Home | History | Annotate | Line # | Download | only in time
tz-link.html revision 1.20
      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="GNUs 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/">ChromiumOS</a>,
     85 <a href="https://cygwin.com">Cygwin</a>,
     86 <a href="https://mariadb.org">MariaDB</a>,
     87 <a href="https://musl.libc.org">musl libc</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>,
    100 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>,
    101 and <a href="https://blackberry.qnx.com/en">QNX</a>.</p>
    102 <p>
    103 Each main entry in the database represents a <dfn>timezone</dfn>
    104 for a set of civil-time clocks that have all agreed since 1970.
    105 Timezones are typically identified by continent or ocean and then by the
    106 name of the largest city within the region containing the clocks.
    107 For example, <code>America/New_York</code>
    108 represents most of the <abbr title="United States">US</abbr> eastern time zone;
    109 <code>America/Phoenix</code> represents most of Arizona, which
    110 uses mountain time without daylight saving time (<abbr>DST</abbr>);
    111 <code>America/Detroit</code> represents most of Michigan, which uses
    112 eastern time but with different <abbr>DST</abbr> rules in 1975;
    113 and other entries represent smaller regions like Starke County,
    114 Indiana, which switched from central to eastern time in 1991
    115 and switched back in 2006.
    116 To use the database on a <a
    117 href="https://en.wikipedia.org/wiki/POSIX"><abbr
    118 title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
    119 implementation set the <code><abbr>TZ</abbr></code>
    120 environment variable to the locations full name,
    121 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    122 <p>
    123 Associated with each timezone is a history of offsets from
    124 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    125 Time</a> (<abbr>UT</abbr>), which is <a
    126 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    127 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    128 for timestamps after 1960 this is more precisely <a
    129 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    130 Universal Time</a> (<abbr>UTC</abbr>).
    131 The database also records when daylight saving time was in use,
    132 along with some time zone abbreviations such as <abbr>EST</abbr>
    133 for Eastern Standard Time in the <abbr>US</abbr>.</p>
    134 </section>
    135 
    136 <section>
    137 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    138 <p>
    139 The following <a
    140 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    141 the latest releases two
    142 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    143 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    144 <pre><code>mkdir tzdb
    145 cd tzdb
    146 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    147 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    148 <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 -
    149 gzip -dc tzdata-latest.tar.gz | tar -xf -
    150 </code></pre>
    151 <p>Alternatively, the following shell commands download the same
    152 release in a single-tarball format containing extra data
    153 useful for regression testing:</p>
    154 <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>
    155 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    156 </code></pre>
    157 <p>These commands use convenience links to the latest release
    158 of the <code><abbr>tz</abbr></code> database hosted by the
    159 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    160 of the <a href="https://www.iana.org">Internet Assigned Numbers
    161 Authority (IANA)</a>.
    162 Older releases are in files named
    163 <code>tzcode<var>V</var>.tar.gz</code>,
    164 <code>tzdata<var>V</var>.tar.gz</code>, and
    165 <code>tzdb-<var>V</var>.tar.lz</code>,
    166 where <code><var>V</var></code> is the version.
    167 Since 1996, each version has been a four-digit year followed by
    168 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    169 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    170 through <samp>zzz</samp>, and so on).
    171 Since version 2022a, each release has been distributed in
    172 <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIX
    173 ustar interchange format</a>, compressed as described above;
    174 older releases use a nearly compatible format.
    175 Since version 2016h, each release has contained a text file named
    176 <code>version</code> whose first (and currently only) line is the version.
    177 <a href="https://ftp.iana.org/tz/releases/">Older archived releases are
    178 available</a> via
    179 <a href="https://en.wikipedia.org/wiki/HTTPS"><abbr
    180 title="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>,
    181 <a href="https://en.wikipedia.org/wiki/Rsync"><abbr
    182 title="remote sync">rsync</abbr></a>, and
    183 <a href="https://en.wikipedia.org/wiki/FTP"><abbr
    184 title="File Transfer Protocol">FTP</abbr></a>.
    185 <p>Alternatively, a development repository of code and data can bem
    186 retrieved from <a href="https://github.com">GitHub</a> via the shell
    187 command:</p>
    188 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    189 </code></pre>
    190 <p>
    191 Since version 2012e, each release has been tagged in development repositories.
    192 Untagged commits are less well tested and probably contain
    193 more errors.</p>
    194 <p>
    195 After obtaining the code and data files, see the
    196 <code>README</code> file for what to do next.
    197 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    198 machine-readable binary files, one for each location. The binary files
    199 are in a special format specified by
    200 <a href="https://www.rfc-editor.org/rfc/rfc9636">The
    201 Time Zone Information Format (<abbr>TZif</abbr>)</a>
    202 (Internet <abbr title="Request For Comments">RFC</abbr> 9636).
    203 The code also lets
    204 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    205 location.</p>
    206 </section>
    207 
    208 <section>
    209 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    210 <p>
    211 The <code><abbr>tz</abbr></code> code and data
    212 are by no means authoritative. If you find errors, please
    213 email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    214 the time zone mailing list.
    215 The mailing list and its archives are public,
    216 so please do not send confidential information.
