 </section>
    714 
    715 <section>
    716 <h2 id="other-dbs">Other time zone databases</h2>
    717 <ul>
    718 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    719 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    720 time zone history atlases
    721 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    722 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
    723 Although these extensive atlases
    724 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    725 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    726 they are unreliable as Shanks appears to have
    727 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    728 sources and do not indicate which entries are guesswork.</li>
    729 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    730 its own <code>tztab</code>(4) format.</li>
    731 <li>Microsoft Windows has proprietary data mentioned
    732 <a href="#System.TimeZoneInfo">above</a>.</li>
    733 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    734 is another time zone database.</li>
    735 <li>The <a
    736 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    737 Schedules Information Manual</a> of the
    738 International Air Transport Association
    739 gives current time zone rules for airports served by commercial aviation.</li>
    740 </ul>
    741 </section>
    742 
    743 <section>
    744 <h2 id="maps">Maps</h2>
    745 <ul>
    746 <li>The <a
    747 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
    748 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
    749 <a href="https://www.cia.gov">US Central Intelligence
    750 Agency (<abbr
    751 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
    752 zone map; the
    753 <a
    754 href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    755 Library Map Collection</a>
    756 of the University of Texas at Austin has copies of
    757 recent editions.
    758 The pictorial quality is good,
    759 but the maps do not indicate daylight saving time,
    760 and parts of the data are a few years out of date.</li>
    761 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    762 with current time</a>
    763 has several fancy time zone maps; it covers Russia particularly well.
    764 The maps' pictorial quality is not quite as good as the
    765 <abbr>CIA</abbr>'s
    766 but the maps are more up to date.</li>
    767 <li><a
    768 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    769 much is time wrong around the world?</a> maps the difference between
    770 mean solar and standard time, highlighting areas such as western China
    771 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    772 </ul>
    773 </section>
    774 
    775 <section>
    776 <h2 id="boundaries">Time zone boundaries</h2>
    777 <p>Geographical boundaries between timezones are available
    778 from several <a
    779 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    780 geolocation</a>
    781 services and other sources.</p>
    782 <ul>
    783 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    784 Boundary Builder</a> extracts
    785 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    786 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    787 Its code is freely available under the <abbr>MIT</abbr> license, and
    788 its data entries are freely available under the
    789 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    790 Open Database License</a>. The borders appear to be quite accurate.
    791 Its main web page lists more than twenty libraries
    792 for looking up a timezone name from a GPS coordinate.</li>
    793 <li>Free access via a network API, if you register a key, is provided by
    794 the <a
    795 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    796 Timezone web service</a>, the <a
    797 href="https://developers.google.com/maps/documentation/timezone/intro">Google
    798 Maps Time Zone API</a>, and
    799 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    800 Commercial network API access is provided
    801 by <a href="https://askgeo.com">AskGeo</a>
    802 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    803 </li>
    804 <li>"<a
    805 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    806 to get a time zone from a location using latitude and longitude
    807 coordinates?</a>" discusses other geolocation possibilities.</li>
    808 <li><a href="http://statoids.com/statoids.html">Administrative
    809 Divisions of Countries ("Statoids")</a> lists
    810 political subdivision data related to time zones.</li>
    811 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
    812 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    813 world time zone boundaries circa 2007, distributed under the
    814 <abbr>GPL</abbr>.</li>
    815 <li>A ship within the <a
    816 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    817 waters</a> of any nation uses that nation's time. In international
    818 waters, time zone boundaries are meridians 15&deg; apart, except that
    819 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    820 wide and are separated by
    821 the 180&deg; meridian (not by the International Date Line, which is
    822 for land and territorial waters only). A captain can change ship's
    823 clocks any time after entering a new time zone; midnight changes are
    824 common.</li>
    825 </ul>
    826 </section>
    827 
    828 <section>
    829 <h2 id="civil">Civil time concepts and history</h2>
    830 <ul>
    831 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    832 Walk through Time</a>
    833 surveys the evolution of timekeeping.</li>
    834 <li>The history of daylight saving time is surveyed in <a
    835 href="http://www.webexhibits.org/daylightsaving/">About Daylight
    836 Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    837 <a href="http://seizethedaylight.com/dst/">A Brief
    838 History of Daylight Saving Time</a>.</li>
    839 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    840 Lords</a> discusses how authoritarians manipulate civil time.</li>
    841 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    842 contains guidelines and best practices for software applications that
    843 deal with civil time.</li>
    844 <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
    845 the International Date Line</a> tells the story of the most important
    846 time zone boundary.</li>
    847 <li><a href="http://statoids.com/tconcept.html">Basic Time
    848 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    849 </ul>
    850 </section>
    851 
    852 <section>
    853 <h2 id="national">National histories of legal time</h2>
    854 <dl>
    855 <dt>Australia</dt>
    856 <dd>The Parliamentary Library commissioned a <a
    857 href="https://parlinfo.aph.gov.au/parlInfo/download/library/prspub/359V6/upload_binary/359v60.pdf">research
    858 paper on daylight saving time in Australia</a>.
