tz-link.html revision 1.2
1<!DOCTYPE html> 2<html lang="en"> 3<head> 4<title>Sources for time zone and daylight saving time data</title> 5<meta charset="UTF-8"> 6<style> 7pre {margin-left: 2em; white-space: pre-wrap;} 8</style> 9</head> 10<body> 11<h1>Sources for time zone and daylight saving time data</h1> 12<p> 13<a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and 14<a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a> 15rules are controlled by individual 16governments. They are sometimes changed with little notice, and their 17histories and planned futures are often recorded only fitfully. Here 18is a summary of attempts to organize and record relevant data in this 19area. 20</p> 21<h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2> 22<p> 23The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a> 24time zone database contains code and data 25that represent the history of local time 26for many representative locations around the globe. 27It is updated periodically to reflect changes made by political bodies 28to time zone boundaries and daylight saving rules. 29This database (known as <code><abbr>tz</abbr></code>, 30<code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>) 31is used by several implementations, 32including 33<a href="https://www.gnu.org/software/libc/">the 34<abbr title="GNU's Not Unix">GNU</abbr> 35C Library</a> (used in 36<a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>), 37<a href="https://www.android.com">Android</a>, 38<a href="https://www.freebsd.org">Free<abbr 39title="Berkeley Software Distribution">BSD</abbr></a>, 40<a href="https://netbsd.org">Net<abbr>BSD</abbr></a>, 41<a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>, 42<a href="https://www.chromium.org/chromium-os">Chromium OS</a>, 43<a href="https://cygwin.com">Cygwin</a>, 44<a href="http://www.delorie.com/djgpp/"><abbr 45title="DJ's GNU Programming Platform">DJGPP</abbr></a>, 46<a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>, 47<a href="https://www.mysql.com">MySQL</a>, 48<a href="https://en.wikipedia.org/wiki/WebOS"><abbr 49title="Web Operating System">webOS</abbr></a>, 50<a href="https://ibm.com/aix"><abbr 51title="Advanced Interactive eXecutive">AIX</abbr></a>, 52<a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>, 53<a href="https://www.apple.com/ios/"><abbr 54title="iPhone OS">iOS</abbr></a>, 55<a href="https://www.apple.com/macos/">macOS</a>, 56<a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>, 57<a href="https://www.hpe.com/info/openvms">Open<abbr 58title="Virtual Memory System">VMS</abbr></a>, 59<a href="https://www.oracle.com/database/index.html">Oracle Database</a>, and 60<a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p> 61<p> 62Each main entry in the database represents a <dfn>timezone</dfn> 63for a set of civil-time clocks that have all agreed since 1970. 64Timezones are typically identified by continent or ocean and then by the 65name of the largest city within the region containing the clocks. 66For example, <code>America/New_York</code> 67represents most of the <abbr title="United States">US</abbr> eastern time zone; 68<code>America/Phoenix</code> represents most of Arizona, which 69uses mountain time without daylight saving time (<abbr>DST</abbr>); 70<code>America/Detroit</code> represents most of Michigan, which uses 71eastern time but with different <abbr>DST</abbr> rules in 1975; 72and other entries represent smaller regions like Starke County, 73Indiana, which switched from central to eastern time in 1991 74and switched back in 2006. 75To use the database on an extended <a 76href="https://en.wikipedia.org/wiki/POSIX"><abbr 77title="Portable Operating System Interface">POSIX</abbr></a> 78implementation set the <code><abbr>TZ</abbr></code> 79environment variable to the location's full name, 80e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p> 81<p> 82Associated with each timezone is a history of offsets from 83<a href="https://en.wikipedia.org/wiki/Universal_Time">Universal 84Time</a> (<abbr>UT</abbr>), which is <a 85href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean 86Time</a> (<abbr>GMT</abbr>) with days beginning at midnight; 87for timestamps after 1960 this is more precisely <a 88href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated 89Universal Time</a> (<abbr>UTC</abbr>). 90The database also records when daylight saving time was in use, 91along with some time zone abbreviations such as <abbr>EST</abbr> 92for Eastern Standard Time in the <abbr>US</abbr>.</p> 93<h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2> 94<p> 95The following <a 96href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download 97the latest release's two 98<a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a> 99to a <abbr>GNU</abbr>/Linux or similar host.</p> 100<pre><code>mkdir tzdb 101cd tzdb 102<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz 103wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz 104<a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf - 105gzip -dc tzdata-latest.tar.gz | tar -xf - 106</code></pre> 107<p>Alternatively, the following shell commands download the same 108release in a single-tarball format containing extra data 109useful for regression testing:</p> 110<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> 111<a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf - 112</code></pre> 113<p>These commands use convenience links to the latest release 114of the <code><abbr>tz</abbr></code> database hosted by the 115<a href="https://www.iana.org/time-zones">Time Zone Database website</a> 116of the <a href="https://www.iana.org">Internet Assigned Numbers 117Authority (IANA)</a>. 118Older releases are in files named 119<code>tzcode<var>V</var>.tar.gz</code>, 120<code>tzdata<var>V</var>.tar.gz</code>, and 121<code>tzdb-<var>V</var>.tar.lz</code>, 122where <code><var>V</var></code> is the version. 123Since 1996, each version has been a four-digit year followed by 124lower-case letter (<samp>a</samp> through <samp>z</samp>, 125then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp> 126through <samp>zzz</samp>, and so on). 