    217 See
    218 <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
    219 lists main page</a> to subscribe or to browse its archive of old messages.
    220 <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
    221 discussions</a> and corresponding data changes can be
    222 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    223 </p>
    224 <p>
    225 Changes to the <code><abbr>tz</abbr></code> code and data are often
    226 propagated to clients via operating system updates, so
    227 client <code><abbr>tz</abbr></code> data can often be corrected by
    228 applying these updates. With GNU/Linux and similar systems, if your
    229 maintenance provider has not yet adopted the
    230 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    231 the process by tailoring the generic instructions in
    232 the <code><abbr>tz</abbr> README</code> file and installing the latest
    233 data yourself. System-specific instructions for installing the
    234 latest <code><abbr>tz</abbr></code> data have also been published
    235 for <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
    236 <a
    237 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
    238 <a
    239 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
    240 title="International Components for Unicode">ICU</abbr></a>,
    241 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
    242 JDK</a>,
    243 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    244 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    245 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    246 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
    247 </p>
    248 <p>Since version 2013a,
    249 sources for the <code><abbr>tz</abbr></code> database have been
    250 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    251 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    252 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    253 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    254 title="linefeed">LF</abbr></a>,
    255 which can be modified by common text editors such
    256 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    257 <a href="https://gedit-text-editor.org">gedit</a>, and
    258 <a href="https://www.vim.org">vim</a>.
    259 Specialized source-file editing can be done via the
    260 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    261 zoneinfo</a> package for <a
    262 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    263 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    264 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    265 Studio Code</a>.
    266 </p>
    267 <p>
    268 For further information about updates, please see
    269 <a href="https://www.rfc-editor.org/rfc/rfc6557">Procedures for
    270 Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
    271 More detail can be
    272 found in <a href="theory.html">Theory and pragmatics of the
    273 <code><abbr>tz</abbr></code> code and data</a>.
    274 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    275 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    276 </p>
    277 </section>
    278 
    279 <section>
    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.
    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/2016/04/23/on-the-timing-of-time-zone-changes/">On
    325 the Timing of Time Zone Changes</a> for examples.
    326 </p>
    327 </section>
    328 
    329 <section>
    330 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    331 <ul>
    332 <li>The article
    333 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    334 an encyclopedic summary.</li>
    335 <li><a href="tz-how-to.html">How to Read the
    336 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    337 database format.</li>
    338 <li><a
    339 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    340 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    341 databases style.</li>
    342 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
    343 managing time in the internet</a> analyzes the database longitudinally.</li>
    344 </ul>
    345 </section>
    346 
    347 <section>
    348 <h2 id="web">Web sites using recent versions of the
    349 <code><abbr>tz</abbr></code> database</h2>
    350 <p>
    351 These are listed roughly in ascending order of complexity and fanciness.
    352 </p>
    353 <ul>
    354 <li><a href="https://time.is">Time.is</a> shows locations
    355 time and zones.</li>
    356 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    357 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    358 <a href="https://www.worldclock.com">The World Clock</a>
    359 are time zone converters.</li>
    360 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
    361 publishes <code><abbr>tzdb</abbr></code>-derived data in
    362 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
    363 title="comma-separated values">CSV</abbr></a> and
    364 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
    365 title="Structured Query Language">SQL</abbr></a> form.</li>
    366 <li><a
    367 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    368 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    369 <li><a
    370 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    371 Time in 1000 Places</a> uses descriptions of the values.</li>
    372 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock 
    373 Worldwide</a> lets you sort zone names and convert times.</li>
    374 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    375 time map and a time converter.</li>
    376 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    377 calculates the current time difference between locations.</li>
    378 <li><a href="https://www.wx-now.com">Weather Now</a> and
    379 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    380 </ul>
    381 </section>
    382 
    383 <section>
    384 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    385 <ul>
    386 <li><a href="https://www.rfc-editor.org/rfc/rfc7808">Time Zone
    387 Data Distribution Service</a> (TZDIST, Internet <abbr>RFC</abbr> 7808)
    388 is associated with
    389 <a href="https://www.rfc-editor.org/rfc/rfc7809">CalDAV</a>
    390 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    391 transferring time zone data by reference.
    392 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
    393 implementations</a> are available.</li>
    394 <li>The <a href="https://www.rfc-editor.org/rfc/rfc5545">iCalendar format</a>
    395 (Internet <abbr>RFC</abbr> 5445)
    396 covers time zone
    397 data; see its VTIMEZONE calendar component.
