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 <h3>Outline</h3> 22 <nav> 23 <ul> 24 <li>The <code><abbr>tz</abbr></code> database product and process 25 <ul> 26 <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li> 27 <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li> 28 <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li> 29 <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li> 30 </ul> 31 </li> 32 <li>Uses of the <code><abbr>tz</abbr></code> database 33 <ul> 34 <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li> 35 <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li> 36 <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li> 37 <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li> 38 <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li> 39 </ul> 40 </li> 41 <li>Related data 42 <ul> 43 <li><a href="#other-dbs">Other time zone databases</a></li> 44 <li><a href="#maps">Maps</a></li> 45 <li><a href="#boundaries">Time zone boundaries</a></li> 46 </ul> 47 </li> 48 <li>Timekeeping concepts 49 <ul> 50 <li><a href="#civil">Civil time concepts and history</a></li> 51 <li><a href="#national">National histories of legal time</a></li> 52 <li><a href="#costs">Costs and benefits of time shifts</a></li> 53 <li><a href="#precision">Precision timekeeping</a></li> 54 <li><a href="#notation">Time notation</a></li> 55 <li><a href="#see-also">See also</a></li> 56 </ul> 57 </li> 58 </ul> 59 </nav> 60 61<section> 62<h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2> 63<p> 64The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a> 65time zone database contains code and data 66that represent the history of local time 67for many representative locations around the globe. 68It is updated periodically to reflect changes made by political bodies 69to time zone boundaries and daylight saving rules. 70This database (known as <code><abbr>tz</abbr></code>, 71<code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>) 72is used by several implementations, 73including 74<a href="https://www.gnu.org/software/libc/">the 75<abbr title="GNU's Not Unix">GNU</abbr> 76C Library</a> (used in 77<a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>), 78<a href="https://www.android.com">Android</a>, 79<a href="https://www.freebsd.org">Free<abbr 80title="Berkeley Software Distribution">BSD</abbr></a>, 81<a href="https://netbsd.org">Net<abbr>BSD</abbr></a>, 82<a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>, 83<a href="https://www.chromium.org/chromium-os">Chromium OS</a>, 84<a href="https://cygwin.com">Cygwin</a>, 85<a href="https://mariadb.org">MariaDB</a>, 86<a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>, 87<a href="https://www.mysql.com">MySQL</a>, 88<a href="https://en.wikipedia.org/wiki/WebOS"><abbr 89title="Web Operating System">webOS</abbr></a>, 90<a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr 91title="Advanced Interactive eXecutive">AIX</abbr></a>, 92<a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>, 93<a href="https://www.apple.com/ios"><abbr 94title="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 98title="Virtual Memory System">VMS</abbr></a>, 99<a href="https://www.oracle.com/database/">Oracle Database</a>, and 100<a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p> 101<p> 102Each main entry in the database represents a <dfn>timezone</dfn> 103for a set of civil-time clocks that have all agreed since 1970. 104Timezones are typically identified by continent or ocean and then by the 105name of the largest city within the region containing the clocks. 106For example, <code>America/New_York</code> 107represents most of the <abbr title="United States">US</abbr> eastern time zone; 108<code>America/Phoenix</code> represents most of Arizona, which 109uses mountain time without daylight saving time (<abbr>DST</abbr>); 110<code>America/Detroit</code> represents most of Michigan, which uses 111eastern time but with different <abbr>DST</abbr> rules in 1975; 112and other entries represent smaller regions like Starke County, 113Indiana, which switched from central to eastern time in 1991 114and switched back in 2006. 115To use the database on an extended <a 116href="https://en.wikipedia.org/wiki/POSIX"><abbr 117title="Portable Operating System Interface">POSIX</abbr></a> 118implementation set the <code><abbr>TZ</abbr></code> 119environment variable to the location's full name, 120e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p> 121<p> 122Associated with each timezone is a history of offsets from 123<a href="https://en.wikipedia.org/wiki/Universal_Time">Universal 124Time</a> (<abbr>UT</abbr>), which is <a 125href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean 126Time</a> (<abbr>GMT</abbr>) with days beginning at midnight; 127for timestamps after 1960 this is more precisely <a 128href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated 129Universal Time</a> (<abbr>UTC</abbr>). 130The database also records when daylight saving time was in use, 131along with some time zone abbreviations such as <abbr>EST</abbr> 132for Eastern Standard Time in the <abbr>US</abbr>.</p> 133</section> 134 135<section> 136<h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2> 137<p> 138The following <a 139href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download 140the latest release's two 141<a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a> 142to a <abbr>GNU</abbr>/Linux or similar host.</p> 143<pre><code>mkdir tzdb 144cd tzdb 145<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz 146wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz 147<a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf - 148gzip -dc tzdata-latest.tar.gz | tar -xf - 149</code></pre> 150<p>Alternatively, the following shell commands download the same 151release in a single-tarball format containing extra data 152useful for regression testing:</p> 153<pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a> 154<a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf - 155</code></pre> 156<p>These commands use convenience links to the latest release 157of the <code><abbr>tz</abbr></code> database hosted by the 158<a href="https://www.iana.org/time-zones">Time Zone Database website</a> 159of the <a href="https://www.iana.org">Internet Assigned Numbers 160Authority (IANA)</a>. 161Older releases are in files named 162<code>tzcode<var>V</var>.tar.gz</code>, 163<code>tzdata<var>V</var>.tar.gz</code>, and 164<code>tzdb-<var>V</var>.tar.lz</code>, 165where <code><var>V</var></code> is the version. 166Since 1996, each version has been a four-digit year followed by 167lower-case letter (<samp>a</samp> through <samp>z</samp>, 168then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp> 169through <samp>zzz</samp>, and so on). 170Since version 2016h, each release has contained a text file named 171"<samp>version</samp>" whose first (and currently only) line is the version. 172Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>, 173and are also available in an 174<a href="ftp://ftp.iana.org/tz/releases/"><abbr 175title="File Transfer Protocol">FTP</abbr> directory</a> via a 176less-secure protocol.</p> 177<p>Alternatively, a development repository of code and data can be 178retrieved from <a href="https://github.com">GitHub</a> via the shell 179command:</p> 180<pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a> 181</code></pre> 182<p> 183Since version 2012e, each release has been tagged in development repositories. 184Untagged commits are less well tested and probably contain 185more errors.</p> 186<p> 187After obtaining the code and data files, see the 188<code>README</code> file for what to do next. 189The code lets you compile the <code><abbr>tz</abbr></code> source files into 190machine-readable binary files, one for each location. The binary files 191are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>) 192specified by <a href="https://tools.ietf.org/html/8536">Internet 193<abbr>RFC</abbr> 8536</a>. 194The code also lets 195you read a <abbr>TZif</abbr> file and interpret timestamps for that 196location.</p> 197</section> 198 199<section> 200<h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2> 201<p> 202The <code><abbr>tz</abbr></code> code and data 203are by no means authoritative. If you find errors, please 204send changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>, 205the time zone mailing list. You can also <a 206href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it 207and browse the <a 208href="https://mm.icann.org/pipermail/tz/">archive of old 209messages</a>. 210<a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list 211discussions</a> and corresponding data changes can be 212generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>. 213</p> 214<p> 215If your government plans to change its time zone boundaries or 216daylight saving rules, inform <code>tz@iana.org</code> well in 217advance, as this will coordinate updates to many cell phones, 218computers, and other devices around the world. 219The change should be officially announced at least a year before it affects 220how clocks operate; otherwise, there is a good chance that some 221clocks will operate incorrectly after the change, due 222to delays in propagating updates to software and data. The shorter 223the notice, the more likely clock problems will arise; see "<a 224href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On 225the Timing of Time Zone Changes</a>" for examples. 226The <code><abbr>tz</abbr></code> data can represent planned changes 227far into the future, and a long-planned change can easily be reverted 228or otherwise altered with a year's notice before the change would have 229affected clocks. 230</p> 231<p> 232Changes to the <code><abbr>tz</abbr></code> code and data are often 233propagated to clients via operating system updates, so 234client <code><abbr>tz</abbr></code> data can often be corrected by 235applying these updates. With GNU/Linux and similar systems, if your 236maintenance provider has not yet adopted the 237latest <code><abbr>tz</abbr></code> data, you can often short-circuit 238the process by tailoring the generic instructions in 239the <code><abbr>tz</abbr> README</code> file and installing the latest 240data yourself. System-specific instructions for installing the 241latest <code><abbr>tz</abbr></code> data have also been published 242for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>, 243<a 244href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>, 245<a 246href="http://userguide.icu-project.org/datetime/timezone"><abbr 247title="International Components for Unicode">ICU</abbr></a>, 248<a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr> 249JDK</a>, 250<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a 251href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>, 252<a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a 253href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html#usage">OpenJDK/Oracle JDK</a>. 254</p> 255<p>Sources for the <code><abbr>tz</abbr></code> database are 256<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr 257title="Unicode Transformation Format 8-bit">UTF-8</abbr></a> 258<a href="https://en.wikipedia.org/wiki/Text_file">text files</a> 259with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr 260title="linefeed">LF</abbr></a>, 261which can be modified by common text editors such 262as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>, 263<a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and 264<a href="https://www.vim.org">vim</a>. 265Specialized source-file editing can be done via the 266<a href="https://packagecontrol.io/packages/zoneinfo">Sublime 267zoneinfo</a> package for <a 268href="https://www.sublimetext.com">Sublime Text</a> and the <a 269href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode 270zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual 271Studio Code</a>. 272</p> 273<p> 274For further information about updates, please see 275<a href="https://tools.ietf.org/html/rfc6557">Procedures for 276Maintaining the Time Zone Database</a> (Internet <abbr 277title="Request For Comments">RFC</abbr> 6557). More detail can be 278found in <a href="theory.html">Theory and pragmatics of the 279<code><abbr>tz</abbr></code> code and data</a>. 280<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a> 281displays changes between recent <code><abbr>tzdb</abbr></code> versions. 282</p> 283</section> 284 285<section> 286<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2> 287<ul> 288<li>The article 289<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is 290an encyclopedic summary.</li> 291<li><a href="tz-how-to.html">How to Read the 292tz Database Source Files</a> explains the <code><abbr>tz</abbr></code> 293database format.</li> 294<li><a 295href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A 296literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the 297database's style.</li> 298</ul> 299</section> 300 301<section> 302<h2 id="web">Web sites using recent versions of the 303<code><abbr>tz</abbr></code> database</h2> 304<p> 305These are listed roughly in ascending order of complexity and fanciness. 306</p> 307<ul> 308<li><a href="https://time.is">Time.is</a> shows locations' 309time and zones.</li> 310<li><a href="https://www.timejones.com">TimeJones.com</a>, 311<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and 312<a href="https://www.worldclock.com">The World Clock</a> 313are time zone converters.</li> 314<li><a href="https://timezonedb.com/download">TimeZoneDB Database</a> 315publishes <code><abbr>tzdb</abbr></code>-derived data in 316<a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr 317title="comma-separated values">CSV</abbr></a> and 318in <a href="https://en.wikipedia.org/wiki/SQL"><abbr 319title="Structured Query Language">SQL</abbr></a> form.