tz-link.html revision 1.8
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></a> 249and <a 250href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a> 251Java, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a 252href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>, 253and <a 254href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below). 255</p> 256<p>Sources for the <code><abbr>tz</abbr></code> database are 257<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr 258title="Unicode Transformation Format 8-bit">UTF-8</abbr></a> 259<a href="https://en.wikipedia.org/wiki/Text_file">text files</a> 260with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr 261title="linefeed">LF</abbr></a>, 262which can be modified by common text editors such 263as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>, 264<a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and 265<a href="https://www.vim.org">vim</a>. 266Specialized source-file editing can be done via the 267<a href="https://packagecontrol.io/packages/zoneinfo">Sublime 268zoneinfo</a> package for <a 269href="https://www.sublimetext.com">Sublime Text</a> and the <a 270href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode 271zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual 272Studio Code</a>. 273</p> 274<p> 275For further information about updates, please see 276<a href="https://tools.ietf.org/html/rfc6557">Procedures for 277Maintaining the Time Zone Database</a> (Internet <abbr 278title="Request For Comments">RFC</abbr> 6557). More detail can be 279found in <a href="theory.html">Theory and pragmatics of the 280<code><abbr>tz</abbr></code> code and data</a>. 281<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a> 282displays changes between recent <code><abbr>tzdb</abbr></code> versions. 283</p> 284</section> 285 286<section> 287<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2> 288<ul> 289<li>The article 290<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is 291an encyclopedic summary.</li> 292<li><a href="tz-how-to.html">How to Read the 293tz Database Source Files</a> explains the <code><abbr>tz</abbr></code> 294database format.</li> 295<li><a 296href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A 297literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the 298database's style.</li> 299</ul> 300</section> 301 302<section> 303<h2 id="web">Web sites using recent versions of the 304<code><abbr>tz</abbr></code> database</h2> 305<p> 306These are listed roughly in ascending order of complexity and fanciness. 307</p> 308<ul> 309<li><a href="https://time.is">Time.is</a> shows locations' 310time and zones.</li> 311<li><a href="https://www.timejones.com">TimeJones.com</a>, 312<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and 313<a href="https://www.worldclock.com">The World Clock</a> 314are time zone converters.</li> 315<li><a href="https://timezonedb.com/download">TimeZoneDB Database</a> 316publishes <code><abbr>tzdb</abbr></code>-derived data in 317<a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr 318title="comma-separated values">CSV</abbr></a> and 319in <a href="https://en.wikipedia.org/wiki/SQL"><abbr 320title="Structured Query Language">SQL</abbr></a> form.</li> 321<li><a 322href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a> 323lets you see the <code><abbr>TZ</abbr></code> values directly.</li> 324<li><a 325href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current 326Time in 1000 Places</a> uses descriptions of the values.</li> 327<li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a> 328uses a pulldown menu.</li> 329<li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete 330timezone information for all countries</a> displays tables of DST rules. 331<li><a href="https://www.timeanddate.com/worldclock/">The World Clock – 332Worldwide</a> lets you sort zone names and convert times.</li> 333<li><a href="https://24timezones.com">24TimeZones</a> has a world 334time map and a time converter.</li> 335<li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a> 336calculates the current time difference between locations.</li> 337<li><a href="http://www.wx-now.com">Weather Now</a> and 338<a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li> 339</ul> 340</section> 341 342<section> 343<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2> 344<ul> 345<li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s 346<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data 347Distribution Service (tzdist) working group</a> defined <a 348href="https://tools.ietf.org/html/rfc7808">TZDIST</a> 349(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service, 350along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a> 351(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for 352transferring time zone data by reference. 353<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST 354implementations</a> are available. 355The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis 356mailing list</a> discussed the Internet draft <a 357id="TZDIST-Geolocate" 358href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST 359Geolocate Extension</a> (now expired), which let a client determine its timezone 360from its geographic location using a <a 361href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li> 362<li>The <a href="https://tools.ietf.org/html/rfc5545"> 363Internet Calendaring and Scheduling Core Object Specification 364(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445) 365covers time zone 366data; see its VTIMEZONE calendar component. 