    398 The iCalendar format requires specialized parsers and generators; a
    399 variant <a href="https://www.rfc-editor.org/rfc/rfc6321">xCal</a>
    400 (Internet <abbr>RFC</abbr> 6321) uses
    401 <a href="https://www.w3.org/XML/"><abbr
    402 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    403 <a href="https://www.rfc-editor.org/rfc/rfc7265">jCal</a>
    404 (Internet <abbr>RFC</abbr> 7265)
    405 uses <a href="https://www.json.org/json-en.html"><abbr
    406 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    407 </ul>
    408 </section>
    409 
    410 <section>
    411 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    412 <p>Although some of these do not fully support
    413 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    414 distributions you can generally work around compatibility problems by
    415 running the command <code>make rearguard_tarballs</code> and compiling
    416 from the resulting tarballs instead.</p>
    417 <ul>
    418 <li><a href="https://github.com/libical/vzic">Vzic</a> is a <a
    419 href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
    420 program that compiles
    421 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    422 Vzic is freely
    423 available under the <a
    424 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    425 General Public License (<abbr
    426 title="General Public License">GPL</abbr>)</a>.</li>
    427 <li><a
    428 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    429 contains a script <code>parse_olson</code> that compiles
    430 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    431 modules. It is part of the Perl <a
    432 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    433 which is freely
    434 available under both the <abbr>GPL</abbr> and the Perl Artistic
    435 License. DateTime::TimeZone also contains a script
    436 <code>tests_from_zdump</code> that generates test cases for each clock
    437 transition in the <code><abbr>tz</abbr></code> database.</li>
    438 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    439 Database Parser</a> is a
    440 <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
    441 runtime library with a <a
    442 href="https://en.cppreference.com/w/cpp/chrono"><code>std::chrono</code> API</a>
    443 that is a standard part of C++.
    444 It is freely available under the
    445 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</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/technetwork/articles/java/jf14-date-time-2125367.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/open-source-tools/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><a href="https://nodatime.org">Noda Time  Date and
    501 time <abbr>API</abbr> for .NET</a>
    502 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    503 It is freely available under the Apache License.</li>
    504 <li>Many modern
    505 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
    506 runtimes support <code><abbr>tz</abbr></code> natively via the
    507 <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>.
    508 This can be used as-is or with most of the following libraries,
    509 many of which also support runtimes lacking the <code>timeZone</code> option.
    510 <ul>
    511 <li>The <a
    512 href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
    513 timezone polyfill</a>
    514 is freely available under a <abbr>BSD</abbr>-style license.</li>
    515 <li>The <a href="https://date-fns.org">date-fns</a>
    516 library manipulates timezone-aware timestamps in browsers and
    517 in <a href="https://nodejs.org/en/">Node.js</a>.
    518 It is freely available under the <abbr>MIT</abbr> license.</li>
    519 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
    520 minimalist replacement for the date and time API of
    521 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
    522 manipulation library.
    523 It is freely available under the <abbr>MIT</abbr> license.</li>
    524 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
    525 timezone support for the <code>Intl</code> API.
    526 It is freely available under the <abbr>MIT</abbr> license.</li>
    527 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    528 Moment.js plugin.
    529 It is freely available under the <abbr>MIT</abbr> license.</li>
    530 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    531 JavaScript library that supports date arithmetic that is time zone
    532 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    533 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
    534 supports live <code><abbr>tzdb</abbr></code> updates,
    535 astronomical and atomic time, a command-line interface,
    536 and full <a
    537 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
    538 Its companion <a
    539 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
    540 can generate <abbr>TZif</abbr> and other files, and a companion website
    541 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
    542 convert timestamps, view transition histories, and download code and data.
    543 It is freely available under the <abbr>MIT</abbr> license.</li>
    544 </ul>
    545 The proposed <a
    546 href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
    547 objects</a> let programs access an abstract view of
    548 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
    549 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScripts
    550 problematic <code>Date</code> objects</a> when working with dates and times.
    551 <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
    552 compiler from <code><abbr>tz</abbr></code> source into
    553 <a href="https://julialang.org">Julia</a>. It is freely available
    554 under the <abbr>MIT</abbr> license.</li>
    555 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> 
    556 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    557 title="Free Pascal Compiler">FPC</abbr></a>
    558 compiles from <code><abbr>tz</abbr></code> source into
    559 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    560 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    561 and <a
    562 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    563 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    564 <li><a href="https://pythonhosted.org/pytz/">pytz  World Timezone
    565 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    566 <a href="https://www.python.org">Python</a>.
    567 It is freely available under a <abbr>BSD</abbr>-style license.
    568 In code that can assume Python 3.6 or later it is largely superseded; see <a
    569 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
    570 The Fastest Footgun in the West</a>.</li>
    571 <li><a href="https://tzinfo.github.io">TZInfo 
    572 Ruby Timezone Library</a>
    573 compiles <code><abbr>tz</abbr></code> source into
    574 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    575 It is freely available under the <abbr>MIT</abbr> license.</li>
    576 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
    577 Library</a> is
    578 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    579 library that compiles <code><abbr>tz</abbr></code> source into a time
    580 zone repository whose format
    581 is either proprietary or an <abbr>XML</abbr>-encoded
    582 representation.</li>
    583 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    584 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    585 source into text files, along with a runtime that can read those
    586 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    587 license.</li>
    588 </ul>
    589 </section>
    590 
    591 <section>
    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 an independent, thread-safe implementation of
    598 a <abbr>TZif</abbr> file reader.
    599 This library is freely available under the LGPL
    600 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    601 <li><a href="https://www.gnome.org">GNOME</a>s
    602 <a href="https://developer.gnome.org/glib/">GLib</a> has
    603 a <abbr>TZif</abbr> file reader written in C that
    604 creates a <code>GTimeZone</code> object representing sets
    605 of <abbr>UT</abbr> offsets.