127Since version 2016h, each release has contained a text file named 128"<samp>version</samp>" whose first (and currently only) line is the version. 129The releases are also available in an 130<a href="ftp://ftp.iana.org/tz/releases/"><abbr 131title="File Transfer Protocol">FTP</abbr> directory</a> via a 132less-secure protocol.</p> 133<p>Alternatively, a development repository of code and data can be 134retrieved from <a href="https://github.com">GitHub</a> via the shell 135command:</p> 136<pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a> 137</code></pre> 138<p> 139Since version 2012e, each release has been tagged in development repositories. 140Untagged commits are less well tested and probably contain 141more errors.</p> 142<p> 143After obtaining the code and data files, see the 144<code>README</code> file for what to do next. 145The code lets you compile the <code><abbr>tz</abbr></code> source files into 146machine-readable binary files, one for each location. The binary files 147are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>). 148The code also lets 149you read a <abbr>TZif</abbr> file and interpret timestamps for that 150location.</p> 151<h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2> 152<p> 153The <code><abbr>tz</abbr></code> code and data 154are by no means authoritative. If you find errors, please 155send changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>, 156the time zone mailing list. You can also <a 157href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it 158and browse the <a 159href="https://mm.icann.org/pipermail/tz/">archive of old 160messages</a>.</p> 161<p> 162If your government plans to change its time zone boundaries or 163daylight saving rules, inform <code>tz@iana.org</code> well in 164advance, as this will coordinate updates to many cell phones, 165computers, and other devices around the world. With 166less than a year's notice there is a good chance that some 167computer-based clocks will operate incorrectly after the change, due 168to delays in propagating updates to software and data. The shorter 169the notice, the more likely clock problems will arise; see "<a 170href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On 171the Timing of Time Zone Changes</a>" for examples. 172</p> 173<p> 174Changes to the <code><abbr>tz</abbr></code> code and data are often 175propagated to clients via operating system updates, so 176client <code><abbr>tz</abbr></code> data can often be corrected by 177applying these updates. With GNU/Linux and similar systems, if your 178maintenance provider has not yet adopted the 179latest <code><abbr>tz</abbr></code> data, you can often short-circuit 180the process by tailoring the generic instructions in 181the <code><abbr>tz</abbr> README</code> file and installing the latest 182data yourself. System-specific instructions for installing the 183latest <code><abbr>tz</abbr></code> data have also been published 184for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>, 185<a 186href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>, 187<a 188href="http://userguide.icu-project.org/datetime/timezone"><abbr 189title="International Components for Unicode">ICU</abbr></a>, 190<a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a> 191and <a 192href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a> 193Java, <a href="http://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a 194href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>, 195and <a 196href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below). 197</p> 198<p>Sources for the <code><abbr>tz</abbr></code> database are 199<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr 200title="Unicode Transformation Format 8-bit">UTF-8</abbr></a> 201<a href="https://en.wikipedia.org/wiki/Text_file">text files</a> 202with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr 203title="linefeed">LF</abbr></a>, 204which can be modified by common text editors such 205as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>, 206<a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and 207<a href="https://www.vim.org">vim</a>. 208Specialized source-file editing can be done via the 209<a href="https://packagecontrol.io/packages/zoneinfo">Sublime 210zoneinfo</a> package for <a 211href="https://www.sublimetext.com">Sublime Text</a> and the <a 212href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode 213zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual 214Studio Code</a>. 215</p> 216<p> 217For further information about updates, please see 218<a href="https://tools.ietf.org/html/rfc6557">Procedures for 219Maintaining the Time Zone Database</a> (Internet <abbr 220title="Request For Comments">RFC</abbr> 6557). More detail can be 221found in <a href="theory.html">Theory and pragmatics of the tz code and data</a>. 222<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a> 223displays changes between recent <code><abbr>tzdb</abbr></code> versions. 224</p> 225<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2> 226<ul> 227<li>The article 228<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is 229an encyclopedic summary.</li> 230<li><a href="tz-how-to.html">How to Read the 231tz Database Source Files</a> explains the <code><abbr>tz</abbr></code> 232database format.</li> 233<li><a 234href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A 235literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the 236database's style.</li> 237</ul> 238<h2 id="web">Web sites using recent versions of the 239<code><abbr>tz</abbr></code> database</h2> 240<p> 241These are listed roughly in ascending order of complexity and fanciness. 242</p> 243<ul> 244<li><a href="https://time.is">Time.is</a> shows locations' 245time and zones.</li> 246<li><a href="https://www.timejones.com">TimeJones.com</a>, 247<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and 248<a href="http://worldclock.com">The World Clock</a> 249are time zone converters.</li> 250<li><a 251href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a> 252lets you see the <code><abbr>TZ</abbr></code> values directly.