367The iCalendar format requires specialized parsers and generators; a 368variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a> 369(Internet <abbr>RFC</abbr> 6321) uses 370<a href="https://www.w3.org/XML/"><abbr 371title="Extensible Markup Language">XML</abbr></a> format, and a variant 372<a href="https://tools.ietf.org/html/rfc7265">jCal</a> 373(Internet <abbr>RFC</abbr> 7265) 374uses <a href="https://www.json.org"><abbr 375title="JavaScript Object Notation">JSON</abbr></a> format.</li> 376</ul> 377</section> 378 379<section> 380<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2> 381<p>Although some of these do not fully support 382<code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code> 383distributions you can generally work around compatibility problems by 384running the command <code>make rearguard_tarballs</code> and compiling 385from the resulting tarballs instead.</p> 386<ul> 387<li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a 388href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a> 389program that compiles 390<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files. 391Vzic is freely 392available under the <a 393href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> 394General Public License (<abbr 395title="General Public License">GPL</abbr>)</a>.</li> 396<li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz 397database conversion utility</a> is like Vzic, except for the <a 398href="https://dotnet.microsoft.com">.NET framework</a> 399and with a <abbr>BSD</abbr>-style license.</li> 400<li><a 401href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a> 402contains a script <code>parse_olson</code> that compiles 403<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a> 404modules. It is part of the Perl <a 405href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>, 406which is freely 407available under both the <abbr>GPL</abbr> and the Perl Artistic 408License. DateTime::TimeZone also contains a script 409<code>tests_from_zdump</code> that generates test cases for each clock 410transition in the <code><abbr>tz</abbr></code> database.</li> 411<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone 412Database Parser</a> is a 413<a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and 414runtime library with API <a 415href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">adopted 416into the draft standard</a> for 417<a href="https://en.wikipedia.org/wiki/C++20">C++20</a>, 418the next iteration of the C++ standard. 419It is freely available under the 420<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li> 421<li><a id="ICU" href="http://site.icu-project.org">International Components for 422Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a 423href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a> 424libraries for internationalization that 425has a compiler from <code><abbr>tz</abbr></code> source 426and from <abbr title="Common Locale Data Repository">CLDR</abbr> data 427(mentioned <a href="#CLDR">below</a>) 428into an <abbr>ICU</abbr>-specific format. 429<abbr>ICU</abbr> is freely available under a 430<abbr>BSD</abbr>-style license.</li> 431<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for 432the <a href="https://elixir-lang.org">Elixir</a> language downloads 433and compiles <code><abbr>tz</abbr></code> source and exposes <abbr 434title="Application Program Interface">API</abbr>s for use. It is 435freely available under the <abbr>MIT</abbr> license.</li> 436<li>Java-based compilers and libraries include: 437<ul> 438<li>The <a 439href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater 440tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by 441Oracle Java.</li> 442<li>The <a 443href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java 444SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a 445href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>, 446which is freely available under a <abbr>BSD</abbr>-style license.</li> 447<li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date 448and time <abbr>API</abbr></a> contains a class 449<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles 450<code><abbr>tz</abbr></code> source into a binary format. It inspired 451Java 8 <code>java.time</code>, which its users should migrate to once 452they can assume Java 8 or later. It is available under the <a 453href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li> 454<li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a>, 455<a href="https://github.com/akashche/tzdbgen">tzdbgen</a>, and <a 456href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a> 457are other alternatives to TZUpdater. IANA Updater's license is unclear; 458the others are licensed under the <abbr>GPL</abbr>.</li> 459<li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and 460time library for Android</a> and 461<a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date, 462time and interval library for Java</a> compile 463<code><abbr>tz</abbr></code> source into a binary format. 464Time4A is available under the Apache License and Time4J is 465available under the <a 466href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser 467General Public License (<abbr title="Lesser General Public 468License">LGPL</abbr>)</a>.</li> 469<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and 470Java-based libraries.</li> 471</ul> 472<li><a href="https://nodatime.org">Noda Time – Date and 473time <abbr>API</abbr> for .NET</a> 474is like Joda-Time and Time4J, but for the .NET framework instead of Java. 475It is freely available under the Apache License.