    606 It is freely available under the <abbr>LGPL</abbr>.</li>
    607 <li>The
    608 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>s
    609 <code>baltzo::TimeZoneUtil</code> component contains a C++
    610 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    611 the Apache License.</li>
    612 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    613 library that translates between <abbr>UT</abbr> and civil time and
    614 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    615 License.</li>
    616 <li>The <a href="https://golang.org">Go programming language</a>
    617 has a <abbr>TZif</abbr> file reader <a
    618 href="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li>
    619 <li>The
    620 <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
    621 package</a> contains Python code
    622 to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
    623 <code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
    624 For example, it maps <code>"Africa/Cairo"</code>
    625 to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
    626 an approximation valid for Cairo timestamps from 2023 on.
    627 This can help porting to platforms that support only proleptic TZ.
    628 The package is freely available under the MIT license.</li>
    629 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    630 library that reads <abbr>TZif</abbr> files and converts
    631 timestamps from one time zone or format to another.
    632 It is used by <a href="https://secure.php.net"><abbr
    633 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    634 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    635 and <a href="https://www.mongodb.com">MongoDB</a>.
    636 It is freely available under the <abbr>MIT</abbr> license.</li>
    637 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    638 <abbr>TZif</abbr> file reader.</li>
    639 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    640 DateTime::TimeZone::Tzfile</a>
    641 is a <abbr>TZif</abbr> file reader written in Perl.
    642 It is freely available under the same terms as Perl
    643 (dual <abbr>GPL</abbr> and Artistic license).</li>
    644 <li>Python has a <a id="python-zoneinfo"
    645 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    646 class</a> that reads <abbr>TZif</abbr> data and creates objects
    647 that represent <code><abbr>tzdb</abbr></code> timezones.
    648 Python is freely available under the
    649 <a href="https://docs.python.org/3/license.html">Python Software Foundation
    650 License</a>.
    651 A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
    652 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    653 supplies TZif data if the underlying system data cannot be found;
    654 it is freely available under the Apache License.</li>
    655 <li>The
    656 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    657 library contains a Python tool that
    658 converts <abbr>TZif</abbr> data into
    659 <abbr>JSON</abbr>-format data suitable for use
    660 in its JavaScript library for time zone conversion. Dates before 1970
    661 are not supported.</li>
    662 <li>The <a
    663 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    664 package contains <a href="https://www.haskell.org">Haskell</a> code that
    665 parses and uses <abbr>TZif</abbr> data. It is freely
    666 available under a <abbr>BSD</abbr>-style license.</li>
    667 </ul>
    668 </section>
    669 
    670 <section>
    671 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    672 <ul>
    673 <li><a href="https://foxclocks.org">FoxClocks</a>
    674 is an extension for <a href="https://www.google.com/chrome/">Google
    675 Chrome</a>, <a
    676 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    677 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    678 It displays multiple clocks in the application window, and has a mapping
    679 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    680 It is freely available under the <abbr>GPL</abbr>.</li>
    681 <li>Microsoft Windows 8.1
    682 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    683 data (mentioned <a href="#CLDR">below</a>) used by the
    684 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    685 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    686 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    687 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    688 <a id="System.TimeZoneInfo"
    689 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    690 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    691 the older, proprietary method of Microsoft Windows 2000 and later,
    692 which stores time zone data in the
    693 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    694 <a
    695 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone
    696  Tzid table</a> or <a
    697 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    698 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    699 to <code><abbr>tz</abbr></code> names.
    700 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    701 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    702 <li><a
    703 href="https://www.oracle.com/java/index.html">Oracle
    704 Java</a> contains a copy of a subset of a recent
    705 <code><abbr>tz</abbr></code> database in a
    706 Java-specific format.</li>
    707 </ul>
    708 </section>
    709 
    710 <section>
    711 <h2 id="other-dbs">Other time zone databases</h2>
    712 <ul>
    713 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    714 is Astrodiensts Web version of Shanks and Pottengers out-of-print
    715 time zone history atlases
    716 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    717 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
    718 Although these extensive atlases
    719 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    720 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    721 they are unreliable as Shanks appears to have
    722 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    723 sources and do not indicate which entries are guesswork.</li>
    724 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    725 its own <code>tztab</code>(4) format.</li>
    726 <li>Microsoft Windows has proprietary data mentioned
    727 <a href="#System.TimeZoneInfo">above</a>.</li>
    728 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    729 is another time zone database.</li>
    730 <li>The <a
    731 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    732 Schedules Information Manual</a> of the
    733 International Air Transport Association
    734 gives current time zone rules for airports served by commercial aviation.</li>
    735 </ul>
    736 </section>
    737 
    738 <section>
    739 <h2 id="maps">Maps</h2>
    740 <ul>
    741 <li>The <a
    742 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
    743 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
    744 <a href="https://www.cia.gov">US Central Intelligence
    745 Agency (<abbr
    746 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
    747 zone map; the
    748 <a
    749 href="https://legacy.lib.utexas.edu/maps/world.html">PerryCastaeda
    750 Library Map Collection</a>
    751 of the University of Texas at Austin has copies of
    752 recent editions.