</li> 253<li><a 254href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current 255Time in 1000 Places</a> uses descriptions of the values.</li> 256<li><a href="http://www.timezoneconverter.com/cgi-bin/tzc.tzc">Time Zone 257Converter</a> 258uses a pulldown menu.</li> 259<li><a href="http://home.kpn.nl/vanadovv/time/TZworld.html">Complete 260timezone information for all countries</a> displays tables of DST rules. 261<li><a href="https://www.timeanddate.com/worldclock/">The World Clock – 262Worldwide</a> lets you sort zone names and convert times.</li> 263<li><a href="https://24timezones.com">24TimeZones</a> has a world 264time map and a time converter.</li> 265<li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a> 266calculates the current time difference between locations.</li> 267<li><a href="http://www.wx-now.com">Weather Now</a> and 268<a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li> 269</ul> 270<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2> 271<ul> 272<li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s 273<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data 274Distribution Service (tzdist) working group</a> defined <a 275href="https://tools.ietf.org/html/rfc7808">TZDIST</a> 276(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service, 277along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a> 278(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for 279transferring time zone data by reference. 280The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis 281mailing list</a> discusses two Internet drafts: <a 282id="TZDIST-Geolocate" 283href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST 284Geolocate Extension</a> lets a client determine its timezone 285from its geographic location using a <a 286href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>, and 287<a href="https://tools.ietf.org/html/draft-murchison-tzdist-tzif">The 288Time Zone Information Format (<abbr>TZif</abbr>)</a> specifies the format of 289<abbr>TZif</abbr> data.</li> 290<li>The <a href="https://tools.ietf.org/html/rfc5545"> 291Internet Calendaring and Scheduling Core Object Specification 292(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445) 293covers time zone 294data; see its VTIMEZONE calendar component. 295The iCalendar format requires specialized parsers and generators; a 296variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a> 297(Internet <abbr>RFC</abbr> 6321) uses 298<a href="https://www.w3.org/XML/"><abbr 299title="Extensible Markup Language">XML</abbr></a> format, and a variant 300<a href="https://tools.ietf.org/html/rfc7265">jCal</a> 301(Internet <abbr>RFC</abbr> 7265) 302uses <a href="https://www.json.org"><abbr 303title="JavaScript Object Notation">JSON</abbr></a> format.</li> 304</ul> 305<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2> 306<ul> 307<li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a 308href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a> 309program that compiles 310<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files. 311Vzic is freely 312available under the <a 313href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> 314General Public License (<abbr 315title="General Public License">GPL</abbr>)</a>.</li> 316<li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz 317database conversion utility</a> is like Vzic, except for the <a 318href="https://www.microsoft.com/net">.NET framework</a> 319and with a <abbr>BSD</abbr>-style license.</li> 320<li><a 321href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a> 322contains a script <code>parse_olson</code> that compiles 323<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a> 324modules. It is part of the Perl <a 325href="http://datetime.perl.org">DateTime Project</a>, which is freely 326available under both the <abbr>GPL</abbr> and the Perl Artistic 327License. DateTime::TimeZone also contains a script 328<code>tests_from_zdump</code> that generates test cases for each clock 329transition in the <code><abbr>tz</abbr></code> database.</li> 330<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone 331Database Parser</a> is a 332<a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and 333runtime library that is <a 334href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0355r4.html">moving 335forward</a> for inclusion in the next iteration of <a 336href="https://isocpp.org/std/the-standard"><em><abbr 337title="International Organization for Standardization">ISO</abbr> 338International Standard ISO/IEC 14882:2017(E) – Programming 339Language C++</em></a>. 340It is freely available under the 341<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li> 342<li><a id="ICU" href="http://site.icu-project.org">International Components for 343Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a 344href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a> 345libraries for internationalization that 346has a compiler from <code><abbr>tz</abbr></code> source 347and from <abbr title="Common Locale Data Repository">CLDR</abbr> data 348(mentioned <a href="#CLDR">below</a>) 349into an <abbr>ICU</abbr>-specific format. 350<abbr>ICU</abbr> is freely available under a 351<abbr>BSD</abbr>-style license.</li> 352<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for 353the <a href="https://elixir-lang.org">Elixir</a> language downloads 354and compiles tz source and exposes <abbr 355title="Application Program Interface">API</abbr>s for use. It is 356freely available under the <abbr>MIT</abbr> license.</li> 357<li>Java-based compilers and libraries include: 358<ul> 359<li>The <a 360href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater 361tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by 362Oracle Java.</li> 363<li>The <a 364href="http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java 3658 <code>java.time</code> <abbr>API</abbr></a> can be supplemented by <a 366href="http://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>, 367which is freely available under a <abbr>BSD</abbr>-style license.