</li> 476<li>Many modern 477<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a> 478runtimes support <code><abbr>tz</abbr></code> natively via the 479<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>. 480On other runtimes, you can use one of the following. 481<ul> 482<li>The <a 483href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp> 484timezone polyfill</a> 485is freely available under a <abbr>BSD</abbr>-style license.</li> 486<li><a 487href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a> 488compiles time zone data into a compact form designed for 489JavaScript. It is freely available under a combination of 490the <abbr>MIT</abbr> license and the Apache License.</li> 491<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a 492plugin for the <a href="https://momentjs.com">Moment.js</a> date 493manipulation library. It is freely available under the <abbr>MIT</abbr> 494license.</li> 495<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a 496JavaScript library that supports date arithmetic that is time zone 497aware. It is freely available under the <abbr>MIT</abbr> license.</li> 498<li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s 499<abbr>API</abbr> is upward compatible with standard JavaScript 500Dates. It is freely available under the Apache License.</li> 501</ul> 502<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a 503compiler from <code><abbr>tz</abbr></code> source into 504<a href="https://julialang.org/">Julia</a>. It is freely available 505under the <abbr>MIT</abbr> license.</li> 506<li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> – 507<abbr>IANA</abbr> Time Zone Database for Delphi/<abbr 508title="Free Pascal Compiler">FPC</abbr></a> 509compiles from <code><abbr>tz</abbr></code> source into 510<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a> 511as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a> 512and <a 513href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>. 514It is freely available under a <abbr>BSD</abbr>-style license.</li> 515<li><a href="http://pytz.sourceforge.net">pytz – World Timezone 516Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into 517<a href="https://www.python.org">Python</a>. 518It is freely available under a <abbr>BSD</abbr>-style license.</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 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 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><a 626href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International 627clock (intclock)</a> is a clock that displays multiple time zones on 628<abbr>GNU</abbr>/Linux and similar systems. It is freely available 629under the <abbr>GPL</abbr>.</li> 630<li>Microsoft Windows 8.1 631and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr> 632data (mentioned <a href="#CLDR">below</a>) used by the 633<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> / 634<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes 635<a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and 636<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>. 637<a id="System.TimeZoneInfo" 638href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring 639Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes 640the older, proprietary method of Microsoft Windows 2000 and later, 641which stores time zone data in the 642<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The 643<a 644href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone → 645Tzid table</a> or <a 646href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr> 647file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs 648to <code><abbr>tz</abbr></code> names. 649These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library, 650or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>. 651<li><a 652href="https://www.oracle.com/java/index.html">Oracle 653Java</a> contains a copy of a subset of a recent 654<code><abbr>tz</abbr></code> database in a 655Java-specific format.</li> 656<li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone 657Master</a> is a Microsoft Windows clock program that can automatically 658download, compile and use <code><abbr>tz</abbr></code> releases. 659The Basic version is free.</li> 660<li><a 661href="http://veladg.com/velaterra.html">VelaTerra</a> is 662a macOS program. Its developers 663<a href="http://veladg.com/tzoffer.html">offer free 664licenses</a> to <code><abbr>tz</abbr></code> contributors.</li> 665</ul> 666</section> 667 668<section> 669<h2 id="other-dbs">Other time zone databases</h2> 670<ul> 671<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a> 672is Astrodienst's Web version of Shanks and Pottenger's out-of-print 673time zone history atlases 674<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and 675<a href="https://www.worldcat.org/oclc/76950459">for the world</a>, 676now published in <a 677href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a> 678form by <a href="https://astrocom.com">ACS-Starcrafts</a>. 679Although these extensive atlases 680<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were 681sources for much of the older <code><abbr>tz</abbr></code> data</a>, 682they are unreliable as Shanks appears to have 683guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no 684sources and do not indicate which entries are guesswork.</li> 685<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in 686its own <code>tztab</code>(4) format.</li> 687<li>Microsoft Windows has proprietary data mentioned 688<a href="#System.TimeZoneInfo">above</a>.