    753 The pictorial quality is good,
    754 but the maps do not indicate daylight saving time,
    755 and parts of the data are a few years out of date.</li>
    756 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    757 with current time</a>
    758 has several fancy time zone maps; it covers Russia particularly well.
    759 The maps pictorial quality is not quite as good as the <abbr>CIA</abbr>s
    760 but the maps are more up to date.</li>
    761 <li><a
    762 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    763 much is time wrong around the world?</a> maps the difference between
    764 mean solar and standard time, highlighting areas such as western China
    765 where the two differ greatly. Its a bit out of date, unfortunately.</li>
    766 </ul>
    767 </section>
    768 
    769 <section>
    770 <h2 id="boundaries">Time zone boundaries</h2>
    771 <p>Geographical boundaries between timezones are available
    772 from several <a
    773 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    774 geolocation</a>
    775 services and other sources.</p>
    776 <ul>
    777 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    778 Boundary Builder</a> extracts
    779 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    780 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    781 Its code is freely available under the <abbr>MIT</abbr> license, and
    782 its data entries are freely available under the
    783 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    784 Open Database License</a>. The borders appear to be quite accurate.
    785 Its main web page lists more than twenty libraries
    786 for looking up a timezone name from a GPS coordinate.</li>
    787 <li>Free access via a network API, if you register a key, is provided by
    788 the <a
    789 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    790 Timezone web service</a>, the <a
    791 href="https://developers.google.com/maps/documentation/timezone/intro">Google
    792 Maps Time Zone API</a>, and
    793 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    794 Commercial network API access is provided
    795 by <a href="https://askgeo.com">AskGeo</a>
    796 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    797 </li>
    798 <li><a
    799 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    800 to get a time zone from a location using latitude and longitude
    801 coordinates?</a> discusses other geolocation possibilities.</li>
    802 <li><a href="http://statoids.com/statoids.html">Administrative
    803 Divisions of Countries (Statoids)</a> lists
    804 political subdivision data related to time zones.</li>
    805 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
    806  GIS and Database Tools</a> includes a Manifold-format map of
    807 world time zone boundaries circa 2007, distributed under the
    808 <abbr>GPL</abbr>.</li>
    809 <li>A ship within the <a
    810 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    811 waters</a> of any nation uses that nations time. In international
    812 waters, time zone boundaries are meridians 15 apart, except that
    813 <abbr>UT</abbr>12 and <abbr>UT</abbr>+12 are each 7.5
    814 wide and are separated by
    815 the 180 meridian (not by the International Date Line, which is
    816 for land and territorial waters only). A captain can change ships
    817 clocks any time after entering a new time zone; midnight changes are
    818 common.</li>
    819 </ul>
    820 </section>
    821 
    822 <section>
    823 <h2 id="civil">Civil time concepts and history</h2>
    824 <ul>
    825 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    826 Walk through Time</a>
    827 surveys the evolution of timekeeping.</li>
    828 <li>The history of daylight saving time is surveyed in <a
    829 href="http://www.webexhibits.org/daylightsaving/">About Daylight
    830 Saving Time  History, rationale, laws &amp; dates</a> and summarized in
    831 <a href="http://seizethedaylight.com/dst/">A Brief
    832 History of Daylight Saving Time</a>.</li>
    833 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    834 Lords</a> discusses how authoritarians manipulate civil time.</li>
    835 <li><a href="https://www.w3.org/TR/timezone/">Working with Time
    836 and Time Zones</a>
    837 contains guidelines and best practices for software applications that
    838 deal with civil time.</li>
    839 <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
    840 the International Date Line</a> tells the story of the most important
    841 time zone boundary.</li>
    842 <li><a href="http://statoids.com/tconcept.html">Basic Time
    843 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    844 </ul>
    845 </section>
    846 
    847 <section>
    848 <h2 id="national">National histories of legal time</h2>
    849 <dl>
    850 <dt>Australia</dt>
    851 <dd>The Bureau of Meteorology publishes a list of <a
    852 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    853 Dates of Daylight Savings Time within Australia</a>.</dd>
    854 <dt>Belgium</dt>
    855 <dd>The Royal Observatory of Belgium maintains a table of time in
    856 Belgium (in
    857 <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
    858 hreflang="nl">Dutch</a> and <a
    859 href="https://robinfo.oma.be/fr/astro-info/heure/"
    860 hreflang="fr">French</a>).</dd>
    861 <dt>Brazil</dt>
    862 <dd>The Time Service Department of the National Observatory
    863 records <a href="http://pcdsh01.on.br/DecHV.html"
    864 hreflang="pt-BR">Brazils daylight saving time decrees (in
    865 Portuguese)</a>.</dd>
    866 <dt>Canada</dt>
    867 <dd>National Research Council Canada publishes current
    868 and some older information about <a
    869 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    870 zones and daylight saving time</a>.</dd>
    871 <dt>Chile</dt>
    872 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    873 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
    874 Chiles official time (in Spanish)</a>.</dd>
    875 <dt>China</dt>
    876 <dd>The Hong Kong Observatory maintains a
    877 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    878  summer time in Hong Kong</a>,
    879 and Macaus Meteorological and Geophysical Bureau maintains a <a
    880 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    881 history for Macau</a>.
