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     15 <h1>Time zone and daylight saving time data</h1>
     16 <p>
     17 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
     18 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
     19 rules are controlled by individual
     20 governments. They are sometimes changed with little notice, and their
     21 histories and planned futures are often recorded only fitfully. Here
     22 is a summary of attempts to organize and record relevant data in this
     23 area.
     24 </p>
     25   <nav>
     26     <ul>
     27       <li>The <code><abbr>tz</abbr></code> database product and process
     28 	<ul>
     29 	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
     30 	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
     31 	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
     32 	  <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
     33 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     34 	</ul>
     35       </li>
     36       <li>Uses of the <code><abbr>tz</abbr></code> database
     37 	<ul>
     38 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     39 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     40 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     41 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     42 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     43 	</ul>
     44       </li>
     45       <li>Related data
     46 	<ul>
     47 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     48 	  <li><a href="#maps">Maps</a></li>
     49 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     50 	</ul>
     51       </li>
     52       <li>Timekeeping concepts
     53 	<ul>
     54 	  <li><a href="#civil">Civil time concepts and history</a></li>
     55 	  <li><a href="#national">National histories of legal time</a></li>
     56 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     57 	  <li><a href="#precision">Precision timekeeping</a></li>
     58 	  <li><a href="#notation">Time notation</a></li>
     59 	</ul>
     60       </li>
     61       <li><a href="#see-also">See also</a></li>
     62     </ul>
     63   </nav>
     64 
     65 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     66 <p>
     67 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     68 time zone database contains code and data
     69 that represent the history of local time
     70 for many representative locations around the globe.
     71 It is updated periodically to reflect changes made by political bodies
     72 to time zone boundaries and daylight saving rules.
     73 This database (known as <code><abbr>tz</abbr></code>,
     74 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     75 is used by several implementations,
     76 including
     77 <a href="https://www.gnu.org/software/libc/">the
     78 <abbr title="GNUs Not Unix">GNU</abbr>
     79 C Library</a> (used in
     80 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     81 <a href="https://www.android.com">Android</a>,
     82 <a href="https://www.freebsd.org">Free<abbr
     83 title="Berkeley Software Distribution">BSD</abbr></a>,
     84 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     85 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     86 <a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>,
     87 <a href="https://en.wikipedia.org/wiki/OpenHarmony">OpenHarmony</a>,
     88 <a href="https://cygwin.com">Cygwin</a>,
     89 <a href="https://mariadb.org">MariaDB</a>,
     90 <a href="https://musl.libc.org">musl libc</a>,
     91 <a href="https://www.mysql.com">MySQL</a>,
     92 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     93 title="Web Operating System">webOS</abbr></a>,
     94 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     95 title="Advanced Interactive eXecutive">AIX</abbr></a>,
     96 <a href="https://www.apple.com/os/ios/"><abbr
     97 title="iPhone OS">iOS</abbr></a>,
     98 <a href="https://www.apple.com/os/macos/">macOS</a>,
     99 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
    100 <a href="https://vmssoftware.com">Open<abbr
    101 title="Virtual Memory System">VMS</abbr></a>,
    102 <a href="https://www.oracle.com/database/">Oracle Database</a>,
    103 <a href="https://www.oracle.com/solaris/solaris11/">Oracle Solaris</a>,
    104 and <a href="https://qnx.software/en">QNX</a>.</p>
    105 <p>
    106 Each main entry in the database represents a <dfn>timezone</dfn>
    107 for a set of civil-time clocks that have all agreed since 1970.
    108 Timezones are typically identified by continent or ocean and then by the
    109 name of the largest city within the region containing the clocks.
    110 For example, <code>America/New_York</code>
    111 represents most of the <abbr title="United States">US</abbr> eastern time zone;
    112 <code>America/Phoenix</code> represents most of Arizona, which
    113 uses mountain time without daylight saving time (<abbr>DST</abbr>);
    114 <code>America/Detroit</code> represents most of Michigan, which uses
    115 eastern time but with different <abbr>DST</abbr> rules in 1975;
    116 and other entries represent smaller regions like Starke County,
    117 Indiana, which switched from central to eastern time in 1991
    118 and switched back in 2006.
    119 To use the database on a <a
    120 href="https://en.wikipedia.org/wiki/POSIX"><abbr
    121 title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
    122 implementation set the <code><abbr>TZ</abbr></code>
    123 environment variable to the locations full name,
    124 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    125 <p>
    126 Associated with each timezone is a history of offsets from
    127 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    128 Time</a> (<abbr>UT</abbr>), which is <a
    129 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    130 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    131 for timestamps after 1960 this is more precisely <a
    132 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    133 Universal Time</a> (<abbr>UTC</abbr>).
    134 The database also records when daylight saving time was in use,
    135 along with some time zone abbreviations such as <abbr>EST</abbr>
    136 for Eastern Standard Time in the <abbr>US</abbr>.</p>
    137 
    138 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    139 <p>
    140 The following <a
    141 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    142 the latest releases two
    143 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    144 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    145 <pre><code>mkdir tzdb
    146 cd tzdb
    147 <a href="https://www.gnu.org/software/wget/">wget</a> https://data.iana.org/time-zones/tzcode-latest.tar.gz
    148 wget https://data.iana.org/time-zones/tzdata-latest.tar.gz
    149 <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 -
    150 gzip -dc tzdata-latest.tar.gz | tar -xf -
    151 </code></pre>
    152 <p>Alternatively, the following shell commands download the same
    153 release in a single-tarball format containing extra data
    154 useful for regression testing:</p>
    155 <pre><code>wget <a href="https://data.iana.org/time-zones/tzdb-latest.tar.lz">https://data.iana.org/time-zones/tzdb-latest.tar.lz</a>
    156 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    157 </code></pre>
    158 <p>These commands use convenience links to the latest release
    159 of the <code><abbr>tz</abbr></code> database hosted by the
    160 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    161 of the <a href="https://www.iana.org">Internet Assigned Numbers
    162 Authority (IANA)</a>.
    163 Older releases are in files named
    164 <code>tzcode<var>V</var>.tar.gz</code>,
    165 <code>tzdata<var>V</var>.tar.gz</code>, and
    166 <code>tzdb-<var>V</var>.tar.lz</code>,
    167 where <code><var>V</var></code> is the version.
    168 Since 1996, each version has been a four-digit year followed by
    169 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    170 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    171 through <samp>zzz</samp>, and so on).
    172 Since version 2022a, each release has been distributed in
    173 <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIX
    174 ustar interchange format</a>, compressed as described above;
    175 older releases use a nearly compatible format.