</li> 320<li><a 321href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a> 322lets you see the <code><abbr>TZ</abbr></code> values directly.</li> 323<li><a 324href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current 325Time in 1000 Places</a> uses descriptions of the values.</li> 326<li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a> 327uses a pulldown menu.</li> 328<li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete 329timezone information for all countries</a> displays tables of DST rules. 330<li><a href="https://www.timeanddate.com/worldclock/">The World Clock – 331Worldwide</a> lets you sort zone names and convert times.</li> 332<li><a href="https://24timezones.com">24TimeZones</a> has a world 333time map and a time converter.</li> 334<li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a> 335calculates the current time difference between locations.</li> 336<li><a href="http://www.wx-now.com">Weather Now</a> and 337<a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li> 338</ul> 339</section> 340 341<section> 342<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2> 343<ul> 344<li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s 345<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data 346Distribution Service (tzdist) working group</a> defined <a 347href="https://tools.ietf.org/html/rfc7808">TZDIST</a> 348(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service, 349along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a> 350(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for 351transferring time zone data by reference. 352<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST 353implementations</a> are available. 354The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis 355mailing list</a> discusses possible extensions.</li> 356<li>The <a href="https://tools.ietf.org/html/rfc5545"> 357Internet Calendaring and Scheduling Core Object Specification 358(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445) 359covers time zone 360data; see its VTIMEZONE calendar component. 361The iCalendar format requires specialized parsers and generators; a 362variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a> 363(Internet <abbr>RFC</abbr> 6321) uses 364<a href="https://www.w3.org/XML/"><abbr 365title="Extensible Markup Language">XML</abbr></a> format, and a variant 366<a href="https://tools.ietf.org/html/rfc7265">jCal</a> 367(Internet <abbr>RFC</abbr> 7265) 368uses <a href="https://www.json.org"><abbr 369title="JavaScript Object Notation">JSON</abbr></a> format.</li> 370</ul> 371</section> 372 373<section> 374<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2> 375<p>Although some of these do not fully support 376<code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code> 377distributions you can generally work around compatibility problems by 378running the command <code>make rearguard_tarballs</code> and compiling 379from the resulting tarballs instead.</p> 380<ul> 381<li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a 382href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a> 383program that compiles 384<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files. 385Vzic is freely 386available under the <a 387href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> 388General Public License (<abbr 389title="General Public License">GPL</abbr>)</a>.</li> 390<li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz 391database conversion utility</a> is like Vzic, except for the <a 392href="https://dotnet.microsoft.com">.NET framework</a> 393and with a <abbr>BSD</abbr>-style license.</li> 394<li><a 395href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a> 396contains a script <code>parse_olson</code> that compiles 397<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a> 398modules. It is part of the Perl <a 399href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>, 400which is freely 401available under both the <abbr>GPL</abbr> and the Perl Artistic 402License. DateTime::TimeZone also contains a script 403<code>tests_from_zdump</code> that generates test cases for each clock 404transition in the <code><abbr>tz</abbr></code> database.</li> 405<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone 406Database Parser</a> is a 407<a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and 408runtime library with <a 409href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a> 410adopted by 411<a href="https://en.wikipedia.org/wiki/C++20">C++20</a>, 412the current iteration of the C++ standard. 413It is freely available under the 414<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li> 415<li><a id="ICU" href="http://site.icu-project.org">International Components for 416Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a 417href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a> 418libraries for internationalization that 419has a compiler from <code><abbr>tz</abbr></code> source 420and from <abbr title="Common Locale Data Repository">CLDR</abbr> data 421(mentioned <a href="#CLDR">below</a>) 422into an <abbr>ICU</abbr>-specific format. 423<abbr>ICU</abbr> is freely available under a 424<abbr>BSD</abbr>-style license.</li> 425<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for 426the <a href="https://elixir-lang.org">Elixir</a> language downloads 427and compiles <code><abbr>tz</abbr></code> source and exposes <abbr 428title="Application Program Interface">API</abbr>s for use. It is 429freely available under the <abbr>MIT</abbr> license.</li> 430<li>Java-based compilers and libraries include: 431<ul> 432<li>The <a 433href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater 434tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by 435<a href="https://openjdk.java.net/">OpenJDK</a> and 436<a href="https://jdk.java.net/">Oracle JDK</a>. 437Although its source code is proprietary, its executable is available under the 438<a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE 439Timezone Updater License Agreement</a>.</li> 440<li>The <a 441href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java 442SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a 443href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>, 444which is freely available under a <abbr>BSD</abbr>-style license.</li> 445<li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date 446and time <abbr>API</abbr></a> contains a class 447<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles 448<code><abbr>tz</abbr></code> source into a binary format. It inspired 449Java 8 <code>java.time</code>, which its users should migrate to once 450they can assume Java 8 or later. It is available under the <a 451href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li> 452<li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a 453href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a> 454are alternatives to TZUpdater. IANA Updater's license is unclear; 455ZIUpdater is licensed under the <abbr>GPL</abbr>.