    882 Unfortunately the latter is incomplete and has errors.</dd>
    883 <dt>Czech Republic</dt>
    884 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    885 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    886 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
    887 <dt>Germany</dt>
    888 <dd>The National Institute for Science and Technology maintains the <a
    889 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    890 of Legal Time in Germany</a>.</dd>
    891 <dt>Israel</dt>
    892 <dd><a
    893 href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
    894 lists official time-change announcements and laws since 1940,
    895 almost all in Hebrew.</dd>
    896 <dt>Malaysia</dt>
    897 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    898 <dt>Mexico</dt>
    899 <dd>The Investigation and Analysis Service of the Mexican Library of
    900 Congress has published a <a
    901 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    902 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    903 <dt>Netherlands</dt>
    904 <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    905 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    906 covers the history of local time in the Netherlands from ancient times.</dd>
    907 <dt>New Zealand</dt>
    908 <dd>The Department of Internal Affairs maintains a brief <a
    909 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    910 Daylight Saving</a>.</dd>
    911 <dt>Palestine</dt>
    912 <dd>The Ministry of Telecom and Digital Economy publishes a <a
    913 href="https://mtde.gov.ps/home/TimeZone"
    914 hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
    915 <dt>Portugal</dt>
    916 <dd>The Lisbon Astronomical Observatory publishes a
    917 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    918 legal time (in Portuguese)</a>.</dd>
    919 <dt>Singapore</dt>
    920 <dd><a id="Singapore"
    921 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    922 is Singapore in the Wrong Time Zone?</a> details the
    923 history of legal time in Singapore and Malaysia.</dd>
    924 <dt>United Kingdom</dt>
    925 <dd><a
    926 href="https://www.polyomino.org.uk/british-time/">History of
    927 legal time in Britain</a> discusses in detail the country
    928 with perhaps the best-documented history of clock adjustments.</dd>
    929 <dt>United States</dt>
    930 <dd>The Department of Transportations <a
    931 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    932 Time Zone Proceedings</a> lists changes to
    933 official written time zone boundaries, and its <a
    934 href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
    935 Zones dataset</a> maps current boundaries.
    936 These boundaries are only for standard time, so the current map puts
    937 all of Arizona in one time zone even though part of Arizona
    938 observes <abbr>DST</abbr> and part does not.</dd>
    939 <dt>Uruguay</dt>
    940 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    941 Navy (SOHMA) publishes an annual <a
    942 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    943 (in Spanish)</a>.</dd>
    944 </dl>
    945 </section>
    946 
    947 <section>
    948 <h2 id="costs">Costs and benefits of time shifts</h2>
    949 <p>Various sources argue for and against daylight saving time and time
    950 zone shifts, and many scientific studies have been conducted. This
    951 section summarizes reviews and position statements based on
    952 scientific literature in the area.</p>
    953 <ul>
    954 <li>Carey RN, Sarma KM.
    955 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    956 daylight saving time on road traffic collision risk: a systematic
    957 review</a>.
    958 <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>.
    959 This reviews research literature and concludes that the evidence
    960 neither supports nor refutes road safety benefits from
    961 shifts in time zones.</li>
    962 <li>Havranek T, Herman D, Irsova D.
    963 Does daylight saving save electricity? A meta-analysis.
    964 <em>Energy J.</em> 2018;39(2):3561.
    965 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    966 This analyzes research literature and concludes, Electricity savings
    967 are larger for countries farther away from the equator, while
    968 subtropical regions consume more electricity because of <abbr>DST</abbr>.</li>
    969 <li>Neumann P, von Blanckenburg K. <a
    970 href="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What
    971 time will it be? A comprehensive literature review on daylight saving time</a>.
    972 <em>Time Soc</em>. 2025-01-21.
    973 doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>.
    974 This reviews DSTs effects on electricity, health, crime, road safety,
    975 and the economy, focusing on research since 2010, and concludes that
    976 year-round standard time is preferable overall.
    977 </ul>
    978 
    979 <p>The following medical societies have taken positions on the
    980 advisability of clock shifts:</p>
    981 
    982 <ul>
    983 <li>In 2022 the American Medical Association issued a
    984 <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
    985 supporting permanent standard time</a> on health grounds.</li>
    986 <li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>.
    987 <a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The
    988 British Sleep Society position statement on Daylight Saving Time in the UK</a>.
    989 <em>J Sleep Res.</em> 2025;34(3):e14352.
    990 doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>.
    991 This recommends that the UK abolish DST for health reasons.</li>
    992 <li>Malow BA. <a
    993 href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
    994 to abolish the clock change and adopt permanent
    995 standard time in the United States:
    996 a Sleep Research Society position statement</a>.
    997 <em>Sleep.</em> 2022;45(12):zsac236.
    998 doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
    999 After reviewing the scientific literature, the Sleep Research Society
   1000 advocates permanent standard time due to its health benefits.</li>
   1001 <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
   1002 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
   1003 is the optimal choice for health and safety:
   1004 an American Academy of Sleep Medicine position statement</a>.
   1005 <em>J Clin Sleep Med.</em> 2024;20(1):121125.