    176 Since version 2016h, each release has contained a text file named
    177 <code>version</code> whose first (and currently only) line is the version.
    178 <a href="https://ftp.iana.org/tz/releases/">Older archived releases are
    179 available</a> via
    180 <a href="https://en.wikipedia.org/wiki/HTTPS"><abbr
    181 title="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>,
    182 <a href="https://en.wikipedia.org/wiki/Rsync"><abbr
    183 title="remote sync">rsync</abbr></a>, and
    184 <a href="https://en.wikipedia.org/wiki/FTP"><abbr
    185 title="File Transfer Protocol">FTP</abbr></a>.</p>
    186 <p>Alternatively, a development repository of code and data can be
    187 retrieved from <a href="https://github.com">GitHub</a> via the shell
    188 command:</p>
    189 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    190 </code></pre>
    191 <p>
    192 Since version 2012e, each release has been tagged in development repositories.
    193 Untagged commits are less well tested and probably contain
    194 more errors.</p>
    195 <p>
    196 After obtaining the code and data files, see the
    197 <code>README</code> file for what to do next.
    198 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    199 machine-readable binary files, one for each location.
    200 The binary file format is specified by
    201 <a href="https://www.rfc-editor.org/info/rfc9636/">The
    202 Time Zone Information Format (<abbr>TZif</abbr>)</a>
    203 (Internet <abbr title="Request For Comments">RFC</abbr> 9636).
    204 The code also lets
    205 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    206 location.</p>
    207 
    208 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    209 <p>
    210 The <code><abbr>tz</abbr></code> code and data
    211 are by no means authoritative. If you find errors, please
    212 email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    213 the time zone mailing list.
    214 The mailing list and its archives are public,
    215 so please do not send confidential information.
    216 See
    217 <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
    218 lists main page</a> to subscribe or to browse its archive of old messages.
    219 <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
    220 discussions</a> and corresponding data changes can be
    221 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    222 </p>
    223 <p>
    224 Changes to the <code><abbr>tz</abbr></code> code and data are often
    225 propagated to clients via operating system updates, so
    226 client <code><abbr>tz</abbr></code> data can often be corrected by
    227 applying these updates. With GNU/Linux and similar systems, if your
    228 maintenance provider has not yet adopted the
    229 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    230 the process by tailoring the generic instructions in
    231 the <code><abbr>tz</abbr> README</code> file and installing the latest
    232 data yourself. System-specific instructions for installing the
    233 latest <code><abbr>tz</abbr></code> data have also been published
    234 for <a href="https://community.ibm.com/community/user/blogs/ravindra-shinde/2024/12/13/time-zone-update-tool-tz"><abbr>AIX</abbr></a>
    235 (and
    236 <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates">older
    237 <abbr>AIX</abbr></a>),
    238 <a
    239 href="https://source.android.com/docs/core/permissions/timezone-rules">Android</a>,
    240 <a
    241 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
    242 title="International Components for Unicode">ICU</abbr></a>,
    243 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
    244 JDK</a>,
    245 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    246 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    247 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    248 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
    249 </p>
    250 <p>Since version 2013a,
    251 sources for the <code><abbr>tz</abbr></code> database have been
    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://gedit-text-editor.org">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://www.rfc-editor.org/info/rfc6557/">Procedures for
    272 Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
    273 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 
    280 <h2 id="coordinating">Coordinating with governments and distributors</h2>
    281 <p>
    282 As discussed in
    283 <a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
    284 Time Zones Are Coordinated</a>, the time zone database relies on
    285 collaboration among governments, the time zone database volunteer
    286 community, and data distributors downstream.</p>
    287 <p>
    288 If your government plans to change its time zone boundaries or
    289 daylight saving rules, please send email as described in
    290 <a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>.
    291 Do this well in advance,
    292 as this will lessen confusion and will coordinate updates to many cell phones,
    293 computers, and other devices around the world.
    294 In your email, please cite the legislation or regulation that specifies
    295 the change, so that it can be checked for details such as the exact times
    296 when clock transitions occur.
    297 It is OK if a rule change is planned to affect clocks
    298 far into the future, as a long-planned change can easily be reverted
    299 or otherwise altered with a years notice before the change would have
    300 affected clocks.</p>
    301 <p>
    302 There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
    303 However, typically a release occurs every few months.
    304 Many downstream timezone data distributors wait for
    305 a <code><abbr>tzdb</abbr></code> release before they produce an update
    306 to time zone behavior in consumer devices and software products.
    307 After a release, various parties must integrate, test,
    308 and roll out an update before <a
    309 href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
    310 These updates can be expensive, for both the <a
    311 href="https://en.wikipedia.org/wiki/Quality_assurance">quality
    312 assurance</a> process and the overall cost of shipping and installing
    313 updates to each devices copy of <code><abbr>tzdb</abbr></code>.
    314 Updates may be batched with other updates and may take substantial
    315 time to reach end users after a release.
    316 Older devices may no longer be supported and thus may never be updated,
    317 which means they will continue to use out-of-date rules.</p>
    318 <p>
    319 For these reasons any rule change should be promulgated at least a
    320 year before it affects how clocks operate; otherwise, there is a good
    321 chance that many clocks will be wrong due to delays in propagating updates,
    322 and that residents will be confused or even actively resist the change.
    323 The shorter the notice, the more likely clock problems will arise; see <a
    324 href="https://codeofmatt.com/on-the-timing-of-time-zone-changes/">On
    325 the Timing of Time Zone Changes</a> for examples.
    326 </p>
    327 
    328 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    329 <ul>
    330 <li>The article
    331 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    332 an encyclopedic summary.</li>
    333 <li><a href="tz-how-to.html">How to Read the
    334 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    335 database source format.</li>
    336 <li><a
    337 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    338 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    339 databases style.</li>