</li> 456<li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and 457time library for Android</a> and 458<a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date, 459time and interval library for Java</a> compile 460<code><abbr>tz</abbr></code> source into a binary format. 461Time4A is available under the Apache License and Time4J is 462available under the <a 463href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser 464General Public License (<abbr title="Lesser General Public 465License">LGPL</abbr>)</a>.</li> 466<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and 467Java-based libraries.</li> 468</ul> 469<li><a href="https://nodatime.org">Noda Time – Date and 470time <abbr>API</abbr> for .NET</a> 471is like Joda-Time and Time4J, but for the .NET framework instead of Java. 472It is freely available under the Apache License.</li> 473<li>Many modern 474<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a> 475runtimes support <code><abbr>tz</abbr></code> natively via the 476<samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>. 477On other runtimes, you can use one of the following. 478<ul> 479<li>The <a 480href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp> 481timezone polyfill</a> 482is freely available under a <abbr>BSD</abbr>-style license.</li> 483<li><a 484href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a> 485compiles time zone data into a compact form designed for 486JavaScript. It is freely available under a combination of 487the <abbr>MIT</abbr> license and the Apache License.</li> 488<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a 489plugin for the <a href="https://momentjs.com">Moment.js</a> date 490manipulation library. It is freely available under the <abbr>MIT</abbr> 491license.</li> 492<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a 493JavaScript library that supports date arithmetic that is time zone 494aware. It is freely available under the <abbr>MIT</abbr> license.</li> 495<li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s 496<abbr>API</abbr> is upward compatible with standard JavaScript 497Dates. It is freely available under the Apache License.</li> 498</ul> 499<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a 500compiler from <code><abbr>tz</abbr></code> source into 501<a href="https://julialang.org/">Julia</a>. It is freely available 502under the <abbr>MIT</abbr> license.</li> 503<li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> – 504<abbr>IANA</abbr> Time Zone Database for Delphi/<abbr 505title="Free Pascal Compiler">FPC</abbr></a> 506compiles from <code><abbr>tz</abbr></code> source into 507<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a> 508as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a> 509and <a 510href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>. 511It is freely available under a <abbr>BSD</abbr>-style license.</li> 512<li><a href="http://pytz.sourceforge.net">pytz – World Timezone 513Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into 514<a href="https://www.python.org">Python</a>. 515It is freely available under a <abbr>BSD</abbr>-style license. 516In code that can assume Python 3.9 or later it is superseded by 517<a href="#python-zoneinfo"><code>zoneinfo.ZoneInfo</code></a> 518and the <a href="#pypi-tzdata"><code>tzdata</code> Python package</a>.</li> 519<li><a href="https://tzinfo.github.io">TZInfo – 520Ruby Timezone Library</a> 521compiles <code><abbr>tz</abbr></code> source into 522<a href="https://www.ruby-lang.org/en/">Ruby</a>. 523It is freely available under the <abbr>MIT</abbr> license.</li> 524<li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time 525Library</a> is 526a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class 527library that compiles <code><abbr>tz</abbr></code> source into a time 528zone repository whose format 529is either proprietary or an <abbr>XML</abbr>-encoded 530representation.</li> 531<li><a id="Tcl" href="https://tcl.tk">Tcl</a> 532contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code> 533source into text files, along with a runtime that can read those 534files. Tcl is freely available under a <abbr>BSD</abbr>-style 535license.</li> 536</ul> 537</section> 538 539<section> 540<h2 id="TZif">Other <abbr>TZif</abbr> readers</h2> 541<ul> 542<li>The <a 543href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C 544Library</a> 545has an independent, thread-safe implementation of 546a <abbr>TZif</abbr> file reader. 547This library is freely available under the LGPL 548and is widely used in <abbr>GNU</abbr>/Linux systems.</li> 549<li><a href="https://www.gnome.org">GNOME</a>'s 550<a href="https://developer.gnome.org/glib/">GLib</a> has 551a <abbr>TZif</abbr> file reader written in C that 552creates a <code>GTimeZone</code> object representing sets 553of <abbr>UT</abbr> offsets. 554It is freely available under the <abbr>LGPL</abbr>.</li> 555<li>The 556<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s 557<code>baltzo::TimeZoneUtil</code> component contains a C++ 558implementation of a <abbr>TZif</abbr> file reader. It is freely available under 559the Apache License.</li> 560<li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++ 561library that translates between <abbr>UT</abbr> and civil time and 562can read <abbr>TZif</abbr> files. It is freely available under the Apache 563License.</li> 564<li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a> 565is a <abbr>TZif</abbr> file reader written in Java. 566It is freely available under the <abbr>LGPL</abbr>.</li> 567<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C 568library that reads <abbr>TZif</abbr> files and converts 569timestamps from one time zone or format to another. 570It is used by <a href="https://secure.php.net"><abbr 571title="PHP: Hypertext Preprocessor">PHP</abbr></a>, 572<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>, 573and <a href="https://www.mongodb.com">MongoDB</a>. 574It is freely available under the <abbr>MIT</abbr> license.</li> 575<li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a 576<abbr>TZif</abbr> file reader.</li> 577<li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile"> 578DateTime::TimeZone::Tzfile</a> 579is a <abbr>TZif</abbr> file reader written in Perl. 580It is freely available under the same terms as Perl 581(dual <abbr>GPL</abbr> and Artistic license).</li> 582<li>Python has a <a id="python-zoneinfo" 583href="https://docs.python.org/3.9/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code> 584class</a> that reads <abbr>TZif</abbr> data and creates objects 585that represent <code><abbr>tzdb</abbr></code> timezones. 