</li> 368<li><a href="http://www.joda.org/joda-time/">Joda-Time – Java date 369and time <abbr>API</abbr></a> contains a class 370<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles 371<code><abbr>tz</abbr></code> source into a binary format. It inspired 372Java 8 <code>java.time</code>, which its users should migrate to once 373they can assume Java 8 or later. It is available under the <a 374href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li> 375<li><a href="https://github.com/MenoData/Time4J/">Time4J – 376Advanced date, time and interval library for Java</a> contains a class 377<code>net.time4j.tool.TimezoneRepositoryCompiler</code> that compiles 378<code><abbr>tz</abbr></code> source into a binary format. Time4J is 379available under the <a 380href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser 381General Public License (<abbr title="Lesser General Public 382License">LGPL</abbr>)</a>.</li> 383<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and 384Java-based libraries.</li> 385</ul> 386<li><a href="https://nodatime.org">Noda Time – Date and 387time <abbr>API</abbr> for .NET</a> 388and <a href="http://www.babiej.demon.nl/Tz4Net/main.htm">TZ4Net</a> 389are similar to Joda-Time and Time4J, but for the .NET framework instead of 390Java. They are freely available under the 391Apache License 392and a <abbr>BSD</abbr>-style license, respectively.</li> 393<li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based 394compilers and libraries include: 395<ul> 396<li><a 397href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a> 398compiles time zone data into a compact form designed for 399JavaScript. It is freely available under a combination of 400the <abbr>MIT</abbr> license and the Apache License.</li> 401<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a 402plugin for the <a href="https://momentjs.com">Moment.js</a> date 403manipulation library. It is freely available under the <abbr>MIT</abbr> 404license.</li> 405<li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s 406<abbr>API</abbr> is upward compatible with standard JavaScript 407Dates. It is freely available under the Apache License.</li> 408<li><a href="https://github.com/sproutsocial/walltime-js">Walltime-js</a> 409translates <abbr>UT</abbr> to local time. It is freely available under 410the <abbr>MIT</abbr> license.</li> 411</ul> 412<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a 413compiler from <code><abbr>tz</abbr></code> source into 414<a href="https://julialang.org/">Julia</a>. It is freely available 415under the <abbr>MIT</abbr> license.</li> 416<li><a href="https://github.com/pavkam/tzdb">Delphi/<abbr 417title="Free Pascal Compiler">FPC</abbr> Time Zone Database</a> 418compiles from <code><abbr>tz</abbr></code> source into 419<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a> 420as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a> 421and <a 422href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>. 423It is freely available under a <abbr>BSD</abbr>-style license.</li> 424<li><a href="http://pytz.sourceforge.net">pytz – World Timezone 425Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into 426<a href="https://www.python.org">Python</a>. 427It is freely available under a <abbr>BSD</abbr>-style license.</li> 428<li><a href="https://tzinfo.github.io">TZInfo – 429Ruby Timezone Library</a> 430compiles <code><abbr>tz</abbr></code> source into 431<a href="https://www.ruby-lang.org/en/">Ruby</a>. 432It is freely available under the <abbr>MIT</abbr> license.</li> 433<li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time 434Library</a> is 435a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class 436library that compiles <code><abbr>tz</abbr></code> source into a time 437zone repository whose format 438is either proprietary or an <abbr>XML</abbr>-encoded 439representation.</li> 440<li><a id="Tcl" href="https://tcl.tk">Tcl</a> 441contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code> 442source into text files, along with a runtime that can read those 443files. Tcl is freely available under a <abbr>BSD</abbr>-style 444license.</li> 445</ul> 446<h2 id="TZif">Other <abbr>TZif</abbr> readers</h2> 447<ul> 448<li>The <a 449href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C 450Library</a> 451has an independent, thread-safe implementation of 452a <abbr>TZif</abbr> file reader. 453This library is freely available under the LGPL 454and is widely used in <abbr>GNU</abbr>/Linux systems.</li> 455<li><a href="https://www.gnome.org">GNOME</a>'s 456<a href="https://developer.gnome.org/glib/">GLib</a> has 457a <abbr>TZif</abbr> file reader written in C that 458creates a <code>GTimeZone</code> object representing sets 459of <abbr>UT</abbr> offsets. 460It is freely available under the <abbr>LGPL</abbr>.</li> 461<li>The 462<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s 463<code>baltzo::TimeZoneUtil</code> component contains a C++ 464implementation of a <abbr>TZif</abbr> file reader. It is freely available under 465the Apache License.</li> 466<li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++ 467library that translates between <abbr>UT</abbr> and civil time and 468can read <abbr>TZif</abbr> files. It is freely available under the Apache 469License.</li> 470<li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a> 471is a <abbr>TZif</abbr> file reader written in Java. 472It is freely available under the <abbr>LGPL</abbr>.</li> 473<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C 474library that reads <abbr>TZif</abbr> files and converts 475timestamps from one time zone or format to another. 476It is used by <a href="https://secure.php.net"><abbr 477title="PHP: Hypertext Preprocessor">PHP</abbr></a>, 478<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>, 479and <a href="https://www.mongodb.com">MongoDB</a>. 480It is freely available under the <abbr>MIT</abbr> license.</li> 481<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a 482JavaScript library that supports date arithmetic that is time zone 483aware. It is freely available under the <abbr>MIT</abbr> license.</li> 484<li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a 485<abbr>TZif</abbr> file reader.