    340 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
    341 managing time in the internet</a> analyzes the database longitudinally.</li>
    342 </ul>
    343 
    344 <h2 id="web">Web sites using recent versions of the
    345 <code><abbr>tz</abbr></code> database</h2>
    346 <p>
    347 These are listed roughly in ascending order of complexity and fanciness.
    348 </p>
    349 <ul>
    350 <li><a href="https://time.is">Time.is</a> shows locations
    351 time and zones.</li>
    352 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    353 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    354 <a href="https://www.worldclock.com">The World Clock</a>
    355 are time zone converters.</li>
    356 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
    357 publishes <code><abbr>tzdb</abbr></code>-derived data in
    358 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
    359 title="comma-separated values">CSV</abbr></a> and
    360 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
    361 title="Structured Query Language">SQL</abbr></a> form.</li>
    362 <li><a
    363 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    364 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    365 <li><a
    366 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    367 Time in 1000 Places</a> uses descriptions of the values.</li>
    368 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock 
    369 Worldwide</a> lets you sort zone names and convert times.</li>
    370 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    371 time map and a time converter.</li>
    372 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    373 calculates the current time difference between locations.</li>
    374 <li><a href="https://www.wx-now.com">Weather Now</a> and
    375 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    376 </ul>
    377 
    378 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    379 <ul>
    380 <li><a href="https://www.rfc-editor.org/info/rfc7808/">Time Zone
    381 Data Distribution Service</a> (Internet <abbr>RFC</abbr> 7808)
    382 is associated with
    383 <a href="https://www.rfc-editor.org/info/rfc7809/">CalDAV</a>
    384 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    385 transferring time zone data by reference.</li>
    386 <li>The <a href="https://www.rfc-editor.org/info/rfc5545/">iCalendar format</a>
    387 (Internet <abbr>RFC</abbr> 5545)
    388 covers time zone
    389 data; see its VTIMEZONE calendar component.
    390 The iCalendar format requires specialized parsers and generators; a
    391 variant <a href="https://www.rfc-editor.org/info/rfc6321/">xCal</a>
    392 (Internet <abbr>RFC</abbr> 6321) uses
    393 <a href="https://www.w3.org/XML/"><abbr
    394 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    395 <a href="https://www.rfc-editor.org/info/rfc7265/">jCal</a>
    396 (Internet <abbr>RFC</abbr> 7265)
    397 uses <a href="https://www.json.org/json-en.html"><abbr
    398 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    399 </ul>
    400 
    401 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    402 <p>Although some of these do not fully support
    403 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    404 distributions you can generally work around compatibility problems by
    405 running the command <code>make rearguard_tarballs</code> and compiling
    406 from the resulting tarballs instead.</p>
    407 <ul>
    408 <li><a href="https://github.com/libical">Libical</a>
    409 contains <code>vzic</code>, a <a
    410 href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
    411 program that compiles
    412 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    413 Vzic is freely
    414 available under the <a
    415 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    416 General Public License (<abbr
    417 title="General Public License">GPL</abbr>)</a>.</li>
    418 <li>The <a id="TZDP"
    419 href="https://howardhinnant.github.io/date/tz.html">Time Zone
    420 Database Parser</a> contains a
    421 <a href="https://en.wikipedia.org/wiki/C++">C++</a> runtime parser
    422 supporting the <a
    423 href="https://en.cppreference.com/cpp/chrono#Time_zone"><code>std::chrono::time_zone</code> API</a>
    424 that is standardized in
    425 <a href="https://en.wikipedia.org/wiki/C%2B%2B20">C++20</a> and later.
    426 It is freely available under the
    427 <a href="https://en.wikipedia.org/wiki/MIT_License"><abbr
    428 title="Massachusetts Institute of Technology">MIT</abbr> license</a>.
    429 This approach is used by the <code>std::chrono::time_zone</code> implementation
    430 of <a href="https://gcc.gnu.org/"><abbr
    431 title="the GNU Compiler Collection">GCC</abbr></a>
    432 <code>libstdc++</code>, which is available under the GPL combined
    433 with the <a href="https://www.gnu.org/licenses/gcc-exception.html">GCC
    434 Runtime Library Exception</a>.</li>
    435 <li><a
    436 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    437 contains a script <code>parse_olson</code> that compiles
    438 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    439 modules. It is part of the Perl <a
    440 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    441 which is freely
    442 available under both the <abbr>GPL</abbr> and the Perl Artistic
    443 License. DateTime::TimeZone also contains a script
    444 <code>tests_from_zdump</code> that generates test cases for each clock
    445 transition in the <code><abbr>tz</abbr></code> database.</li>
    446 <li><a id="ICU" href="https://icu.unicode.org">International Components for
    447 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    448 href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
    449 libraries for internationalization that
    450 has a compiler from <code><abbr>tz</abbr></code> source
    451 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    452 (mentioned <a href="#CLDR">below</a>)
    453 into an <abbr>ICU</abbr>-specific format.
    454 <abbr>ICU</abbr> is freely available under a
    455 <abbr>BSD</abbr>-style license.</li>
    456 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    457 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    458 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    459 title="Application Program Interface">API</abbr>s for use. It is
    460 freely available under the <abbr>MIT</abbr> license.</li>
    461 <li>Java-based compilers and libraries include:
    462 <ul>
    463 <li>The <a
    464 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
    465 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    466 <a href="https://openjdk.org">OpenJDK</a> and
    467 <a href="https://jdk.java.net">Oracle JDK</a>.
    468 Although its source code is proprietary, its executable is available under the
    469 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
    470 Timezone Updater License Agreement</a>.</li>
    471 <li>The <a
    472 href="https://www.oracle.com/technical-resources/articles/java/jf14-date-time.html">Java
    473 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    474 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    475 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    476 <li><a href="https://www.joda.org/joda-time/">Joda-Time  Java date
    477 and time <abbr>API</abbr></a> contains a class
    478 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    479 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    480 Java 8 <code>java.time</code>, which its users should migrate to once
    481 they can assume Java 8 or later. It is available under the <a
    482 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    483 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
    484 href="https://www.azul.com/products/components/ziupdater-time-zone-tool/">ZIUpdater</a>
    485 are alternatives to TZUpdater. IANA Updaters license is unclear;
    486 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
    487 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    488 time library for Android</a> and
    489 <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,
    490 time and interval library for Java</a> compile
    491 <code><abbr>tz</abbr></code> source into a binary format.
    492 Time4A is available under the Apache License and Time4J is
    493 available under the <a
    494 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    495 General Public License (<abbr title="Lesser General Public
    496 License">LGPL</abbr>)</a>.</li>
    497 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    498 Java-based libraries.</li>
    499 </ul>
    500 </li>
    501 <li><a href="https://nodatime.org">Noda Time  Date and
    502 time <abbr>API</abbr> for .NET</a>
    503 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    504 It is freely available under the Apache License.</li>
    505 <li>Many modern
    506 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
    507 runtimes support <code><abbr>tz</abbr></code> natively via the
    508 <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>.
    509 This can be used as-is or with most of the following libraries,
    510 many of which also support runtimes lacking the <code>timeZone</code> option.