    859 The Bureau of Meteorology publishes a list of <a
    860 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    861 Dates of Daylight Savings Time within Australia</a>.</dd>
    862 <dt>Belgium</dt>
    863 <dd>The Royal Observatory of Belgium maintains a table of time in
    864 Belgium (in
    865 <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
    866 hreflang="nl">Dutch</a> and <a
    867 href="https://robinfo.oma.be/fr/astro-info/heure/"
    868 hreflang="fr">French</a>).</dd>
    869 <dt>Brazil</dt>
    870 <dd>The Time Service Department of the National Observatory
    871 records <a href="http://pcdsh01.on.br/DecHV.html"
    872 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    873 Portuguese)</a>.</dd>
    874 <dt>Canada</dt>
    875 <dd>National Research Council Canada publishes current
    876 and some older information about <a
    877 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    878 zones and daylight saving time</a>.</dd>
    879 <dt>Chile</dt>
    880 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    881 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
    882 Chile's official time (in Spanish)</a>.</dd>
    883 <dt>China</dt>
    884 <dd>The Hong Kong Observatory maintains a
    885 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    886  summer time in Hong Kong</a>,
    887 and Macau's Meteorological and Geophysical Bureau maintains a <a
    888 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    889 history for Macau</a>.
    890 Unfortunately the latter is incomplete and has errors.</dd>
    891 <dt>Czech Republic</dt>
    892 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    893 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    894 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
    895 <dt>Germany</dt>
    896 <dd>The National Institute for Science and Technology maintains the <a
    897 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    898 of Legal Time in Germany</a>.</dd>
    899 <dt>Israel</dt>
    900 <dd>The Interior Ministry periodically issues <a
    901 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    902 hreflang="he">announcements (in Hebrew)</a>.</dd>
    903 <dt>Malaysia</dt>
    904 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    905 <dt>Mexico</dt>
    906 <dd>The Investigation and Analysis Service of the Mexican Library of
    907 Congress has published a <a
    908 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    909 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    910 <dt>Netherlands</dt>
    911 <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    912 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    913 covers the history of local time in the Netherlands from ancient times.</dd>
    914 <dt>New Zealand</dt>
    915 <dd>The Department of Internal Affairs maintains a brief <a
    916 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    917 Daylight Saving</a>.</dd>
    918 <dt>Palestine</dt>
    919 <dd>The Ministry of Telecom and IT publishes a <a
    920 href="https://mtit.pna.ps/home/TimeZone"
    921 hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
    922 <dt>Portugal</dt>
    923 <dd>The Lisbon Astronomical Observatory publishes a
    924 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    925 legal time (in Portuguese)</a>.</dd>
    926 <dt>Singapore</dt>
    927 <dd><a id="Singapore"
    928 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    929 is Singapore in the "Wrong" Time Zone?</a> details the
    930 history of legal time in Singapore and Malaysia.</dd>
    931 <dt>United Kingdom</dt>
    932 <dd><a
    933 href="https://www.polyomino.org.uk/british-time/">History of
    934 legal time in Britain</a> discusses in detail the country
    935 with perhaps the best-documented history of clock adjustments.</dd>
    936 <dt>United States</dt>
    937 <dd>The Department of Transportation's <a
    938 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    939 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    940 <dt>Uruguay</dt>
    941 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    942 Navy (SOHMA) publishes an annual <a
    943 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    944 (in Spanish)</a>.</dd>
    945 </dl>
    946 </section>
    947 
    948 <section>
    949 <h2 id="costs">Costs and benefits of time shifts</h2>
    950 <p>Various sources argue for and against daylight saving time and time
    951 zone shifts, and many scientific studies have been conducted. This
    952 section summarizes reviews and position statements based on
    953 scientific literature in the area.</p>
    954 <ul>
    955 <li>In 2022 the American Medical Association issued a
    956 <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
    957 supporting permanent standard time</a> on health grounds.</li>
    958 <li>Carey RN, Sarma KM.
    959 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    960 daylight saving time on road traffic collision risk: a systematic
    961 review</a>.