</li> 689<li><a href="https://www.worldtimeserver.com">World Time Server</a> 690is another time zone database.</li> 691<li>The <a 692href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard 693Schedules Information Manual</a> of the 694International Air Transport Association 695gives current time zone rules for airports served by commercial aviation.</li> 696</ul> 697</section> 698 699<section> 700<h2 id="maps">Maps</h2> 701<ul> 702<li>The <a href="https://www.cia.gov/index.html">United States Central 703Intelligence Agency (<abbr 704title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a 705href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time 706zone map</a>; the 707<a 708href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda 709Library Map Collection</a> 710of the University of Texas at Austin has copies of 711recent editions. 712The pictorial quality is good, 713but the maps do not indicate daylight saving time, 714and parts of the data are a few years out of date.</li> 715<li><a href="https://www.worldtimezone.com">World Time Zone Map 716with current time</a> 717has several fancy time zone maps; it covers Russia particularly well. 718The maps' pictorial quality is not quite as good as the 719<abbr>CIA</abbr>'s 720but the maps are more up to date.</li> 721<li><a 722href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How 723much is time wrong around the world?</a> maps the difference between 724mean solar and standard time, highlighting areas such as western China 725where the two differ greatly. It's a bit out of date, unfortunately.</li> 726</ul> 727</section> 728 729<section> 730<h2 id="boundaries">Time zone boundaries</h2> 731<p>Geographical boundaries between timezones are available 732from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a> 733services and other sources.</p> 734<ul> 735<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone 736Boundary Builder</a> extracts 737<a href="https://www.openstreetmap.org">Open Street Map</a> data to build 738boundaries of <code><abbr>tzdb</abbr></code> timezones. 739Its code is freely available under the <abbr>MIT</abbr> license, and 740its data entries are freely available under the 741<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons 742Open Database License</a>. The maps' borders appear to be quite accurate.</li> 743<li>Programmatic interfaces that map geographical coordinates via tz_world to 744<code><abbr>tzdb</abbr></code> timezones include: 745<ul> 746<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is 747written in <a 748href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a> 749and is freely available under the <abbr>MIT</abbr> license.</li> 750<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a> 751is written in Go and is freely available under the Apache License.</li> 752<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>, 753in both Java and 754<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a> 755form, is freely available under the MIT license.</li> 756<li>For <a href="https://nodejs.org/en/">Node.js</a>, 757the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a> 758is freely available under the MIT license, and 759the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a> 760is in the public domain.</li> 761<li>The <a 762href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a> 763library for Python is freely available under the MIT license. 764<li>The <a 765href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a> 766library for Ruby is freely available under the MIT license.</li> 767</ul></li> 768<li>Free access via a network API, if you register a key, is provided by 769the <a 770href="https://www.geonames.org/export/web-services.html#timezone">GeoNames 771Timezone web service</a>, the <a 772href="https://developers.google.com/maps/documentation/timezone/intro">Google 773Maps Time Zone API</a>, and 774the <a href="https://timezonedb.com/api">TimeZoneDB API</a>. 775Commercial network API access is provided 776by <a href="https://askgeo.com">AskGeo</a> 777and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>. 778</li> 779<li>"<a 780href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How 781to get a time zone from a location using latitude and longitude 782coordinates?</a>" discusses other geolocation possibilities.</li> 783<li><a href="http://statoids.com/statoids.html">Administrative 784Divisions of Countries ("Statoids")</a> lists 785political subdivision data related to time zones.</li> 786<li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time 787zone boundaries for multizone countries</a> summarizes legal 788boundaries between time zones within countries.</li> 789<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software 790– GIS and Database Tools</a> includes a Manifold-format map of 791world time zone boundaries distributed under the 792<abbr>GPL</abbr>.</li> 793<li>A ship within the <a 794href="https://en.wikipedia.org/wiki/Territorial_waters">territorial 795waters</a> of any nation uses that nation's time. In international 796waters, time zone boundaries are meridians 15° apart, except that 797<abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5° 798wide and are separated by 799the 180° meridian (not by the International Date Line, which is 800for land and territorial waters only). A captain can change ship's 801clocks any time after entering a new time zone; midnight changes are 802common.</li> 803</ul> 804</section> 805 806<section> 807<h2 id="civil">Civil time concepts and history</h2> 808<ul> 809<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A 810Walk through Time</a> 811surveys the evolution of timekeeping.