586Python is freely available under the 587<a href="https://docs.python.org/3.9/license.html">Python Software Foundation 588License</a>. 589A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module 590<a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a> 591supplies TZif data if the underlying system data cannot be found; 592it is freely available under the Apache License.</li> 593<li>The 594public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a> 595library contains a Python tool that 596converts <abbr>TZif</abbr> data into 597<abbr>JSON</abbr>-format data suitable for use 598in its JavaScript library for time zone conversion. Dates before 1970 599are not supported.</li> 600<li>The <a 601href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a> 602package contains <a href="https://www.haskell.org">Haskell</a> code that 603parses and uses <abbr>TZif</abbr> data. It is freely 604available under a <abbr>BSD</abbr>-style license.</li> 605</ul> 606</section> 607 608<section> 609<h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2> 610<ul> 611<li><a href="https://foxclocks.org">FoxClocks</a> 612is an extension for <a href="https://www.google.com/chrome/">Google 613Chrome</a> and for <a 614href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla 615Toolkit</a> applications like <a 616href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a 617href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>. 618It displays multiple clocks in the application window, and has a mapping 619interface to <a href="https://www.google.com/earth/">Google Earth</a>. 620It is freely available under the <abbr>GPL</abbr>.</li> 621<li><a href="https://golang.org">Go programming language</a> 622implementations contain a copy of a 32-bit subset of a recent 623<code><abbr>tz</abbr></code> database in a 624Go-specific format.</li> 625<li>Microsoft Windows 8.1 626and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr> 627data (mentioned <a href="#CLDR">below</a>) used by the 628<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> / 629<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes 630<a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and 631<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>. 632<a id="System.TimeZoneInfo" 633href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring 634Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes 635the older, proprietary method of Microsoft Windows 2000 and later, 636which stores time zone data in the 637<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The 638<a 639href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone → 640Tzid table</a> or <a 641href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr> 642file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs 643to <code><abbr>tz</abbr></code> names. 644These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library, 645or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>. 646<li><a 647href="https://www.oracle.com/java/index.html">Oracle 648Java</a> contains a copy of a subset of a recent 649<code><abbr>tz</abbr></code> database in a 650Java-specific format.</li> 651<li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone 652Master</a> is a Microsoft Windows clock program that can automatically 653download, compile and use <code><abbr>tz</abbr></code> releases. 654The Basic version is free.</li> 655<li><a 656href="http://veladg.com/velaterra.html">VelaTerra</a> is 657a macOS program. Its developers 658<a href="http://veladg.com/tzoffer.html">offer free 659licenses</a> to <code><abbr>tz</abbr></code> contributors.</li> 660</ul> 661</section> 662 663<section> 664<h2 id="other-dbs">Other time zone databases</h2> 665<ul> 666<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a> 667is Astrodienst's Web version of Shanks and Pottenger's out-of-print 668time zone history atlases 669<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and 670<a href="https://www.worldcat.org/oclc/76950459">for the world</a>. 671Although these extensive atlases 672<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were 673sources for much of the older <code><abbr>tz</abbr></code> data</a>, 674they are unreliable as Shanks appears to have 675guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no 676sources and do not indicate which entries are guesswork.</li> 677<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in 678its own <code>tztab</code>(4) format.</li> 679<li>Microsoft Windows has proprietary data mentioned 680<a href="#System.TimeZoneInfo">above</a>.</li> 681<li><a href="https://www.worldtimeserver.com">World Time Server</a> 682is another time zone database.</li> 683<li>The <a 684href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard 685Schedules Information Manual</a> of the 686International Air Transport Association 687gives current time zone rules for airports served by commercial aviation.</li> 688</ul> 689</section> 690 691<section> 692<h2 id="maps">Maps</h2> 693<ul> 694<li>The <a 695href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World 696and Regional Maps section</a> of <em>The World Factbook</em>, published by the 697<a href="https://www.cia.gov">US Central Intelligence 698Agency (<abbr 699title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time 700zone map; the 701<a 702href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda 703Library Map Collection</a> 704of the University of Texas at Austin has copies of 705recent editions. 706The pictorial quality is good, 707but the maps do not indicate daylight saving time, 708and parts of the data are a few years out of date.</li> 709<li><a href="https://www.worldtimezone.com">World Time Zone Map 710with current time</a> 711has several fancy time zone maps; it covers Russia particularly well. 712The maps' pictorial quality is not quite as good as the 713<abbr>CIA</abbr>'s 714but the maps are more up to date.</li> 715<li><a 716href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How 717much is time wrong around the world?</a> maps the difference between 718mean solar and standard time, highlighting areas such as western China 719where the two differ greatly. It's a bit out of date, unfortunately.</li> 720</ul> 721</section> 722 723<section> 724<h2 id="boundaries">Time zone boundaries</h2> 725<p>Geographical boundaries between timezones are available 726from several <a 727href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet 728geolocation</a> 729services and other sources.</p> 730<ul> 731<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone 732Boundary Builder</a> extracts 733<a href="https://www.openstreetmap.org">Open Street Map</a> data to build 734boundaries of <code><abbr>tzdb</abbr></code> timezones. 735Its code is freely available under the <abbr>MIT</abbr> license, and 736its data entries are freely available under the 737<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons 738Open Database License</a>. The maps' borders appear to be quite accurate.