    511 <ul>
    512 <li>The <a
    513 href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
    514 timezone polyfill</a>
    515 is freely available under a <abbr>BSD</abbr>-style license.</li>
    516 <li>The <a href="https://date-fns.org">date-fns</a>
    517 library manipulates timezone-aware timestamps in browsers and
    518 in <a href="https://nodejs.org/en/">Node.js</a>.
    519 It is freely available under the <abbr>MIT</abbr> license.</li>
    520 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
    521 minimalist replacement for the date and time API of
    522 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
    523 manipulation library.
    524 It is freely available under the <abbr>MIT</abbr> license.</li>
    525 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
    526 timezone support for the <code>Intl</code> API.
    527 It is freely available under the <abbr>MIT</abbr> license.</li>
    528 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    529 Moment.js plugin.
    530 It is freely available under the <abbr>MIT</abbr> license.</li>
    531 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    532 JavaScript library that supports date arithmetic that is time zone
    533 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    534 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
    535 supports live <code><abbr>tzdb</abbr></code> updates,
    536 astronomical and atomic time, a command-line interface,
    537 and full <a
    538 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
    539 Its companion <a
    540 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
    541 can generate <abbr>TZif</abbr> and other files, and a companion website
    542 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
    543 convert timestamps, view transition histories, and download code and data.
    544 It is freely available under the <abbr>MIT</abbr> license.</li>
    545 </ul>
    546 The proposed <a
    547 href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
    548 objects</a> let programs access an abstract view of
    549 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
    550 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScripts
    551 problematic <code>Date</code> objects</a> when working with dates and times.
    552 </li>
    553 <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
    554 compiler from <code><abbr>tz</abbr></code> source into
    555 <a href="https://julialang.org">Julia</a>. It is freely available
    556 under the <abbr>MIT</abbr> license.</li>
    557 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> 
    558 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    559 title="Free Pascal Compiler">FPC</abbr></a>
    560 compiles from <code><abbr>tz</abbr></code> source into
    561 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    562 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    563 and <a
    564 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    565 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    566 <li><a href="https://pythonhosted.org/pytz/">pytz  World Timezone
    567 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    568 <a href="https://www.python.org">Python</a>.
    569 It is freely available under a <abbr>BSD</abbr>-style license.
    570 In code that can assume Python 3.6 or later it is largely superseded; see <a
    571 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
    572 The Fastest Footgun in the West</a>.</li>
    573 <li><a href="https://tzinfo.github.io">TZInfo 
    574 Ruby Timezone Library</a>
    575 compiles <code><abbr>tz</abbr></code> source into
    576 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    577 It is freely available under the <abbr>MIT</abbr> license.</li>
    578 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
    579 Library</a> is
    580 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    581 library that compiles <code><abbr>tz</abbr></code> source into a time
    582 zone repository whose format
    583 is either proprietary or an <abbr>XML</abbr>-encoded
    584 representation.</li>
    585 <li><a id="Tcl" href="https://www.tcl-lang.org">Tcl</a>
    586 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    587 source into text files, along with a runtime that can read those
    588 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    589 license.</li>
    590 </ul>
    591 
    592 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    593 <ul>
    594 <li>The <a
    595 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    596 Library</a>
    597 has a thread-safe <abbr>TZif</abbr> file reader.
    598 This library is freely available under the LGPL
    599 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    600 <li><a href="https://www.gnome.org">GNOME</a>s
    601 <a href="https://docs.gtk.org/glib/">GLib</a> has
    602 a <abbr>TZif</abbr> file reader written in C that
    603 creates a <code>GTimeZone</code> object representing sets
    604 of <abbr>UT</abbr> offsets.
    605 It is freely available under the <abbr>LGPL</abbr>.</li>
    606 <li>The <a href="https://en.wikipedia.org/wiki/Musl">musl</a> C library
    607 also has an <abbr>TZif</abbr> reader.
    608 It is freely available under the <abbr>MIT</abbr> license.</li>
    609 <li>The Time Zone Database Parser for C++, mentioned <a href="#TZDP">above,</a>
    610 can be configured to read TZif binary files.
    611 A similar approach is taken in
    612 <a href="https://www.llvm.org/"><abbr>LLVM</abbr></a>
    613 <code>std::chrono::time_zone</code>, which is available
    614 under the Apache License with
    615 <a href="https://llvm.org/docs/DeveloperPolicy.html#open-source-licensing-terms">LLVM
    616 Exceptions</a>.</li>
    617 <li>The
    618 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>s
    619 <code>baltzo::TimeZoneUtil</code> component contains a C++
    620 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    621 the Apache License.</li>
    622 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    623 library that translates between <abbr>UT</abbr> and civil time and
    624 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    625 License.</li>
    626 <li>The <a href="https://go.dev">Go programming language</a>
    627 has a <abbr>TZif</abbr> file reader <a
    628 href="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li>
    629 <li>The
    630 <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
    631 package</a> contains Python code
    632 to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
    633 <code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
    634 For example, it maps <code>"Africa/Cairo"</code>
    635 to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
    636 an approximation valid for Cairo timestamps from 2023 on.
    637 This can help porting to platforms that support only proleptic TZ.
    638 The package is freely available under the MIT license.</li>
    639 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    640 library that reads <abbr>TZif</abbr> files and converts
    641 timestamps from one time zone or format to another.
    642 It is used by <a href="https://www.php.net"><abbr
    643 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    644 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    645 and <a href="https://www.mongodb.com">MongoDB</a>.
    646 It is freely available under the <abbr>MIT</abbr> license.</li>
    647 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    648 <abbr>TZif</abbr> file reader.</li>
    649 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    650 DateTime::TimeZone::Tzfile</a>
    651 is a <abbr>TZif</abbr> file reader written in Perl.
    652 It is freely available under the same terms as Perl
    653 (dual <abbr>GPL</abbr> and Artistic license).</li>
    654 <li>Python has a <a id="python-zoneinfo"
    655 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    656 class</a> that reads <abbr>TZif</abbr> data and creates objects
    657 that represent <code><abbr>tzdb</abbr></code> timezones.
    658 Python is freely available under the
    659 <a href="https://docs.python.org/3/license.html">Python Software Foundation
    660 License</a>.
    661 A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
    662 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    663 supplies TZif data if the underlying system data cannot be found;
    664 it is freely available under the Apache License.</li>
    665 <li>The
    666 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    667 library contains a Python tool that
    668 converts <abbr>TZif</abbr> data into
    669 <abbr>JSON</abbr>-format data suitable for use
    670 in its JavaScript library for time zone conversion. Dates before 1970
    671 are not supported.</li>
    672 <li>The <a href="https://crates.io/crates/jiff">jiff</a> datetime library
    673 for <a href="https://rust-lang.org/">Rust</a> either compiles
    674 <code><abbr>tz</abbr></code> source into Rust,
    675 or consults <abbr>TZif</abbr> data at runtime.