</li> 812<li>The history of daylight saving time is surveyed in <a 813href="http://www.webexhibits.org/daylightsaving/">About Daylight 814Saving Time – History, rationale, laws & dates</a> and summarized in 815<a href="http://seizethedaylight.com/dst/">A Brief 816History of Daylight Saving Time</a>.</li> 817<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time 818Lords</a> discusses how authoritarians manipulate civil time.</li> 819<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a> 820contains guidelines and best practices for software applications that 821deal with civil time.</li> 822<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of 823the International Date Line</a> tells the story of the most important 824time zone boundary.</li> 825<li><a href="http://statoids.com/tconcept.html">Basic Time 826Zone Concepts</a> discusses terminological issues behind time zones.</li> 827</ul> 828</section> 829 830<section> 831<h2 id="national">National histories of legal time</h2> 832<dl> 833<dt>Australia</dt> 834<dd>The Parliamentary Library commissioned a <a 835href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research 836paper on daylight saving time in Australia</a>. 837The Bureau of Meteorology publishes a list of <a 838href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation 839Dates of Daylight Savings Time within Australia</a>.</dd> 840<dt>Belgium</dt> 841<dd>The Royal Observatory of Belgium maintains a table of time in 842Belgium (in 843<a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html" 844hreflang="nl">Dutch</a> and <a 845href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html" 846hreflang="fr">French</a>).</dd> 847<dt>Brazil</dt> 848<dd>The Time Service Department of the National Observatory 849records <a href="http://pcdsh01.on.br/DecHV.html" 850hreflang="pt-BR">Brazil's daylight saving time decrees (in 851Portuguese)</a>.</dd> 852<dt>Canada</dt> 853<dd>National Research Council Canada publishes current 854and some older information about <a 855href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time 856zones and daylight saving time</a>.</dd> 857<dt>Chile</dt> 858<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a 859<a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of 860Chile's official time (in Spanish)</a>.</dd> 861<dt>China</dt> 862<dd>The Hong Kong Observatory maintains a 863<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of 864 summer time in Hong Kong</a>, 865and Macau's Meteorological and Geophysical Bureau maintains a <a 866href="https://www.smg.gov.mo/en/subpage/224/page/174">similar 867history for Macau</a>. 868Unfortunately the latter is incomplete and has errors.</dd> 869<dt>Czech Republic</dt> 870<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas" 871hreflang="cs">When daylight saving time starts and ends (in Czech)</a> 872summarizes and cites historical DST regulations.</dd> 873<dt>Germany</dt> 874<dd>The National Institute for Science and Technology maintains the <a 875href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation 876of Legal Time in Germany</a>.</dd> 877<dt>Israel</dt> 878<dd>The Interior Ministry periodically issues <a 879href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements" 880hreflang="he">announcements (in Hebrew)</a>.</dd> 881<dt>Italy</dt> 882<dd>The National Institute of Metrological Research publishes a 883<a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table 884of civil time (in Italian)</a>.</dd> 885<dt>Malaysia</dt> 886<dd>See Singapore <a href="#Singapore">below</a>.</dd> 887<dt>Mexico</dt> 888<dd>The Investigation and Analysis Service of the Mexican Library of 889Congress has published a <a 890href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm" 891hreflang="es">history of Mexican local time (in Spanish)</a>.</dd> 892<dt>Netherlands</dt> 893<dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm" 894hreflang="nl">Legal time in the Netherlands (in Dutch)</a> 895covers the history of local time in the Netherlands from ancient times.</dd> 896<dt>New Zealand</dt> 897<dd>The Department of Internal Affairs maintains a brief <a 898href="https://www.dia.govt.nz/Daylight-Saving-History">History of 899Daylight Saving</a>.</dd> 900<dt>Singapore</dt> 901<dd><a id="Singapore" 902href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why 903is Singapore in the "Wrong" Time Zone?</a> details the 904history of legal time in Singapore and Malaysia.</dd> 905<dt>United Kingdom</dt> 906<dd><a 907href="https://www.polyomino.org.uk/british-time/">History of 908legal time in Britain</a> discusses in detail the country 909with perhaps the best-documented history of clock adjustments.</dd> 910<dt>United States</dt> 911<dd>The Department of Transportation's <a 912href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent 913Time Zone Proceedings</a> lists changes to time zone boundaries.</dd> 914<dt>Uruguay</dt> 915<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan 916Navy (SOHMA) publishes an annual <a 917href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac 918(in Spanish)</a>.</dd> 919</dl> 920</section> 921 922<section> 923<h2 id="costs">Costs and benefits of time shifts</h2> 924<p>Various sources argue for and against daylight saving time and time 925zone shifts, and many scientific studies have been conducted. This 926section summarizes reviews and position statements based on 927scientific literature in the area.