</li> 486<li><a href="http://search.cpan.org/perldoc?DateTime::TimeZone::Tzfile"> 487DateTime::TimeZone::Tzfile</a> 488is a <abbr>TZif</abbr> file reader written in Perl. 489It is freely available under the same terms as Perl 490(dual <abbr>GPL</abbr> and Artistic license).</li> 491<li>The 492public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a> 493library contains a Python tool that 494converts <abbr>TZif</abbr> data into 495<abbr>JSON</abbr>-format data suitable for use 496in its JavaScript library for time zone conversion. Dates before 1970 497are not supported.</li> 498<li>The <a 499href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a> 500package contains <a href="https://www.haskell.org">Haskell</a> code that 501parses and uses <abbr>TZif</abbr> data. It is freely 502available under a <abbr>BSD</abbr>-style license.</li> 503</ul> 504<h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2> 505<ul> 506<li><a href="https://foxclocks.org">FoxClocks</a> 507is an extension for <a href="https://www.google.com/chrome/">Google 508Chrome</a> and for <a 509href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla 510Toolkit</a> applications like <a 511href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a 512href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>. 513It displays multiple clocks in the application window, and has a mapping 514interface to <a href="https://www.google.com/earth/">Google Earth</a>. 515It is freely available under the <abbr>GPL</abbr>.</li> 516<li><a href="https://golang.org">Go programming language</a> 517implementations contain a copy of a 32-bit subset of a recent 518<code><abbr>tz</abbr></code> database in a 519Go-specific format.</li> 520<li><a 521href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International 522clock (intclock)</a> is a clock that displays multiple time zones on 523<abbr>GNU</abbr>/Linux and similar systems. It is freely available 524under the <abbr>GPL</abbr>.</li> 525<li>Microsoft Windows 8.1 526and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr> 527data (mentioned <a href="#CLDR">below</a>) used by 528<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> 529classes such as <a 530href="https://msdn.microsoft.com/en-us/library/windows/apps/windows.globalization.datetimeformatting.datetimeformatter.aspx"><code>DateTimeFormatter</code></a>. 531<a id="System.TimeZoneInfo" 532href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring 533Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes 534the older, proprietary method of Microsoft Windows 2000 and later, 535which stores time zone data in the 536<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The 537<a 538href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone → 539Tzid table</a> or <a 540href="https://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml"><abbr>XML</abbr> 541file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs 542to <code><abbr>tz</abbr></code> names. 543<li><a 544href="https://www.oracle.com/java/index.html">Oracle 545Java</a> contains a copy of a subset of a recent 546<code><abbr>tz</abbr></code> database in a 547Java-specific format.</li> 548<li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone 549Master</a> is a Microsoft Windows clock program that can automatically 550download, compile and use <code>tz</code> releases. The Basic version 551is free.</li> 552<li><a 553href="http://veladg.com/velaterra.html">VelaTerra</a> is 554a macOS program. Its developers 555<a href="http://veladg.com/tzoffer.html">offer free 556licenses</a> to <code><abbr>tz</abbr></code> contributors.</li> 557</ul> 558<h2 id="other-dbs">Other time zone databases</h2> 559<ul> 560<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a> 561is Astrodienst's Web version of Shanks and Pottenger's 562time zone history atlases also published in <a 563href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a> 564form by <a href="https://astrocom.com">ACS-Starcrafts</a>. 565These atlases are extensive but unreliable, as Shanks appears to have 566guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no 567sources and do not indicate which entries are guesswork.</li> 568<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in 569its own <code>tztab</code>(4) format.</li> 570<li>Microsoft Windows has proprietary data mentioned 571<a href="#System.TimeZoneInfo">above</a>.</li> 572<li><a href="https://www.worldtimeserver.com">World Time Server</a> 573is another time zone database.</li> 574<li>The <a 575href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard 576Schedules Information Manual</a> of the 577International Air Transport Association 578gives current time zone rules for airports served by commercial aviation.</li> 579</ul> 580<h2 id="maps">Maps</h2> 581<ul> 582<li>The <a href="https://www.cia.gov/index.html">United States Central 583Intelligence Agency (<abbr 584title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a 585href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time 586zone map</a>; the 587<a 588href="https://www.lib.utexas.edu/maps/world.html">Perry–Castañeda 589Library Map Collection</a> 590of the University of Texas at Austin has copies of 591recent editions. 592The pictorial quality is good, 593but the maps do not indicate daylight saving time, 594and parts of the data are a few years out of date.</li> 595<li><a href="https://www.worldtimezone.com">Current time around the world 596and standard time zones map of the world</a> 597has several fancy time zone maps; it covers Russia particularly well. 598The maps' pictorial quality is not quite as good as the 599<abbr>CIA</abbr>'s 600but the maps are more up to date.</li> 601<li><a 602href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How 603much is time wrong around the world?</a> maps the difference between 604mean solar and standard time, highlighting areas such as western China 605where the two differ greatly. It's a bit out of date, unfortunately.</li> 606</ul> 607<h2 id="boundaries">Time zone boundaries</h2> 608<p>Geographical boundaries between timezones are available 609from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a> 610services and other sources.