    676 It is in the public domain.
    677 It intends to supplant the soft-deprecated
    678 <a href="https://crates.io/crates/chrono-tz">Chrono-TZ</a>.</li>
    679 <li>The <a
    680 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    681 package contains <a href="https://www.haskell.org">Haskell</a> code that
    682 parses and uses <abbr>TZif</abbr> data. It is freely
    683 available under a <abbr>BSD</abbr>-style license.</li>
    684 </ul>
    685 
    686 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    687 <ul>
    688 <li><a href="https://foxclocks.org">FoxClocks</a>
    689 is an extension for <a href="https://www.google.com/chrome/">Google
    690 Chrome</a>, <a
    691 href="https://www.firefox.com/en-US/">Firefox</a> and <a
    692 href="https://www.thunderbird.net/en-US/">Thunderbird</a>.
    693 It displays multiple clocks in the application window, and has a mapping
    694 interface to <a href="https://earth.google.com/web/">Google Earth</a>.
    695 It is freely available under the <abbr>GPL</abbr>.</li>
    696 <li>Microsoft Windows 8.1
    697 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    698 data (mentioned <a href="#CLDR">below</a>) used by the
    699 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    700 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    701 <a href="https://learn.microsoft.com/en-us/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    702 <a href="https://learn.microsoft.com/en-us/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>,
    703 and with updates discussed in the
    704 <a href="https://techcommunity.microsoft.com/category/windows/blog/dstblog">Daylight
    705 Saving Time &amp; Time Zone Blog</a>.
    706 <a id="System.TimeZoneInfo"
    707 href="https://learn.microsoft.com/en-us/archive/blogs/bclteam/exploring-windows-time-zones-with-system-timezoneinfo-josh-free">Exploring
    708 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    709 the older, proprietary method of Microsoft Windows 2000 and later,
    710 which stores time zone data in the
    711 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    712 <a
    713 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone
    714  Tzid table</a> or <a
    715 href="https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    716 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    717 to <code><abbr>tz</abbr></code> names.
    718 These mappings can be performed programmatically via the
    719 <a href="https://github.com/mattjohnsonpint/TimeZoneConverter">TimeZoneConverter</a>
    720 .NET library,
    721 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.</li>
    722 <li><a
    723 href="https://www.oracle.com/java/">Oracle
    724 Java</a> contains a copy of a subset of a recent
    725 <code><abbr>tz</abbr></code> database in a
    726 Java-specific format.</li>
    727 </ul>
    728 
    729 <h2 id="other-dbs">Other time zone databases</h2>
    730 <ul>
    731 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    732 is Astrodiensts Web version of Shanks and Pottengers out-of-print
    733 time zone history atlases
    734 <a href="https://search.worldcat.org/title/468828649">for the US</a> and
    735 <a href="https://search.worldcat.org/title/76950459">for the world</a>.
    736 Although these extensive atlases
    737 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    738 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    739 they are unreliable as Shanks appears to have
    740 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    741 sources and do not indicate which entries are guesswork.</li>
    742 <li>Microsoft Windows has proprietary data mentioned
    743 <a href="#System.TimeZoneInfo">above</a>.</li>
    744 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    745 is another time zone database.</li>
    746 <li>The <a
    747 href="https://www.iata.org/en/publications/manuals/standard-schedules-information">Standard
    748 Schedules Information Manual</a> of the
    749 International Air Transport Association
    750 gives current time zone rules for airports served by commercial aviation.</li>
    751 </ul>
    752 
    753 <h2 id="maps">Maps</h2>
    754 <ul>
    755 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    756 with current time</a>
    757 has several fancy time zone maps; it covers Russia particularly well.</li>
    758 <li><a
    759 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    760 much is time wrong around the world?</a> maps the difference between
    761 mean solar and standard time, highlighting areas such as western China
    762 where the two differ greatly. Its a bit out of date, unfortunately.</li>
    763 <li>The
    764 <a
    765 href="https://maps.lib.utexas.edu/maps/world.html">PerryCastaeda
    766 Library Map Collection</a> of the University of Texas at Austin has
    767 copies of old maps taken from <a
    768 href="https://en.wikipedia.org/wiki/The_World_Factbook"><em>The
    769 World Factbook</em></a>, formerly published by the
    770 <a href="https://www.cia.gov">US Central Intelligence Agency</a>.
    771 Although the maps pictorial quality is good,
    772 the maps do not indicate daylight saving time.</li>
    773 </ul>
    774 
    775 <h2 id="boundaries">Time zone boundaries</h2>
    776 <p>Geographical boundaries between timezones are available
    777 from several <a
    778 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    779 geolocation</a>
    780 services and other sources.</p>
    781 <ul>
    782 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    783 Boundary Builder</a> extracts
    784 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    785 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    786 Its code is freely available under the <abbr>MIT</abbr> license, and
    787 its data entries are freely available under the
    788 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    789 Open Database License</a>. The borders appear to be quite accurate.