</p> 928<ul> 929<li>Carey RN, Sarma KM. 930<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of 931daylight saving time on road traffic collision risk: a systematic 932review</a>. 933<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>. 934This reviews research literature and concludes that the evidence 935neither supports nor refutes road safety benefits from 936shifts in time zones.</li> 937<li>Havranek T, Herman D, Irsova D. 938<a href="https://www.econstor.eu/handle/10419/174191">Does daylight 939saving save electricity? A meta-analysis</a>. <em>Energy J.</em> 9402018;39(2). 941doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>. 942This analyzes research literature and concludes, "Electricity savings 943are larger for countries farther away from the equator, while 944subtropical regions consume more electricity because of DST."</li> 945<li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>. 946<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time: 947an American Academy of Sleep Medicine position statement</a>. 948<em>J Clin Sleep Med.</em> 9492020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>. 950This argues for permanent standard time due to health risks of both 951DST transitions and permanent DST.</li> 952<li>Roenneberg T, Winnebeck EC, Klerman EB. 953<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight 954saving time and artificial time zones – a battle between 955biological and social times</a>. <em>Front Physiol.</em> 2019;10:944. 956doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>. 957This reviews evidence about the health effects of DST and concludes, 958"In summary, the scientific literature strongly argues against the 959switching between DST and Standard Time and even more so against 960adopting DST permanently."</li> 961</ul> 962</section> 963 964<section> 965<h2 id="precision">Precision timekeeping</h2> 966<ul> 967<li><a 968href="http://leapsecond.com/hpan/an1289.pdf">The 969Science of Timekeeping</a> is a thorough introduction 970to the theory and practice of precision timekeeping.</li> 971<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of 972Time 2016</a> contains several freely-readable papers.</li> 973<li><a href="http://www.ntp.org"><abbr 974title="Network Time Protocol">NTP</abbr>: The Network 975Time Protocol</a> (Internet <abbr>RFC</abbr> 5905) 976discusses how to synchronize clocks of 977Internet hosts.</li> 978<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a> 979family of software algorithms can achieve accuracy to a few tens of 980nanoseconds in scalable server farms without special hardware.</li> 981<li>The <a 982href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision 983Time Protocol</a> (<abbr 984title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588) 985can achieve submicrosecond clock accuracy on a local area network 986with special-purpose hardware.</li> 987<li><a 988href="https://tools.ietf.org/html/rfc4833">Timezone 989Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> 990(Internet <abbr>RFC</abbr> 4833) 991specifies a <a 992href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a> 993option for a server to configure 994a client's time zone and daylight saving settings automatically.</li> 995<li><a 996href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical 997Times</a> explains more abstruse astronomical time scales like 998<abbr title="Terrestrial Dynamic Time">TDT</abbr>, 999<abbr title="Geocentric Coordinate Time">TCG</abbr>, and 1000<abbr title="Barycentric Dynamic Time">TDB</abbr>. 1001<a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time 1002Scales</a> goes into more detail, particularly for historical variants.</li> 1003<li>The <a href="https://www.iau.org"><abbr 1004title="International Astronomical Union">IAU</abbr></a>'s <a 1005href="http://www.iausofa.org"><abbr 1006title="Standards Of Fundamental Astronomy">SOFA</abbr></a> 1007collection contains C and <a 1008href="https://en.wikipedia.org/wiki/Fortran">Fortran</a> 1009code for converting among time scales like 1010<abbr title="International Atomic Time">TAI</abbr>, 1011<abbr>TDB</abbr>, <abbr>TDT</abbr> and 1012<abbr>UTC</abbr>.</li> 1013<li><a 1014href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock 1015– Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the 1016diverse local time 1017scales used by each landed mission on Mars.</li> 1018<li><a href="http://leapsecond.com">LeapSecond.com</a> is 1019dedicated not only to leap seconds but to precise time and frequency 1020in general. It covers the state of the art in amateur timekeeping, and 1021how the art has progressed over the past few decades.</li> 1022<li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a 1023href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency 1024and time-signal emissions</a>, International Telecommunication Union – 1025Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li> 1026<li><a 1027href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr 1028title="International Earth Rotation and Reference Systems Service">IERS</abbr> 1029Bulletins</a> contains official publications of the International 1030Earth Rotation and Reference Systems Service, which decides when leap 1031seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds 1032via an optional "<code>right</code>" configuration, as opposed to the 1033default "<code>posix</code>" configuration.