   1006 doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
   1007 The AASM argues for permanent standard time due to health and safety risks
   1008 and economic costs of both <abbr>DST</abbr> transitions and
   1009 permanent <abbr>DST</abbr>.</li>
   1010 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
   1011 <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
   1012 should we abolish Daylight Saving Time?</a>
   1013 <em>J Biol Rhythms.</em> 2019;34(3):227230.
   1014 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
   1015 The Society for Research on Biological Rhythms
   1016 opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
   1017 and advocates that governments adopt
   1018 permanent Standard Time for the health and safety of their citizens.</li>
   1019 </ul>
   1020 </section>
   1021 
   1022 <section>
   1023 <h2 id="precision">Precision timekeeping</h2>
   1024 <ul>
   1025 <li><a
   1026 href="http://leapsecond.com/hpan/an1289.pdf">The
   1027 Science of Timekeeping</a> is a thorough introduction
   1028 to the theory and practice of precision timekeeping.</li>
   1029 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
   1030 Time 2016</a> contains several freely readable papers.</li>
   1031 <li><a href="https://www.ntp.org"><abbr
   1032 title="Network Time Protocol">NTP</abbr>: The Network
   1033 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
   1034 discusses how to synchronize clocks of
   1035 Internet hosts.</li>
   1036 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
   1037 family of software algorithms can achieve accuracy to a few tens of
   1038 nanoseconds in scalable server farms without special hardware.</li>
   1039 <li>The <a
   1040 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
   1041 Time Protocol</a> (<abbr
   1042 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
   1043 can achieve submicrosecond clock accuracy on a local area network
   1044 with special-purpose hardware.</li>
   1045 <li><a
   1046 href="https://www.rfc-editor.org/rfc/rfc4833">Timezone
   1047 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
   1048 (Internet <abbr>RFC</abbr> 4833)
   1049 specifies a <a
   1050 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1051 option for a server to configure
   1052 a clients time zone and daylight saving settings automatically.</li>
   1053 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1054 Scales</a> describes astronomical time scales like
   1055 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1056 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1057 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
   1058 <li>The <a href="https://www.iau.org"><abbr
   1059 title="International Astronomical Union">IAU</abbr></a>s <a
   1060 href="https://www.iausofa.org"><abbr
   1061 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1062 collection contains C and <a
   1063 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1064 code for converting among time scales like
   1065 <abbr title="International Atomic Time">TAI</abbr>,
   1066 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1067 <abbr>UTC</abbr>. It is freely available under the
   1068 <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
   1069 <li><a
   1070 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1071  Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1072 diverse local time
   1073 scales used by each landed mission on Mars.</li>
   1074 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1075 dedicated not only to leap seconds but to precise time and frequency
   1076 in general. It covers the state of the art in amateur timekeeping, and
   1077 how the art has progressed over the past few decades.</li>
   1078 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1079 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1080 and time-signal emissions</a>, International Telecommunication Union 
   1081 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1082 <li><a
   1083 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1084 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1085 Bulletins</a> contains official publications of the International
   1086 Earth Rotation and Reference Systems Service, which decides when leap
   1087 seconds occur.
   1088 The <code><abbr>tz</abbr></code> code and data support leap seconds
   1089 via an optional <code>"right"</code> configuration where a computers internal
   1090 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1091 as opposed to the default <code>"posix"</code> configuration
   1092 where the internal clock ignores leap seconds.
   1093 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1094 <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1095 timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1096 which corresponds to the first leap second
   1097 1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration,
   1098 and to
   1099 1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration.
   1100 In practice the two configurations also agree for timestamps before
   1101 1972 even though the historical situation is messy, partly because
   1102 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1103 is well-defined for sufficiently old timestamps.</li>
   1104 <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
   1105 <abbr>NTP</abbr> Leap Second File</a> covers the text file
   1106 <code>leap-seconds.list</code>, which lists the currently known leap seconds.
   1107 The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
   1108 <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
   1109 <a href="https://www.ntp.org">classic
   1110 <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
   1111 and <a href="https://ntpsec.org">NTPsec</a>.
   1112 The <code><abbr>tz</abbr></code> database also distributes leap second
   1113 information in a differently-formatted <code>leapseconds</code> text file,
   1114 as well as in the <code>"right"</code> configuration in binary form; for
   1115 example, <code>right/UTC</code> can be used
   1116 by <a href="https://chrony-project.org"><code>chrony</code></a>,
   1117 another <abbr>NTP</abbr> implementation.</li>
   1118 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1119 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1120 leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1121 half second, even though every <abbr>POSIX</abbr> minute has exactly
   1122 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1123 <code>"posix"</code> configuration, is <a
   1124 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
   1125 the abovementioned <abbr>NTP</abbr> implementations, <a
   1126 href="https://github.com/google/unsmear">supports</a> conversion between
   1127 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
   1128 cloud service providers. However, according to
   1129 <a href="https://www.rfc-editor.org/rfc/rfc8633#section-3.7.1">3.7.1 of
   1130 Network Time Protocol Best Current Practices</a>
   1131 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1132 applications requiring accurate <abbr>UTC</abbr> or civil time,
   1133 and is intended for use only in single, well-controlled environments.</li>
   1134 <li>The <a
   1135 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1136 Second Discussion List</a> covers <a
   1137 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
   1138 and Klepczynskis 1999 proposal to discontinue leap seconds</a>,
   1139 discussed further in
   1140 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1141 leap second: its history and possible future</a>.