    962 <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>.
    963 This reviews research literature and concludes that the evidence
    964 neither supports nor refutes road safety benefits from
    965 shifts in time zones.</li>
    966 <li>Havranek T, Herman D, Irsova D.
    967 <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
    968 daylight saving save electricity? A meta-analysis</a>.
    969 <em>Energy J.</em> 2018;39(2):35&ndash;61.
    970 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    971 This analyzes research literature and concludes, "Electricity savings
    972 are larger for countries farther away from the equator, while
    973 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
    974 <li>Malow BA. <a
    975 href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
    976 to abolish the clock change and adopt permanent
    977 standard time in the United States:
    978 a Sleep Research Society position statement</a>.
    979 <em>Sleep.</em> 2022;45(12):zsac236.
    980 doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
    981 After reviewing the scientific literature, the Sleep Research Society
    982 advocates permanent standard time due to its health benefits.
    983 <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
    984 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
    985 is the optimal choice for health and safety:
    986 an American Academy of Sleep Medicine position statement</a>.
    987 <em>J Clin Sleep Med.</em> 2024;20(1):121&ndash;125.
    988 doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
    989 The AASM argues for permanent standard time due to health and safety risks
    990 and economic costs of both <abbr>DST</abbr> transitions and
    991 permanent <abbr>DST</abbr>.</li>
    992 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
    993 <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
    994 should we abolish Daylight Saving Time?</a>
    995 <em>J Biol Rhythms.</em> 2019;34(3):227&ndash;230.
    996 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
    997 The Society for Research on Biological Rhythms
    998 opposes DST changes and permanent DST, and advocates that governments adopt
    999 "permanent Standard Time for the health and safety of their citizens".</li>
   1000 </ul>
   1001 </section>
   1002 
   1003 <section>
   1004 <h2 id="precision">Precision timekeeping</h2>
   1005 <ul>
   1006 <li><a
   1007 href="http://leapsecond.com/hpan/an1289.pdf">The
   1008 Science of Timekeeping</a> is a thorough introduction
   1009 to the theory and practice of precision timekeeping.</li>
   1010 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
   1011 Time 2016</a> contains several freely readable papers.</li>
   1012 <li><a href="https://www.ntp.org"><abbr
   1013 title="Network Time Protocol">NTP</abbr>: The Network
   1014 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
   1015 discusses how to synchronize clocks of
   1016 Internet hosts.</li>
   1017 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
   1018 family of software algorithms can achieve accuracy to a few tens of
   1019 nanoseconds in scalable server farms without special hardware.</li>
   1020 <li>The <a
   1021 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
   1022 Time Protocol</a> (<abbr
   1023 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
   1024 can achieve submicrosecond clock accuracy on a local area network
   1025 with special-purpose hardware.</li>
   1026 <li><a
   1027 href="https://datatracker.ietf.org/doc/html/rfc4833">Timezone
   1028 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
   1029 (Internet <abbr>RFC</abbr> 4833)
   1030 specifies a <a
   1031 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1032 option for a server to configure
   1033 a client's time zone and daylight saving settings automatically.</li>
   1034 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1035 Scales</a> describes astronomical time scales like
   1036 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1037 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1038 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
   1039 <li>The <a href="https://www.iau.org"><abbr
   1040 title="International Astronomical Union">IAU</abbr></a>'s <a
   1041 href="https://www.iausofa.org"><abbr
   1042 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1043 collection contains C and <a
   1044 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1045 code for converting among time scales like
   1046 <abbr title="International Atomic Time">TAI</abbr>,
   1047 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1048 <abbr>UTC</abbr>. It is freely available under the
   1049 <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
   1050 <li><a
   1051 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1052 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1053 diverse local time
   1054 scales used by each landed mission on Mars.</li>
   1055 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1056 dedicated not only to leap seconds but to precise time and frequency
   1057 in general. It covers the state of the art in amateur timekeeping, and
   1058 how the art has progressed over the past few decades.</li>
   1059 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1060 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1061 and time-signal emissions</a>, International Telecommunication Union &ndash;
   1062 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1063 <li><a
   1064 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1065 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1066 Bulletins</a> contains official publications of the International
   1067 Earth Rotation and Reference Systems Service, which decides when leap
   1068 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
   1069 via an optional "<code>right</code>" configuration where a computer's internal
   1070 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1071 as opposed to the default "<code>posix</code>" configuration
   1072 where the internal clock ignores leap seconds.