</li> 739<li>Programmatic interfaces that map geographical coordinates via tz_world to 740<code><abbr>tzdb</abbr></code> timezones include: 741<ul> 742<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is 743written in <a 744href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a> 745and is freely available under the <abbr>MIT</abbr> license.</li> 746<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a> 747is written in Go and is freely available under the Apache License.</li> 748<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>, 749in both Java and 750<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a> 751form, is freely available under the MIT license.</li> 752<li>For <a href="https://nodejs.org/en/">Node.js</a>, 753the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a> 754is freely available under the MIT license, and 755the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a> 756is in the public domain.</li> 757<li>The <a 758href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a> 759library for Python is freely available under the MIT license. 760<li>The <a 761href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a> 762library for Ruby is freely available under the MIT license.</li> 763</ul></li> 764<li>Free access via a network API, if you register a key, is provided by 765the <a 766href="https://www.geonames.org/export/web-services.html#timezone">GeoNames 767Timezone web service</a>, the <a 768href="https://developers.google.com/maps/documentation/timezone/intro">Google 769Maps Time Zone API</a>, and 770the <a href="https://timezonedb.com/api">TimeZoneDB API</a>. 771Commercial network API access is provided 772by <a href="https://askgeo.com">AskGeo</a> 773and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>. 774</li> 775<li>"<a 776href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How 777to get a time zone from a location using latitude and longitude 778coordinates?</a>" discusses other geolocation possibilities.</li> 779<li><a href="http://statoids.com/statoids.html">Administrative 780Divisions of Countries ("Statoids")</a> lists 781political subdivision data related to time zones.</li> 782<li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time 783zone boundaries for multizone countries</a> summarizes legal 784boundaries between time zones within countries.</li> 785<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software 786– GIS and Database Tools</a> includes a Manifold-format map of 787world time zone boundaries distributed under the 788<abbr>GPL</abbr>.</li> 789<li>A ship within the <a 790href="https://en.wikipedia.org/wiki/Territorial_waters">territorial 791waters</a> of any nation uses that nation's time. In international 792waters, time zone boundaries are meridians 15° apart, except that 793<abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5° 794wide and are separated by 795the 180° meridian (not by the International Date Line, which is 796for land and territorial waters only). A captain can change ship's 797clocks any time after entering a new time zone; midnight changes are 798common.</li> 799</ul> 800</section> 801 802<section> 803<h2 id="civil">Civil time concepts and history</h2> 804<ul> 805<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A 806Walk through Time</a> 807surveys the evolution of timekeeping.</li> 808<li>The history of daylight saving time is surveyed in <a 809href="http://www.webexhibits.org/daylightsaving/">About Daylight 810Saving Time – History, rationale, laws & dates</a> and summarized in 811<a href="http://seizethedaylight.com/dst/">A Brief 812History of Daylight Saving Time</a>.</li> 813<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time 814Lords</a> discusses how authoritarians manipulate civil time.</li> 815<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a> 816contains guidelines and best practices for software applications that 817deal with civil time.</li> 818<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of 819the International Date Line</a> tells the story of the most important 820time zone boundary.</li> 821<li><a href="http://statoids.com/tconcept.html">Basic Time 822Zone Concepts</a> discusses terminological issues behind time zones.</li> 823</ul> 824</section> 825 826<section> 827<h2 id="national">National histories of legal time</h2> 828<dl> 829<dt>Australia</dt> 830<dd>The Parliamentary Library commissioned a <a 831href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research 832paper on daylight saving time in Australia</a>. 833The Bureau of Meteorology publishes a list of <a 834href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation 835Dates of Daylight Savings Time within Australia</a>.</dd> 836<dt>Belgium</dt> 837<dd>The Royal Observatory of Belgium maintains a table of time in 838Belgium (in 839<a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html" 840hreflang="nl">Dutch</a> and <a 841href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html" 842hreflang="fr">French</a>).</dd> 843<dt>Brazil</dt> 844<dd>The Time Service Department of the National Observatory 845records <a href="http://pcdsh01.on.br/DecHV.html" 846hreflang="pt-BR">Brazil's daylight saving time decrees (in 847Portuguese)</a>.</dd> 848<dt>Canada</dt> 849<dd>National Research Council Canada publishes current 850and some older information about <a 851href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time 852zones and daylight saving time</a>.</dd> 853<dt>Chile</dt> 854<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a 855<a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of 856Chile's official time (in Spanish)</a>.</dd> 857<dt>China</dt> 858<dd>The Hong Kong Observatory maintains a 859<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of 860 summer time in Hong Kong</a>, 861and Macau's Meteorological and Geophysical Bureau maintains a <a 862href="https://www.smg.gov.mo/en/subpage/224/page/174">similar 863history for Macau</a>. 864Unfortunately the latter is incomplete and has errors.</dd> 865<dt>Czech Republic</dt> 866<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas" 867hreflang="cs">When daylight saving time starts and ends (in Czech)</a> 868summarizes and cites historical DST regulations.</dd> 869<dt>Germany</dt> 870<dd>The National Institute for Science and Technology maintains the <a 871href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation 872of Legal Time in Germany</a>.</dd> 873<dt>Israel</dt> 874<dd>The Interior Ministry periodically issues <a 875href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements" 876hreflang="he">announcements (in Hebrew)</a>.</dd> 877<dt>Malaysia</dt> 878<dd>See Singapore <a href="#Singapore">below</a>.</dd> 879<dt>Mexico</dt> 880<dd>The Investigation and Analysis Service of the Mexican Library of 881Congress has published a <a 882href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm" 883hreflang="es">history of Mexican local time (in Spanish)</a>.