</li> 1034<li><a href="https://developers.google.com/time/smear">Leap Smear</a> 1035discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a 1036leap second so that they disagree with <abbr>UTC</abbr> by at most a 1037half second, even though every <abbr>POSIX</abbr> minute has exactly 1038sixty seconds. This approach works with the default <code><abbr>tz</abbr></code> 1039"<code>posix</code>" configuration, is <a 1040href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by 1041the <abbr>NTP</abbr> reference implementation, and is used by major 1042cloud service providers. However, according to 1043<a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of 1044Network Time Protocol Best Current Practices</a> 1045(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for 1046applications requiring accurate UTC or civil time, 1047and is intended for use only in single, well-controlled environments.</li> 1048<li>The <a 1049href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap 1050Second Discussion List</a> covers <a 1051href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy 1052and Klepczynski's 1999 proposal to discontinue leap seconds</a>, 1053discussed further in 1054<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The 1055leap second: its history and possible future</a>. 1056<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr> 1057might be redefined 1058without Leap Seconds</a> gives pointers on this 1059contentious issue, which was active until 2015 and could become active 1060again.</li> 1061</ul> 1062</section> 1063 1064<section> 1065<h2 id="notation">Time notation</h2> 1066<ul> 1067<li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data 1068Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time 1069zone names, abbreviations, identifiers, and formats. For example, it 1070contains French translations for "Eastern European Summer Time", 1071"<abbr title="Eastern European Summer Time">EEST</abbr>", and 1072"Bucharest". Its 1073<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type 1074charts</a> show these values for many locales. Data values are available in 1075both <abbr title="Locale Data Markup Language">LDML</abbr> 1076(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>. 1077<li> 1078<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of 1079the international standard date and time notation</a> covers 1080<a 1081href="https://www.iso.org/standard/70907.html"><em><abbr 1082title="International Organization for Standardization">ISO</abbr> 10838601-1:2019 – Date and time – Representations for information 1084interchange – Part 1: Basic rules</em></a>.</li> 1085<li> 1086<a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr> 1087Schema: Datatypes – dateTime</a> specifies a format inspired by 1088<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li> 1089<li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of 1090Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322) 1091specifies the time notation used in email and <a 1092href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a> 1093headers.</li> 1094<li> 1095<a href="https://tools.ietf.org/html/rfc3339">Date and Time 1096on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) 1097specifies an <abbr>ISO</abbr> 8601 1098profile for use in new Internet 1099protocols.</li> 1100<li> 1101<a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time 1102Formats on the Web</a> surveys web- and Internet-oriented date and time 1103formats.</li> 1104<li>Alphabetic time zone abbreviations should not be used as unique 1105identifiers for <abbr>UT</abbr> offsets as they are ambiguous in 1106practice. For example, in English-speaking North America 1107"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>, 1108but in China it denotes 8 hours ahead of <abbr>UT</abbr>, 1109and French-speaking North Americans prefer 1110"<abbr title="Heure Normale du Centre">HNC</abbr>" to 1111"<abbr>CST</abbr>". The <code><abbr>tz</abbr></code> 1112database contains English abbreviations for many timestamps; 1113unfortunately some of these abbreviations were merely the database maintainers' 1114inventions, and these have been removed when possible.</li> 1115<li>Numeric time zone abbreviations typically count hours east of 1116<abbr>UT</abbr>, e.g., +09 for Japan and 1117−10 for Hawaii. However, the <abbr>POSIX</abbr> 1118<code><abbr>TZ</abbr></code> environment variable uses the opposite convention. 1119For example, one might use <code><abbr>TZ</abbr>="<abbr 1120title="Japan Standard Time">JST</abbr>-9"</code> and 1121<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code> 1122for Japan and Hawaii, respectively. If the 1123<code><abbr>tz</abbr></code> database is available, it is usually better to use 1124settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and 1125<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid 1126confusion, handle old timestamps better, and insulate you better from 1127any future changes to the rules. One should never set 1128<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like 1129<code>"GMT-9"</code>, though, since this would incorrectly imply that 1130local time is nine hours ahead of <abbr>UT</abbr> and the time zone 1131is called "<abbr>GMT</abbr>".</li> 1132</ul> 1133</section> 1134 1135<section> 1136<h2 id="see-also">See also</h2> 1137<ul> 1138<li><a href="theory.html">Theory and pragmatics of the 1139<code><abbr>tz</abbr></code> code and data</a></li> 1140<li><a href="tz-art.html">Time and the Arts</a></li> 1141</ul> 1142</section> 1143 1144<footer> 1145<hr> 1146This web page is in the public domain, so clarified as of 11472009-05-17 by Arthur David Olson. 1148<br> 1149Please send corrections to this web page to the 1150<a href="mailto:tz@iana.org">time zone mailing list</a>. 1151</footer> 1152</body> 1153</html> 1154