   1073 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1074 <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1075 timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1076 which corresponds to the first leap second
   1077 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
   1078 and to
   1079 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
   1080 In practice the two configurations also agree for timestamps before
   1081 1972 even though the historical situation is messy, partly because
   1082 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1083 is well-defined for sufficiently old timestamps.</li>
   1084 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1085 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1086 leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1087 half second, even though every <abbr>POSIX</abbr> minute has exactly
   1088 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1089 "<code>posix</code>" configuration, is <a
   1090 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
   1091 the <abbr>NTP</abbr> reference implementation, <a
   1092 href="https://github.com/google/unsmear">supports</a> conversion between
   1093 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
   1094 cloud service providers. However, according to
   1095 <a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">&sect;3.7.1 of
   1096 Network Time Protocol Best Current Practices</a>
   1097 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1098 applications requiring accurate <abbr>UTC</abbr> or civil time,
   1099 and is intended for use only in single, well-controlled environments.</li>
   1100 <li>The <a
   1101 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1102 Second Discussion List</a> covers <a
   1103 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
   1104 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
   1105 discussed further in
   1106 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1107 leap second: its history and possible future</a>.
   1108 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1109 might be redefined
   1110 without Leap Seconds</a> gives pointers on this
   1111 contentious issue.
   1112 The General Conference on Weights and Measures
   1113 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
   1114 to discontinue the use of leap seconds by 2035, replacing them with an
   1115 as-yet-undetermined scheme some time after the year 2135.
   1116 The World Radiocommunication Conference <a
   1117 href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
   1118 in 2023</a> to cooperate with this process.
   1119 </li>
   1120 </ul>
   1121 </section>
   1122 
   1123 <section>
   1124 <h2 id="notation">Time notation</h2>
   1125 <ul>
   1126 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
   1127 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1128 zone names, abbreviations, identifiers, and formats. For example, it
   1129 contains French translations for "Eastern European Summer Time",
   1130 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
   1131 "Bucharest". Its
   1132 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1133 charts</a> show these values for many locales. Data values are available in
   1134 both <abbr title="Locale Data Markup Language">LDML</abbr>
   1135 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1136 <li>
   1137 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1138 the international standard date and time notation</a> covers
   1139 <a
   1140 href="https://www.iso.org/standard/70907.html"><em><abbr
   1141 title="International Organization for Standardization">ISO</abbr>
   1142 8601-1:2019 &ndash; Date and time &ndash; Representations for information
   1143 interchange &ndash; Part 1: Basic rules</em></a>.</li>
   1144 <li>
   1145 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1146 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
   1147 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1148 <li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">&sect;3.3 of
   1149 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1150 specifies the time notation used in email and <a
   1151 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1152 headers.</li>
   1153 <li>
   1154 <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time
   1155 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1156 specifies an <abbr>ISO</abbr> 8601
   1157 profile for use in new Internet
   1158 protocols.</li>
   1159 <li>
   1160 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1161 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1162 formats.</li>
   1163 <li>Alphabetic time zone abbreviations should not be used as unique
   1164 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1165 practice. For example, in English-speaking North America
   1166 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
   1167 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1168 and French-speaking North Americans prefer
   1169 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
   1170 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
   1171 database contains English abbreviations for many timestamps;
   1172 unfortunately some of these abbreviations were merely the database maintainers'
   1173 inventions, and these have been removed when possible.</li>
   1174 <li>Numeric time zone abbreviations typically count hours east of
   1175 <abbr>UT</abbr>, e.g., +09 for Japan and
   1176 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
   1177 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
   1178 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1179 title="Japan Standard Time">JST</abbr>-9"</code> and
   1180 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1181 for Japan and Hawaii, respectively. If the
   1182 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1183 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1184 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1185 confusion, handle old timestamps better, and insulate you better from
   1186 any future changes to the rules. One should never set
   1187 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1188 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1189 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1190 is called "<abbr>GMT</abbr>".</li>
   1191 </ul>
   1192 </section>
   1193 
   1194 <section>
   1195 <h2 id="see-also">See also</h2>
   1196 <ul>
   1197 <li><a href="theory.html">Theory and pragmatics of the
   1198 <code><abbr>tz</abbr></code> code and data</a></li>
   1199 <li><a href="tz-art.html">Time and the Arts</a></li>
   1200 </ul>
   1201 </section>
   1202 
   1203 <footer>
   1204 <hr>
   1205 This web page is in the public domain, so clarified as of
   1206 2009-05-17 by Arthur David Olson.
   1207 <br>
   1208 Please send corrections to this web page to the
   1209 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1210 </footer>
   1211 </body>
   1212 </html>
   1213