    790 Its main web page lists more than twenty libraries
    791 for looking up a timezone name from a GPS coordinate.</li>
    792 <li>Free access via a network API, if you register a key, is provided by
    793 the <a
    794 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    795 Timezone web service</a>, the <a
    796 href="https://developers.google.com/maps/documentation/timezone/overview">Google
    797 Maps Time Zone API</a>, and
    798 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    799 Commercial network API access is provided
    800 by <a href="https://askgeo.com">AskGeo</a>
    801 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    802 </li>
    803 <li><a
    804 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    805 to get a time zone from a location using latitude and longitude
    806 coordinates?</a> discusses other geolocation possibilities.</li>
    807 <li><a href="https://statoids.com/statoids.html">Administrative
    808 Divisions of Countries (Statoids)</a> lists
    809 political subdivision data related to time zones.</li>
    810 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
    811  GIS and Database Tools</a> includes a Manifold-format map of
    812 world time zone boundaries circa 2007, distributed under the
    813 <abbr>GPL</abbr>.</li>
    814 <li>A ship within the <a
    815 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    816 waters</a> of any nation uses that nations time. In international
    817 waters, time zone boundaries are meridians 15 apart, except that
    818 <abbr>UT</abbr>12 and <abbr>UT</abbr>+12 are each 7.5
    819 wide and are separated by
    820 the 180 meridian (not by the International Date Line, which is
    821 for land and territorial waters only). A captain can change ships
    822 clocks any time after entering a new time zone; midnight changes are
    823 common.</li>
    824 </ul>
    825 
    826 <h2 id="civil">Civil time concepts and history</h2>
    827 <ul>
    828 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    829 Walk through Time</a>
    830 surveys the evolution of timekeeping.</li>
    831 <li>The history of daylight saving time is surveyed in <a
    832 href="https://www.webexhibits.org/daylightsaving/">About Daylight
    833 Saving Time  History, rationale, laws &amp; dates</a> and summarized in
    834 <a href="http://seizethedaylight.com/dst/">A Brief
    835 History of Daylight Saving Time</a>.</li>
    836 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    837 Lords</a> discusses how authoritarians manipulate civil time.</li>
    838 <li><a href="https://www.w3.org/TR/timezone/">Working with Time
    839 and Time Zones</a>
    840 contains guidelines and best practices for software applications that
    841 deal with civil time.</li>
    842 <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
    843 the International Date Line</a> tells the story of the most important
    844 time zone boundary.</li>
    845 <li><a href="https://statoids.com/tconcept.html">Basic Time
    846 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    847 </ul>
    848 
    849 <h2 id="national">National histories of legal time</h2>
    850 <dl>
    851 <dt>Australia</dt>
    852 <dd>The Bureau of Meteorology publishes a list of <a
    853 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    854 Dates of Daylight Savings Time within Australia</a>.</dd>
    855 <dt>Belgium</dt>
    856 <dd>The Royal Observatory of Belgium maintains a table of time in
    857 Belgium (in
    858 <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
    859 hreflang="nl">Dutch</a> and <a
    860 href="https://robinfo.oma.be/fr/astro-info/heure/"
    861 hreflang="fr">French</a>).</dd>
    862 <dt>Brazil</dt>
    863 <dd>The Time Service Department of the National Observatory
    864 records <a href="http://pcdsh01.on.br/DecHV.html"
    865 hreflang="pt-BR">Brazils daylight saving time decrees (in
    866 Portuguese)</a>.</dd>
    867 <dt>Canada</dt>
    868 <dd>National Research Council Canada publishes current
    869 and some older information about <a
    870 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    871 zones and daylight saving time</a>.</dd>
    872 <dt>Chile</dt>
    873 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    874 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
    875 Chiles official time (in Spanish)</a>.</dd>
    876 <dt>China</dt>
    877 <dd>The Hong Kong Observatory maintains a
    878 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    879  summer time in Hong Kong</a>,
    880 and Macaus Meteorological and Geophysical Bureau maintains a <a
    881 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    882 history for Macau</a>.
    883 Unfortunately the latter is incomplete and has errors.</dd>
    884 <dt>Czech Republic</dt>
    885 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    886 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    887 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
    888 <dt>Germany</dt>
    889 <dd>The National Institute for Science and Technology maintains the <a
    890 href="https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    891 of Legal Time in Germany</a>.</dd>
    892 <dt>Israel</dt>
    893 <dd><a
    894 href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
    895 lists official time-change announcements and laws since 1940,
    896 almost all in Hebrew.</dd>
    897 <dt>Malaysia</dt>
    898 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    899 <dt>Mexico</dt>
    900 <dd>The Investigation and Analysis Service of the Mexican Library of
    901 Congress has published a <a
    902 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    903 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    904 <dt>Netherlands</dt>
    905 <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    906 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    907 covers the history of local time in the Netherlands from ancient times.</dd>
    908 <dt>New Zealand</dt>
    909 <dd>The Department of Internal Affairs maintains a brief <a
    910 href="https://www.govt.nz/browse/recreation-and-the-environment/daylight-saving/history-of-daylight-saving-in-nz/">History
    911 of Daylight Saving in NZ</a>.</dd>
    912 <dt>Palestine</dt>
    913 <dd>The Ministry of Telecom and Digital Economy publishes a <a
    914 href="https://mtde.gov.ps/home/TimeZone"
    915 hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
    916 <dt>Portugal</dt>
    917 <dd>The Lisbon Astronomical Observatory publishes a
    918 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    919 legal time (in Portuguese)</a>.</dd>
    920 <dt>Singapore</dt>
    921 <dd><a id="Singapore"
    922 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    923 is Singapore in the Wrong Time Zone?</a> details the
    924 history of legal time in Singapore and Malaysia.</dd>
    925 <dt>United Kingdom</dt>
    926 <dd><a
    927 href="https://www.polyomino.org.uk/british-time/">History of
    928 legal time in Britain</a> discusses in detail the country
    929 with perhaps the best-documented history of clock adjustments.</dd>
    930 <dt>United States</dt>
    931 <dd>The Department of Transportations <a
    932 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    933 Time Zone Proceedings</a> lists changes to
    934 official written time zone boundaries, and its <a
    935 href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
    936 Zones dataset</a> maps current boundaries.
    937 These boundaries are only for standard time, so the current map puts
    938 all of Arizona in one time zone even though part of Arizona
    939 observes <abbr>DST</abbr> and part does not.</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 
    947 <h2 id="costs">Costs and benefits of time shifts</h2>
    948 <p>Various sources argue for and against daylight saving time and time
    949 zone shifts, and many scientific studies have been conducted. This
    950 section summarizes reviews and position statements based on
    951 scientific literature in the area.</p>
    952 <ul>
    953 <li>Carey RN, Sarma KM.
    954 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    955 daylight saving time on road traffic collision risk: a systematic
    956 review</a>.
    957 <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>.
    958 This reviews research literature and concludes that the evidence
    959 neither supports nor refutes road safety benefits from
    960 shifts in time zones.</li>
    961 <li>Havranek T, Herman D, Irsova D.
    962 Does daylight saving save electricity? A meta-analysis.
    963 <em>Energy J.</em> 2018;39(2):3561.
    964 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    965 This analyzes research literature and concludes, Electricity savings
    966 are larger for countries farther away from the equator, while
    967 subtropical regions consume more electricity because of <abbr>DST</abbr>.</li>
    968 <li>Neumann P, von Blanckenburg K. <a
    969 href="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What
    970 time will it be? A comprehensive literature review on daylight saving time</a>.
    971 <em>Time Soc</em>. 2025;34(4):684745.
    972 doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>.
    973 This reviews <abbr>DST</abbr>s effects on electricity, health, crime, road
    974 safety, and the economy, focusing on research since 2010, and concludes that
    975 year-round standard time is preferable overall.</li>
    976 <li>Romigi A, Franco V, Scoditti E <em>et al</em>.
