tz-link.html revision 1.4 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>
7 pre {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>
15 rules are controlled by individual
16 governments. They are sometimes changed with little notice, and their
17 histories and planned futures are often recorded only fitfully. Here
18 is a summary of attempts to organize and record relevant data in this
19 area.
20 </p>
21 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
22 <p>
23 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
24 time zone database contains code and data
25 that represent the history of local time
26 for many representative locations around the globe.
27 It is updated periodically to reflect changes made by political bodies
28 to time zone boundaries and daylight saving rules.
29 This database (known as <code><abbr>tz</abbr></code>,
30 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
31 is used by several implementations,
32 including
33 <a href="https://www.gnu.org/software/libc/">the
34 <abbr title="GNU's Not Unix">GNU</abbr>
35 C Library</a> (used in
36 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
37 <a href="https://www.android.com">Android</a>,
38 <a href="https://www.freebsd.org">Free<abbr
39 title="Berkeley Software Distribution">BSD</abbr></a>,
40 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
41 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
42 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
43 <a href="https://cygwin.com">Cygwin</a>,
44 <a href="http://www.delorie.com/djgpp/"><abbr
45 title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
46 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
47 <a href="https://www.mysql.com">MySQL</a>,
48 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
49 title="Web Operating System">webOS</abbr></a>,
50 <a href="https://ibm.com/aix"><abbr
51 title="Advanced Interactive eXecutive">AIX</abbr></a>,
52 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
53 <a href="https://www.apple.com/ios/"><abbr
54 title="iPhone OS">iOS</abbr></a>,
55 <a href="https://www.apple.com/macos/">macOS</a>,
56 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
57 <a href="https://www.hpe.com/info/openvms">Open<abbr
58 title="Virtual Memory System">VMS</abbr></a>,
59 <a href="https://www.oracle.com/database/index.html">Oracle Database</a>, and
60 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
61 <p>
62 Each main entry in the database represents a <dfn>timezone</dfn>
63 for a set of civil-time clocks that have all agreed since 1970.
64 Timezones are typically identified by continent or ocean and then by the
65 name of the largest city within the region containing the clocks.
66 For example, <code>America/New_York</code>
67 represents most of the <abbr title="United States">US</abbr> eastern time zone;
68 <code>America/Phoenix</code> represents most of Arizona, which
69 uses mountain time without daylight saving time (<abbr>DST</abbr>);
70 <code>America/Detroit</code> represents most of Michigan, which uses
71 eastern time but with different <abbr>DST</abbr> rules in 1975;
72 and other entries represent smaller regions like Starke County,
73 Indiana, which switched from central to eastern time in 1991
74 and switched back in 2006.
75 To use the database on an extended <a
76 href="https://en.wikipedia.org/wiki/POSIX"><abbr
77 title="Portable Operating System Interface">POSIX</abbr></a>
78 implementation set the <code><abbr>TZ</abbr></code>
79 environment variable to the location's full name,
80 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
81 <p>
82 Associated with each timezone is a history of offsets from
83 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
84 Time</a> (<abbr>UT</abbr>), which is <a
85 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
86 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
87 for timestamps after 1960 this is more precisely <a
88 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
89 Universal Time</a> (<abbr>UTC</abbr>).
90 The database also records when daylight saving time was in use,
91 along with some time zone abbreviations such as <abbr>EST</abbr>
92 for Eastern Standard Time in the <abbr>US</abbr>.</p>
93 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
94 <p>
95 The following <a
96 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
97 the latest release's two
98 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
99 to a <abbr>GNU</abbr>/Linux or similar host.</p>
100 <pre><code>mkdir tzdb
101 cd tzdb
102 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
103 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
104 <a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf -
105 gzip -dc tzdata-latest.tar.gz | tar -xf -
106 </code></pre>
107 <p>Alternatively, the following shell commands download the same
108 release in a single-tarball format containing extra data
109 useful for regression testing:</p>
110 <pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
111 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
112 </code></pre>
113 <p>These commands use convenience links to the latest release
114 of the <code><abbr>tz</abbr></code> database hosted by the
115 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
116 of the <a href="https://www.iana.org">Internet Assigned Numbers
117 Authority (IANA)</a>.
118 Older releases are in files named
119 <code>tzcode<var>V</var>.tar.gz</code>,
120 <code>tzdata<var>V</var>.tar.gz</code>, and
121 <code>tzdb-<var>V</var>.tar.lz</code>,
122 where <code><var>V</var></code> is the version.