</p> 611<ul> 612<li>Databases of time zone boundaries include: 613<ul> 614<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone 615Boundary Builder</a> extracts 616<a href="https://www.openstreetmap.org">Open Street Map</a> data to build 617boundaries of <code><abbr>tzdb</abbr></code> timezones. 618Its code is freely available under the <abbr>MIT</abbr> license, and 619its data entries are freely available under the 620<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons 621Open Database License</a>. The maps' borders appear to be quite accurate.</li> 622<li><a href="http://efele.net/maps/tz/"><abbr>TZ</abbr> timezones 623maps</a> contains <a 624href="https://en.wikipedia.org/wiki/Shapefile">shapefiles</a> of 625sets of <code><abbr>tzdb</abbr></code> timezones. This includes 626<a href="http://efele.net/maps/tz/world/">tz_world</a>, a shapefile 627for all the world's timezones. These maps are no longer maintained and 628are superseded by the Timezone Boundary Builder.</li> 629<li><a 630href="https://github.com/straup/whereonearth-timezone">Whereonearth-timezone</a> 631is in <a href="https://tools.ietf.org/html/rfc7946">GeoJSON</a> format 632(Internet <abbr>RFC</abbr> 7946), and combines the 633the tz_world shapefiles with the 634<a href="https://developer.yahoo.com/geo/geoplanet/">GeoPlanet</a> 635dataset.</li> 636</ul></li> 637<li>Programmatic interfaces that map geographical coordinates via tz_world to 638<code><abbr>tzdb</abbr></code> timezones include: 639<ul> 640<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is 641written in <a 642href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a> 643and is freely available under the <abbr>MIT</abbr> license.</li> 644<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a> 645is written in Go and is freely available under the Apache License.</li> 646<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>, 647in both Java and 648<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a> 649form, is freely available under the MIT license.</li> 650<li>For <a href="https://nodejs.org/en/">Node.js</a>, 651the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a> 652is freely available under the MIT license, and 653the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a> 654is in the public domain.</li> 655<li>The <a 656href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a> 657library for Python is freely available under the MIT license. 658<li>The <a 659href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a> 660library for Ruby is freely available under the MIT license.</li> 661<li><a href="https://derickrethans.nl/what-time-is-it.html">What Time 662is It Here?</a> applies MongoDB 663geospatial query operators to shapefiles' data.</li> 664</ul></li> 665<li>Free access via a network API, if you register a key, is provided by 666the <a href="http://www.geonames.org/export/web-services.html#timezone">GeoNames Timezone web service</a>, 667the <a href="https://developers.google.com/maps/documentation/timezone/intro">Google Maps Time Zone API</a>, and 668the <a href="https://timezonedb.com">Time Zone Database & API</a>. 669Commercial network API access is provided 670by <a href="https://askgeo.com">AskGeo</a> 671and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>. 672</li> 673<li>"<a 674href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How 675to get a time zone from a location using latitude and longitude 676coordinates?</a>" discusses other geolocation possibilities.</li> 677<li><a href="http://statoids.com/statoids.html">Administrative 678Divisions of Countries ("Statoids")</a> lists 679political subdivision data related to time zones.</li> 680<li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time 681zone boundaries for multizone countries</a> summarizes legal 682boundaries between time zones within countries.</li> 683<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software 684– GIS and Database Tools</a> includes a Manifold-format map of 685world time zone boundaries distributed under the 686<abbr>GPL</abbr>.</li> 687<li>A ship within the <a 688href="https://en.wikipedia.org/wiki/Territorial_waters">territorial 689waters</a> of any nation uses that nation's time. In international 690waters, time zone boundaries are meridians 15° apart, except that 691<abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5° 692wide and are separated by 693the 180° meridian (not by the International Date Line, which is 694for land and territorial waters only). A captain can change ship's 695clocks any time after entering a new time zone; midnight changes are 696common.</li> 697</ul> 698<h2 id="civil">Civil time concepts and history</h2> 699<ul> 700<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A 701Walk through Time</a> 702surveys the evolution of timekeeping.</li> 703<li><a href="http://www.webexhibits.org/daylightsaving/">About Daylight 704Saving Time – History, rationale, laws & dates</a> 705is an overall history of <abbr>DST</abbr>.</li> 706<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a> 707contains guidelines and best practices for software applications that 708deal with civil time.</li> 709<li><a href="http://seizethedaylight.com/dst/">A Brief 710History of Daylight Saving Time</a> summarizes some of the contentious 711history of <abbr>DST</abbr>.</li> 712<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of 713the International Date Line</a> tells the story of the most important 714time zone boundary.</li> 715<li><a href="http://statoids.com/tconcept.html">Basic Time 716Zone Concepts</a> discusses terminological issues behind time zones.</li> 717</ul> 718<h2 id="national">National histories of legal time</h2> 719<dl> 720<dt>Australia</dt> 721<dd>The Parliamentary Library has commissioned a <a 722href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research 723paper on daylight saving time in Australia</a>. 724The Bureau of Meteorology publishes a list of <a 725href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation 726Dates of Daylight Savings Time within Australia</a>.</dd> 727<dt>Belgium</dt> 728<dd>The Royal Observatory of Belgium maintains a table of <a 729href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html" 730hreflang="nl">time in Belgium (in Dutch)</a>.