    977 The effects of daylight saving time and clock time transitions on sleep and
    978 sleepiness: a systematic review. <em>Sleep Med Rev.</em> 2025;84:102161. doi:<a
    979 href="https://doi.org/10.1016/j.smrv.2025.102161">10.1016/j.smrv.2025.102161</a>.
    980 This reviews <abbr>DST</abbr> and <abbr>DST</abbr> transitions,
    981 and concludes that they both harm sleep, health and behavior.</li>
    982 </ul>
    983 
    984 <p>The following medical societies have taken positions on the
    985 advisability of clock shifts:</p>
    986 
    987 <ul>
    988 <li>In 2022 the American Medical Association issued a
    989 <a href="https://www.ama-assn.org/press-center/ama-press-releases/ama-calls-permanent-standard-time">statement
    990 supporting permanent standard time</a> on health grounds.</li>
    991 <li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>.
    992 <a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The
    993 British Sleep Society position statement on Daylight Saving Time in the UK</a>.
    994 <em>J Sleep Res.</em> 2025;34(3):e14352.
    995 doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>.
    996 This recommends that the UK abolish <abbr>DST</abbr> for health reasons.</li>
    997 <li>Malow BA. <a
    998 href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
    999 to abolish the clock change and adopt permanent
   1000 standard time in the United States:
   1001 a Sleep Research Society position statement</a>.
   1002 <em>Sleep.</em> 2022;45(12):zsac236.
   1003 doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
   1004 The Sleep Research Society
   1005 advocates permanent standard time due to its health benefits.</li>
   1006 <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
   1007 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
   1008 is the optimal choice for health and safety:
   1009 an American Academy of Sleep Medicine position statement</a>.
   1010 <em>J Clin Sleep Med.</em> 2024;20(1):121125.
   1011 doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
   1012 The AASM argues for permanent standard time due to health and safety risks
   1013 and economic costs of both <abbr>DST</abbr> transitions and
   1014 permanent <abbr>DST</abbr>.</li>
   1015 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
   1016 <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
   1017 should we abolish Daylight Saving Time?</a>
   1018 <em>J Biol Rhythms.</em> 2019;34(3):227230.
   1019 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
   1020 The Society for Research on Biological Rhythms
   1021 opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
   1022 and advocates that governments adopt
   1023 permanent Standard Time for the health and safety of their citizens.</li>
   1024 </ul>
   1025 
   1026 <h2 id="precision">Precision timekeeping</h2>
   1027 <ul>
   1028 <li><a
   1029 href="http://leapsecond.com/hpan/an1289.pdf">The
   1030 Science of Timekeeping</a> is a thorough introduction
   1031 to the theory and practice of precision timekeeping.</li>
   1032 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
   1033 Time 2016</a> contains several freely readable papers.</li>
   1034 <li><a href="https://www.ntp.org"><abbr
   1035 title="Network Time Protocol">NTP</abbr>: The Network
   1036 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
   1037 discusses how to synchronize clocks of
   1038 Internet hosts.</li>
   1039 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
   1040 family of software algorithms can achieve accuracy to a few tens of
   1041 nanoseconds in scalable server farms without special hardware.</li>
   1042 <li>The <a
   1043 href="https://www.nist.gov/el/intelligent-systems-division-73500/ieee-1588">Precision
   1044 Time Protocol</a> (<abbr
   1045 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
   1046 can achieve submicrosecond clock accuracy on a local area network
   1047 with special-purpose hardware.</li>
   1048 <li><a
   1049 href="https://www.rfc-editor.org/info/rfc4833/">Timezone
   1050 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
   1051 (Internet <abbr>RFC</abbr> 4833)
   1052 specifies a <a
   1053 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1054 option for a server to configure
   1055 a clients time zone and daylight saving settings automatically.</li>
   1056 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1057 Scales</a> describes astronomical time scales like
   1058 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1059 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1060 <abbr title="Barycentric Dynamic Time">TDB</abbr>.</li>
   1061 <li>The <a href="https://www.iau.org"><abbr
   1062 title="International Astronomical Union">IAU</abbr></a>s <a
   1063 href="https://www.iausofa.org"><abbr
   1064 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1065 collection contains C and <a
   1066 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1067 code for converting among time scales like
   1068 <abbr title="International Atomic Time">TAI</abbr>,
   1069 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1070 <abbr>UTC</abbr>. It is freely available under the
   1071 <a href="https://www.iausofa.org/terms-and-conditions">SOFA license</a>.</li>
   1072 <li><a
   1073 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1074  Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1075 diverse local time
   1076 scales used by each landed mission on Mars.</li>
   1077 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1078 dedicated not only to leap seconds but to precise time and frequency
   1079 in general. It covers the state of the art in amateur timekeeping, and
   1080 how the art has progressed over the past few decades.</li>
   1081 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1082 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1083 and time-signal emissions</a>, International Telecommunication Union 
   1084 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1085 <li><a
   1086 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1087 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1088 Bulletins</a> contains official publications of the International
   1089 Earth Rotation and Reference Systems Service, which decides when leap
   1090 seconds occur.
   1091 The <code><abbr>tz</abbr></code> code and data support leap seconds
   1092 via an optional <code>"right"</code> configuration where a computers internal
   1093 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1094 as opposed to the default <code>"posix"</code> configuration
   1095 where the internal clock ignores leap seconds.
   1096 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1097 <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1098 timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1099 which corresponds to the first leap second
   1100 1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration,
   1101 and to
   1102 1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration.
   1103 In practice the two configurations also agree for timestamps before
   1104 1972 even though the historical situation is messy, partly because
   1105 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1106 is well-defined for sufficiently old timestamps.</li>
   1107 <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
   1108 <abbr>NTP</abbr> Leap Second File</a> covers the text file
   1109 <code>leap-seconds.list</code>, which lists the currently known leap seconds.
   1110 The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
   1111 <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
   1112 <a href="https://www.ntp.org">classic
   1113 <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
   1114 and <a href="https://ntpsec.org">NTPsec</a>.