   1142 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1143 might be redefined
   1144 without Leap Seconds</a> gives pointers on this
   1145 contentious issue.
   1146 The General Conference on Weights and Measures
   1147 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
   1148 to discontinue the use of leap seconds by 2035, and requested that no
   1149 discontinuous adjustments be made to UTC for at least a century.
   1150 The World Radiocommunication Conference <a
   1151 href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
   1152 in 2023</a> to cooperate with this process. One proposal to implement this
   1153 would replace leap seconds with seven 13-second leap smears occurring once per
   1154 decade until 2100, with leap smears after that gradually increasing in size.
   1155 See:
   1156 <ul>
   1157 <li>Levine J. <a href="https://www.preprints.org/manuscript/202406.0043/v1">A
   1158 proposal to change the leap-second adjustments to
   1159 coordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a
   1160 href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>.</li>
   1161 </ul>
   1162 However, there is still no consensus on whether this is the best way
   1163 to replace leap seconds.
   1164 </li>
   1165 </ul>
   1166 </section>
   1167 
   1168 <section>
   1169 <h2 id="notation">Time notation</h2>
   1170 <ul>
   1171 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
   1172 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1173 zone names, abbreviations, identifiers, and formats. For example, it
   1174 contains French translations for Eastern European Summer Time,
   1175 <abbr title="Eastern European Summer Time">EEST</abbr>, and Bucharest. Its
   1176 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1177 charts</a> show these values for many locales. Data values are available in
   1178 both <abbr title="Locale Data Markup Language">LDML</abbr>
   1179 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1180 <li>
   1181 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1182 the international standard date and time notation</a> covers
   1183 <a
   1184 href="https://www.iso.org/standard/70907.html"><em><abbr
   1185 title="International Organization for Standardization">ISO</abbr>
   1186 8601-1:2019  Date and time  Representations for information
   1187 interchange  Part 1: Basic rules</em></a>.</li>
   1188 <li>
   1189 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1190 Schema: Datatypes  dateTime</a> specifies a format inspired by
   1191 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1192 <li><a href="https://www.rfc-editor.org/rfc/rfc5322#section-3.3">3.3 of
   1193 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1194 specifies the time notation used in email and <a
   1195 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1196 headers.</li>
   1197 <li>
   1198 <a href="https://www.rfc-editor.org/rfc/rfc3339">Date and Time
   1199 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1200 specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
   1201 An extension, <a href="https://www.rfc-editor.org/rfc/rfc9557">Date
   1202 and Time on the Internet: Timestamps with Additional Information</a>
   1203 (Internet <abbr>RFC</abbr> 9557) extends this profile
   1204 to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
   1205 via suffixes like <code>[Asia/Tokyo]</code>.
   1206 <li>
   1207 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1208 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1209 formats.</li>
   1210 <li>Alphabetic time zone abbreviations should not be used as unique
   1211 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1212 practice. For example, in English-speaking North America
   1213 <abbr>CST</abbr> denotes 6 hours behind <abbr>UT</abbr>,
   1214 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1215 and French-speaking North Americans prefer
   1216 <abbr title="Heure Normale du Centre">HNC</abbr> to
   1217 <abbr>CST</abbr>. The <code><abbr>tz</abbr></code>
   1218 database contains English abbreviations for many timestamps;
   1219 unfortunately some of these abbreviations were merely the database maintainers
   1220 inventions, and these have been removed when possible.</li>
   1221 <li>Numeric time zone abbreviations typically count hours east of
   1222 <abbr>UT</abbr>, e.g., +09 for Japan and
   1223 10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
   1224 <code><abbr>TZ</abbr></code> settings use the opposite convention.
   1225 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1226 title="Japan Standard Time">JST</abbr>-9"</code> and
   1227 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1228 for Japan and Hawaii, respectively. If the
   1229 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1230 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1231 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1232 confusion, handle old timestamps better, and insulate you better from
   1233 any future changes to the rules. One should never set
   1234 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1235 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1236 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1237 is called <abbr>GMT</abbr>.</li>
   1238 </ul>
   1239 </section>
   1240 
   1241 <section>
   1242 <h2 id="see-also">See also</h2>
   1243 <ul>
   1244 <li><a href="theory.html">Theory and pragmatics of the
   1245 <code><abbr>tz</abbr></code> code and data</a></li>
   1246 <li><a href="tz-art.html">Time and the Arts</a></li>
   1247 </ul>
   1248 </section>
   1249 
   1250 <footer>
   1251 <hr>
   1252 This web page is in the public domain, so clarified as of
   1253 2009-05-17 by Arthur David Olson.
   1254 <br>
   1255 Please send corrections to this web page to the
   1256 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1257 The mailing list and its archives are public,
   1258 so please do not send confidential information.
   1259 </footer>
   1260 </body>
   1261 </html>
   1262