</dd> 731<dt>Brazil</dt> 732<dd>The Time Service Department of the National Observatory 733records <a href="http://pcdsh01.on.br/DecHV.html" 734hreflang="pt-BR">Brazil's daylight saving time decrees (in 735Portuguese)</a>.</dd> 736<dt>Canada</dt> 737<dd>National Research Council Canada publishes current 738and some older information about <a 739href="https://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time 740zones & daylight saving time</a>.</dd> 741<dt>Chile</dt> 742<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a 743<a href="http://www.horaoficial.cl/historia_hora.html" hreflang="es">history of 744Chile's official time (in Spanish)</a>.</dd> 745<dt>Czech Republic</dt> 746<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas" 747hreflang="cs">When daylight saving time starts and ends (in Czech)</a> 748summarizes and cites historical DST regulations.</dd> 749<dt>Germany</dt> 750<dd>The National Institute for Science and Technology maintains the <a 751href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation 752of Legal Time in Germany</a>.</dd> 753<dt>Israel</dt> 754<dd>The Interior Ministry periodically issues <a 755href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements" 756hreflang="he">announcements (in Hebrew)</a>.</dd> 757<dt>Italy</dt> 758<dd>The National Institute of Metrological Research maintains a 759<a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml">table of civil time 760(in Italian)</a>.</dd> 761<dt>Macau</dt> 762<dd>The Meteorological and Geophysical Bureau maintains a 763<a href="http://www.smg.gov.mo/smg/geophysics/e_t_Summer%20Time.htm">history 764of summer time</a>. Unfortunately it is incomplete and has errors.</dd> 765<dt>Malaysia</dt> 766<dd>See Singapore <a href="#Singapore">below</a>.</dd> 767<dt>Mexico</dt> 768<dd>The Investigation and Analysis Service of the Mexican Library of 769Congress has published a <a 770href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm" 771hreflang="es">history of Mexican local time (in Spanish)</a>.</dd> 772<dt>Netherlands</dt> 773<dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm" 774hreflang="nl">Legal time in the Netherlands (in Dutch)</a> 775covers the history of local time in the Netherlands from ancient times.</dd> 776<dt>New Zealand</dt> 777<dd>The Department of Internal Affairs maintains a brief <a 778href="https://www.dia.govt.nz/Daylight-Saving-History">History of 779Daylight Saving</a>. The privately-maintained <a 780href="http://astrologyschool.com/nztime.html">History of New Zealand 781time</a> has more details.</dd> 782<dt>Singapore</dt> 783<dd><a id="Singapore" 784href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why 785is Singapore in the "Wrong" Time Zone?</a> details the 786history of legal time in Singapore and Malaysia.</dd> 787<dt>United Kingdom</dt> 788<dd><a 789href="https://www.polyomino.org.uk/british-time/">History of 790legal time in Britain</a> discusses in detail the country 791with perhaps the best-documented history of clock adjustments. 792The National Physical Laboratory also maintains an <a 793href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates">Archive 794of Summer time dates</a>.</dd> 795<dt>United States</dt> 796<dd>The Department of Transportation's <a 797href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent 798Time Zone Proceedings</a> lists changes to time zone boundaries.</dd> 799<dt>Uruguay</dt> 800<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan 801Navy (SOHMA) publishes an annual <a 802href="http://www.armada.mil.uy/Pagina/institucion/dimat/sohma/almanaque.html" hreflang="es">almanac 803(in Spanish)</a>.</dd> 804</dl> 805<h2 id="precision">Precision timekeeping</h2> 806<ul> 807<li><a 808href="http://leapsecond.com/hpan/an1289.pdf">The 809Science of Timekeeping</a> is a thorough introduction 810to the theory and practice of precision timekeeping.</li> 811<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of 812Time 2016</a> contains several freely-readable papers.</li> 813<li><a href="http://www.ntp.org"><abbr 814title="Network Time Protocol">NTP</abbr>: The Network 815Time Protocol</a> (Internet <abbr>RFC</abbr> 5905) 816discusses how to synchronize clocks of 817Internet hosts.</li> 818<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a> 819family of software algorithms can achieve accuracy to a few tens of 820nanoseconds in scalable server farms without special hardware.</li> 821<li>The <a 822href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision 823Time Protocol</a> (<abbr 824title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588) 825can achieve submicrosecond clock accuracy on a local area network 826with special-purpose hardware.</li> 827<li><a 828href="https://tools.ietf.org/html/rfc4833">Timezone 829Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> 830(Internet <abbr>RFC</abbr> 4833) 831specifies a <a 832href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a> 833option for a server to configure 834a client's time zone and daylight saving settings automatically.</li> 835<li><a 836href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical 837Times</a> explains more abstruse astronomical time scales like 838<abbr title="Terrestrial Dynamic Time">TDT</abbr>, 839<abbr title="Geocentric Coordinate Time">TCG</abbr>, and 840<abbr title="Barycentric Dynamic Time">TDB</abbr>. 841<a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time 842Scales</a> goes into more detail, particularly for historical variants.</li> 843<li>The <a href="https://www.iau.org"><abbr 844title="International Astronomical Union">IAU</abbr></a>'s <a 845href="http://www.iausofa.org"><abbr 846title="Standards Of Fundamental Astronomy">SOFA</abbr></a> 847collection contains C and <a 848href="https://en.wikipedia.org/wiki/Fortran">Fortran</a> 849code for converting among time scales like 850<abbr title="International Atomic Time">TAI</abbr>, 851<abbr>TDB</abbr>, <abbr>TDT</abbr> and 852<abbr>UTC</abbr>.