</dd> 884<dt>Netherlands</dt> 885<dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm" 886hreflang="nl">Legal time in the Netherlands (in Dutch)</a> 887covers the history of local time in the Netherlands from ancient times.</dd> 888<dt>New Zealand</dt> 889<dd>The Department of Internal Affairs maintains a brief <a 890href="https://www.dia.govt.nz/Daylight-Saving-History">History of 891Daylight Saving</a>.</dd> 892<dt>Singapore</dt> 893<dd><a id="Singapore" 894href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why 895is Singapore in the "Wrong" Time Zone?</a> details the 896history of legal time in Singapore and Malaysia.</dd> 897<dt>United Kingdom</dt> 898<dd><a 899href="https://www.polyomino.org.uk/british-time/">History of 900legal time in Britain</a> discusses in detail the country 901with perhaps the best-documented history of clock adjustments.</dd> 902<dt>United States</dt> 903<dd>The Department of Transportation's <a 904href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent 905Time Zone Proceedings</a> lists changes to time zone boundaries.</dd> 906<dt>Uruguay</dt> 907<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan 908Navy (SOHMA) publishes an annual <a 909href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac 910(in Spanish)</a>.</dd> 911</dl> 912</section> 913 914<section> 915<h2 id="costs">Costs and benefits of time shifts</h2> 916<p>Various sources argue for and against daylight saving time and time 917zone shifts, and many scientific studies have been conducted. This 918section summarizes reviews and position statements based on 919scientific literature in the area.</p> 920<ul> 921<li>Carey RN, Sarma KM. 922<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of 923daylight saving time on road traffic collision risk: a systematic 924review</a>. 925<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>. 926This reviews research literature and concludes that the evidence 927neither supports nor refutes road safety benefits from 928shifts in time zones.</li> 929<li>Havranek T, Herman D, Irsova D. 930<a href="https://www.econstor.eu/handle/10419/174191">Does daylight 931saving save electricity? A meta-analysis</a>. <em>Energy J.</em> 9322018;39(2). 933doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>. 934This analyzes research literature and concludes, "Electricity savings 935are larger for countries farther away from the equator, while 936subtropical regions consume more electricity because of DST."</li> 937<li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>. 938<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time: 939an American Academy of Sleep Medicine position statement</a>. 940<em>J Clin Sleep Med.</em> 9412020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>. 942This argues for permanent standard time due to health risks of both 943DST transitions and permanent DST.</li> 944<li>Roenneberg T, Winnebeck EC, Klerman EB. 945<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight 946saving time and artificial time zones – a battle between 947biological and social times</a>. <em>Front Physiol.</em> 2019;10:944. 948doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>. 949This reviews evidence about the health effects of DST and concludes, 950"In summary, the scientific literature strongly argues against the 951switching between DST and Standard Time and even more so against 952adopting DST permanently."</li> 953</ul> 954</section> 955 956<section> 957<h2 id="precision">Precision timekeeping</h2> 958<ul> 959<li><a 960href="http://leapsecond.com/hpan/an1289.pdf">The 961Science of Timekeeping</a> is a thorough introduction 962to the theory and practice of precision timekeeping.</li> 963<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of 964Time 2016</a> contains several freely-readable papers.</li> 965<li><a href="http://www.ntp.org"><abbr 966title="Network Time Protocol">NTP</abbr>: The Network 967Time Protocol</a> (Internet <abbr>RFC</abbr> 5905) 968discusses how to synchronize clocks of 969Internet hosts.</li> 970<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a> 971family of software algorithms can achieve accuracy to a few tens of 972nanoseconds in scalable server farms without special hardware.</li> 973<li>The <a 974href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision 975Time Protocol</a> (<abbr 976title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588) 977can achieve submicrosecond clock accuracy on a local area network 978with special-purpose hardware.</li> 979<li><a 980href="https://tools.ietf.org/html/rfc4833">Timezone 981Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> 982(Internet <abbr>RFC</abbr> 4833) 983specifies a <a 984href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a> 985option for a server to configure 986a client's time zone and daylight saving settings automatically.</li> 987<li><a 988href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical 989Times</a> explains more abstruse astronomical time scales like 990<abbr title="Terrestrial Dynamic Time">TDT</abbr>, 991<abbr title="Geocentric Coordinate Time">TCG</abbr>, and 992<abbr title="Barycentric Dynamic Time">TDB</abbr>. 993<a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time 994Scales</a> goes into more detail, particularly for historical variants.</li> 995<li>The <a href="https://www.iau.org"><abbr 996title="International Astronomical Union">IAU</abbr></a>'s <a 997href="http://www.iausofa.org"><abbr 998title="Standards Of Fundamental Astronomy">SOFA</abbr></a> 999collection contains C and <a 1000href="https://en.wikipedia.org/wiki/Fortran">Fortran</a> 1001code for converting among time scales like 1002<abbr title="International Atomic Time">TAI</abbr>, 1003<abbr>TDB</abbr>, <abbr>TDT</abbr> and 1004<abbr>UTC</abbr>.</li> 1005<li><a 1006href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock 1007– Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the 1008diverse local time 1009scales used by each landed mission on Mars.</li> 1010<li><a href="http://leapsecond.com">LeapSecond.com</a> is 1011dedicated not only to leap seconds but to precise time and frequency 1012in general. It covers the state of the art in amateur timekeeping, and 1013how the art has progressed over the past few decades.</li> 1014<li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a 1015href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency 1016and time-signal emissions</a>, International Telecommunication Union – 1017Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li> 1018<li><a 1019href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr 1020title="International Earth Rotation and Reference Systems Service">IERS</abbr> 1021Bulletins</a> contains official publications of the International 1022Earth Rotation and Reference Systems Service, which decides when leap 1023seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds 1024via an optional "<code>right</code>" configuration where a computer's internal 1025<code>time_t</code> integer clock counts every <abbr>TAI</abbr> second, 1026as opposed to the default "<code>posix</code>" configuration 1027where the internal clock ignores leap seconds. 