   1115 The <code><abbr>tz</abbr></code> database also distributes leap second
   1116 information in a differently-formatted <code>leapseconds</code> text file,
   1117 as well as in the <code>"right"</code> configuration in binary form; for
   1118 example, <code>right/UTC</code> can be used
   1119 by <a href="https://chrony-project.org"><code>chrony</code></a>,
   1120 another <abbr>NTP</abbr> implementation.</li>
   1121 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1122 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1123 leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1124 half second, even though every <abbr>POSIX</abbr> minute has exactly
   1125 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1126 <code>"posix"</code> configuration, is <a
   1127 href="https://gitlab.com/NTPsec/ntpsec/-/blob/master/docs/leapsmear.adoc">supported</a>
   1128 by the <abbr>NTP</abbr> implementations mentioned above, <a
   1129 href="https://github.com/google/unsmear">supports</a> conversion between
   1130 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
   1131 cloud service providers. However, according to
   1132 <a href="https://www.rfc-editor.org/info/rfc8633#section-3.7.1">3.7.1 of
   1133 Network Time Protocol Best Current Practices</a>
   1134 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1135 applications requiring accurate <abbr>UTC</abbr> or civil time,
   1136 and is intended for use only in single, well-controlled environments.</li>
   1137 <li>The <a
   1138 href="https://groups.io/g/LEAPSECS">LEAPSECS List</a> covers <a
   1139 href="https://gge.ext.unb.ca/Resources/gpsworld.november99.pdf">McCarthy
   1140 and Klepczynskis 1999 proposal to discontinue leap seconds</a>,
   1141 discussed further in
   1142 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1143 leap second: its history and possible future</a>.
   1144 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1145 might be redefined
   1146 without Leap Seconds</a> gives pointers on this
   1147 contentious issue.
   1148 The General Conference on Weights and Measures (CGPM)
   1149 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
   1150 to discontinue the use of leap seconds by 2035, and requested that no
   1151 discontinuous adjustments be made to UTC for at least a century.
   1152 The World Radiocommunication Conference <a
   1153 href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
   1154 in 2023</a> to cooperate with this process. A draft <a
   1155 href="https://www.bipm.org/documents/d/guest/cgpm-2026-draft-resolutions">Resolution
   1156 C to make continuous UTC effective on 2027-05-20</a>,
   1157 and thereby discontinue leap seconds,
   1158 has been scheduled for the 28th CGPM starting 2026-10-13 in Paris.
   1159 One proposal to implement this
   1160 would replace leap seconds with seven 13-second leap smears occurring once per
   1161 decade until 2100, with leap smears after that gradually increasing in size.
   1162 See:
   1163 <ul>
   1164 <li>Levine J. <a href="https://tf.nist.gov/general/pdf/3242.pdf">A
   1165 proposal to change the leap-second adjustments to
   1166 coordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a
   1167 href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>
   1168 with followups in doi:<a
   1169 href="https://doi.org/10.1088/1681-7575/ade314">10.1088/1681-7575/ade314</a>
   1170 and doi:<a
   1171 href="https://doi.org/10.1088/1681-7575/ade315">10.1088/1681-7575/ade315</a>.</li>
   1172 </ul>
   1173 However, there is still no consensus on whether this is the best way
   1174 to replace leap seconds.
   1175 </li>
   1176 </ul>
   1177 
   1178 <h2 id="notation">Time notation</h2>
   1179 <ul>
   1180 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
   1181 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1182 zone names, abbreviations, identifiers, and formats. For example, it
   1183 contains French translations for Eastern European Summer Time,
   1184 <abbr title="Eastern European Summer Time">EEST</abbr>, and Bucharest. Its
   1185 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1186 charts</a> show these values for many locales. Data values are available in
   1187 both <abbr title="Locale Data Markup Language">LDML</abbr>
   1188 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.</li>
   1189 <li>
   1190 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1191 the international standard date and time notation</a> covers
   1192 <a
   1193 href="https://www.iso.org/standard/70907.html"><em><abbr
   1194 title="International Organization for Standardization">ISO</abbr>
   1195 8601-1:2019  Date and time  Representations for information
   1196 interchange  Part 1: Basic rules</em></a>.</li>
   1197 <li>
   1198 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1199 Schema: Datatypes  dateTime</a> specifies a format inspired by
   1200 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1201 <li><a href="https://www.rfc-editor.org/info/rfc5322#section-3.3">3.3 of
   1202 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1203 specifies the time notation used in email and <a
   1204 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1205 headers.</li>
   1206 <li>
   1207 <a href="https://www.rfc-editor.org/info/rfc3339/">Date and Time
   1208 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1209 specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
   1210 An extension, <a href="https://www.rfc-editor.org/info/rfc9557/">Date
   1211 and Time on the Internet: Timestamps with Additional Information</a>
   1212 (Internet <abbr>RFC</abbr> 9557) extends this profile
   1213 to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
   1214 via suffixes like <code>[Asia/Tokyo]</code>.</li>
   1215 <li>
   1216 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1217 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1218 formats.</li>
   1219 <li>Alphabetic time zone abbreviations should not be used as unique
   1220 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1221 practice. For example, in English-speaking North America
   1222 <abbr>CST</abbr> denotes 6 hours behind <abbr>UT</abbr>,
   1223 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1224 and French-speaking North Americans prefer
   1225 <abbr title="Heure Normale du Centre">HNC</abbr> to
   1226 <abbr>CST</abbr>. The <code><abbr>tz</abbr></code>
   1227 database contains English abbreviations for many timestamps;
   1228 unfortunately some of these abbreviations were merely the database maintainers
   1229 inventions, and these have been removed when possible.</li>
   1230 <li>Numeric time zone abbreviations typically count hours east of
   1231 <abbr>UT</abbr>, e.g., +09 for Japan and
   1232 10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
   1233 <code><abbr>TZ</abbr></code> settings use the opposite convention.
   1234 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1235 title="Japan Standard Time">JST</abbr>-9"</code> and
   1236 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1237 for Japan and Hawaii, respectively. If the
   1238 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1239 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1240 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1241 confusion, handle old timestamps better, and insulate you better from
   1242 any future changes to the rules. One should never set
   1243 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1244 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1245 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1246 is called <abbr>GMT</abbr>.</li>
   1247 </ul>
   1248 </main>
   1249 
   1250 <aside>
   1251 <h2 id="see-also">See also</h2>
   1252 <ul>
   1253 <li><a href="theory.html">Theory and pragmatics of the
   1254 <code><abbr>tz</abbr></code> code and data</a></li>
   1255 <li><a href="tz-art.html">Time and the arts</a></li>
   1256 </ul>
   1257 </aside>
   1258 
   1259 <footer>
   1260 <hr>
   1261 This web page is in the public domain, so clarified as of
   1262 2009-05-17 by Arthur David Olson.
   1263 <br>
   1264 Please send corrections to this web page to the
   1265 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1266 The mailing list and its archives are public,
   1267 so please do not send confidential information.
   1268 </footer>
   1269 </body>
   1270 </html>
   1271