</li> 853<li><a 854href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock 855– Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the 856diverse local time 857scales used by each landed mission on Mars.</li> 858<li><a href="http://leapsecond.com">LeapSecond.com</a> is 859dedicated not only to leap seconds but to precise time and frequency 860in general. It covers the state of the art in amateur timekeeping, and 861how the art has progressed over the past few decades.</li> 862<li><a 863href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr 864title="International Earth Rotation and Reference Systems Service">IERS</abbr> 865Bulletins</a> contains official publications of the International 866Earth Rotation and Reference Systems Service, which decides when leap 867seconds occur. The <code>tz</code> code and data support leap seconds 868via an optional "<code>right</code>" configuration, as opposed to the 869default "<code>posix</code>" configuration.</li> 870<li><a href="https://developers.google.com/time/smear">Leap Smear</a> 871discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a 872leap second so that they disagree with <abbr>UTC</abbr> by at most a 873half second, even though every <abbr>POSIX</abbr> minute has exactly 874sixty seconds. This approach works with the default <code>tz</code> 875"<code>posix</code>" configuration, is <a 876href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by 877the <abbr>NTP</abbr> reference implementation, and is used by major 878cloud service providers.</li> 879<li>The <a 880href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap 881Second Discussion List</a> covers <a 882href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy 883and Klepczynski's 1999 proposal to discontinue leap seconds</a>, 884discussed further in 885<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The 886leap second: its history and possible future</a>. 887<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr> 888might be redefined 889without Leap Seconds</a> gives pointers on this 890contentious issue, which was active until 2015 and could become active 891again.</li> 892</ul> 893<h2 id="notation">Time notation</h2> 894<ul> 895<li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data 896Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time 897zone names, abbreviations, identifiers, and formats. For example, it 898contains French translations for "Eastern European Summer Time", 899"<abbr title="Eastern European Summer Time">EEST</abbr>", and 900"Bucharest". Its 901<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type 902charts</a> show these values for many locales. Data values are available in 903both <abbr title="Locale Data Markup Language">LDML</abbr> 904(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>. 905<li> 906<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of 907the international standard date and time notation</a> is a good 908summary of 909<a 910href="https://www.iso.org/standard/40874.html"><em><abbr>ISO</abbr> 9118601:2004 – Data elements and interchange formats – Information 912interchange – Representation of dates and times</em></a>.</li> 913<li> 914<a href="https://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr> 915Schema: Datatypes – dateTime</a> specifies a format inspired by 916<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li> 917<li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of 918Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322) 919specifies the time notation used in email and <a 920href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a> 921headers.</li> 922<li> 923<a href="https://tools.ietf.org/html/rfc3339">Date and Time 924on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) 925specifies an <abbr>ISO</abbr> 8601 926profile for use in new Internet 927protocols.</li> 928<li> 929<a href="https://www.hackcraft.net/web/datetime/">Date & Time 930Formats on the Web</a> surveys web- and Internet-oriented date and time 931formats.</li> 932<li>Alphabetic time zone abbreviations should not be used as unique 933identifiers for <abbr>UT</abbr> offsets as they are ambiguous in 934practice. For example, in English-speaking North America 935"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>, 936but in China it denotes 8 hours ahead of <abbr>UT</abbr>, 937and French-speaking North Americans prefer 938"<abbr title="Heure Normale du Centre">HNC</abbr>" to 939"<abbr>CST</abbr>". The <code><abbr>tz</abbr></code> 940database contains English abbreviations for many timestamps; 941unfortunately some of these abbreviations were merely the database maintainers' 942inventions, and these have been removed when possible.</li> 943<li>Numeric time zone abbreviations typically count hours east of 944<abbr>UT</abbr>, e.g., +09 for Japan and 945−10 for Hawaii. However, the <abbr>POSIX</abbr> 946<code><abbr>TZ</abbr></code> environment variable uses the opposite convention. 947For example, one might use <code><abbr>TZ</abbr>="<abbr 948title="Japan Standard Time">JST</abbr>-9"</code> and 949<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code> 950for Japan and Hawaii, respectively. If the 951<code><abbr>tz</abbr></code> database is available, it is usually better to use 952settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and 953<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid 954confusion, handle old timestamps better, and insulate you better from 955any future changes to the rules. One should never set 956<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like 957<code>"GMT-9"</code>, though, since this would incorrectly imply that 958local time is nine hours ahead of <abbr>UT</abbr> and the time zone 959is called "<abbr>GMT</abbr>".</li> 960</ul> 961<h2 id="see-also">See also</h2> 962<ul> 963<li><a href="theory.html">Theory and pragmatics of the tz code and data</a></li> 964<li><a href="tz-art.html">Time and the Arts</a></li> 965</ul> 966<hr> 967<address> 968This web page is in the public domain, so clarified as of 9692009-05-17 by Arthur David Olson. 970<br> 971Please send corrections to this web page to the 972<a href="mailto:tz@iana.org">time zone mailing list</a>. 973</address> 974</body> 975</html> 976