1028The two configurations agree for timestamps starting with 1972-01-01 00:00:00 1029<abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for 1030timestamps starting with <code>time_t</code> 78 796 800, 1031which corresponds to the first leap second 10321972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration, 1033and to 10341972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration. 1035In practice the two configurations also agree for timestamps before 10361972 even though the historical situation is messy, partly because 1037neither <abbr>UTC</abbr> nor <abbr>TAI</abbr> 1038is well-defined for sufficiently-old timestamps.</li> 1039<li><a href="https://developers.google.com/time/smear">Leap Smear</a> 1040discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a 1041leap second so that they disagree with <abbr>UTC</abbr> by at most a 1042half second, even though every <abbr>POSIX</abbr> minute has exactly 1043sixty seconds. This approach works with the default <code><abbr>tz</abbr></code> 1044"<code>posix</code>" configuration, is <a 1045href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by 1046the <abbr>NTP</abbr> reference implementation, and is used by major 1047cloud service providers. However, according to 1048<a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of 1049Network Time Protocol Best Current Practices</a> 1050(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for 1051applications requiring accurate <abbr>UTC</abbr> or civil time, 1052and is intended for use only in single, well-controlled environments.</li> 1053<li>The <a 1054href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap 1055Second Discussion List</a> covers <a 1056href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy 1057and Klepczynski's 1999 proposal to discontinue leap seconds</a>, 1058discussed further in 1059<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The 1060leap second: its history and possible future</a>. 1061<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr> 1062might be redefined 1063without Leap Seconds</a> gives pointers on this 1064contentious issue, which was active until 2015 and could become active 1065again.</li> 1066</ul> 1067</section> 1068 1069<section> 1070<h2 id="notation">Time notation</h2> 1071<ul> 1072<li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data 1073Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time 1074zone names, abbreviations, identifiers, and formats. For example, it 1075contains French translations for "Eastern European Summer Time", 1076"<abbr title="Eastern European Summer Time">EEST</abbr>", and 1077"Bucharest". Its 1078<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type 1079charts</a> show these values for many locales. Data values are available in 1080both <abbr title="Locale Data Markup Language">LDML</abbr> 1081(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>. 1082<li> 1083<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of 1084the international standard date and time notation</a> covers 1085<a 1086href="https://www.iso.org/standard/70907.html"><em><abbr 1087title="International Organization for Standardization">ISO</abbr> 10888601-1:2019 – Date and time – Representations for information 1089interchange – Part 1: Basic rules</em></a>.</li> 1090<li> 1091<a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr> 1092Schema: Datatypes – dateTime</a> specifies a format inspired by 1093<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li> 1094<li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of 1095Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322) 1096specifies the time notation used in email and <a 1097href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a> 1098headers.</li> 1099<li> 1100<a href="https://tools.ietf.org/html/rfc3339">Date and Time 1101on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) 1102specifies an <abbr>ISO</abbr> 8601 1103profile for use in new Internet 1104protocols.</li> 1105<li> 1106<a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time 1107Formats on the Web</a> surveys web- and Internet-oriented date and time 1108formats.</li> 1109<li>Alphabetic time zone abbreviations should not be used as unique 1110identifiers for <abbr>UT</abbr> offsets as they are ambiguous in 1111practice. For example, in English-speaking North America 1112"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>, 1113but in China it denotes 8 hours ahead of <abbr>UT</abbr>, 1114and French-speaking North Americans prefer 1115"<abbr title="Heure Normale du Centre">HNC</abbr>" to 1116"<abbr>CST</abbr>". The <code><abbr>tz</abbr></code> 1117database contains English abbreviations for many timestamps; 1118unfortunately some of these abbreviations were merely the database maintainers' 1119inventions, and these have been removed when possible.</li> 1120<li>Numeric time zone abbreviations typically count hours east of 1121<abbr>UT</abbr>, e.g., +09 for Japan and 1122−10 for Hawaii. However, the <abbr>POSIX</abbr> 1123<code><abbr>TZ</abbr></code> environment variable uses the opposite convention. 1124For example, one might use <code><abbr>TZ</abbr>="<abbr 1125title="Japan Standard Time">JST</abbr>-9"</code> and 1126<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code> 1127for Japan and Hawaii, respectively. If the 1128<code><abbr>tz</abbr></code> database is available, it is usually better to use 1129settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and 1130<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid 1131confusion, handle old timestamps better, and insulate you better from 1132any future changes to the rules. One should never set 1133<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like 1134<code>"GMT-9"</code>, though, since this would incorrectly imply that 1135local time is nine hours ahead of <abbr>UT</abbr> and the time zone 1136is called "<abbr>GMT</abbr>".</li> 1137</ul> 1138</section> 1139 1140<section> 1141<h2 id="see-also">See also</h2> 1142<ul> 1143<li><a href="theory.html">Theory and pragmatics of the 1144<code><abbr>tz</abbr></code> code and data</a></li> 1145<li><a href="tz-art.html">Time and the Arts</a></li> 1146</ul> 1147</section> 1148 1149<footer> 1150<hr> 1151This web page is in the public domain, so clarified as of 11522009-05-17 by Arthur David Olson. 1153<br> 1154Please send corrections to this web page to the 1155<a href="mailto:tz@iana.org">time zone mailing list</a>. 1156</footer> 1157</body> 1158</html> 1159