123 Since 1996, each version has been a four-digit year followed by
124 lower-case letter (<samp>a</samp> through <samp>z</samp>,
125 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
126 through <samp>zzz</samp>, and so on).
127 Since version 2016h, each release has contained a text file named
128 "<samp>version</samp>" whose first (and currently only) line is the version.
129 The releases are also available in an
130 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
131 title="File Transfer Protocol">FTP</abbr> directory</a> via a
132 less-secure protocol.</p>
133 <p>Alternatively, a development repository of code and data can be
134 retrieved from <a href="https://github.com">GitHub</a> via the shell
135 command:</p>
136 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
137 </code></pre>
138 <p>
139 Since version 2012e, each release has been tagged in development repositories.
140 Untagged commits are less well tested and probably contain
141 more errors.</p>
142 <p>
143 After obtaining the code and data files, see the
144 <code>README</code> file for what to do next.
145 The code lets you compile the <code><abbr>tz</abbr></code> source files into
146 machine-readable binary files, one for each location. The binary files
147 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>).
148 The code also lets
149 you read a <abbr>TZif</abbr> file and interpret timestamps for that
150 location.</p>
151 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
152 <p>
153 The <code><abbr>tz</abbr></code> code and data
154 are by no means authoritative. If you find errors, please
155 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
156 the time zone mailing list. You can also <a
157 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
158 and browse the <a
159 href="https://mm.icann.org/pipermail/tz/">archive of old
160 messages</a>.</p>
161 <p>
162 If your government plans to change its time zone boundaries or
163 daylight saving rules, inform <code>tz (a] iana.org</code> well in
164 advance, as this will coordinate updates to many cell phones,
165 computers, and other devices around the world. With
166 less than a year's notice there is a good chance that some
167 computer-based clocks will operate incorrectly after the change, due
168 to delays in propagating updates to software and data. The shorter
169 the notice, the more likely clock problems will arise; see "<a
170 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
171 the Timing of Time Zone Changes</a>" for examples.
172 </p>
173 <p>
174 Changes to the <code><abbr>tz</abbr></code> code and data are often
175 propagated to clients via operating system updates, so
176 client <code><abbr>tz</abbr></code> data can often be corrected by
177 applying these updates. With GNU/Linux and similar systems, if your
178 maintenance provider has not yet adopted the
179 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
180 the process by tailoring the generic instructions in
181 the <code><abbr>tz</abbr> README</code> file and installing the latest
182 data yourself. System-specific instructions for installing the
183 latest <code><abbr>tz</abbr></code> data have also been published
184 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
185 <a
186 href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
187 <a
188 href="http://userguide.icu-project.org/datetime/timezone"><abbr
189 title="International Components for Unicode">ICU</abbr></a>,
190 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
191 and <a
192 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
193 Java, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
194 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
195 and <a
196 href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
197 </p>
198 <p>Sources for the <code><abbr>tz</abbr></code> database are
199 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
200 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
201 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
202 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
203 title="linefeed">LF</abbr></a>,
204 which can be modified by common text editors such
205 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
206 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
207 <a href="https://www.vim.org">vim</a>.
208 Specialized source-file editing can be done via the
209 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
210 zoneinfo</a> package for <a
211 href="https://www.sublimetext.com">Sublime Text</a> and the <a
212 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
213 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
214 Studio Code</a>.
215 </p>
216 <p>
217 For further information about updates, please see
218 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
219 Maintaining the Time Zone Database</a> (Internet <abbr
220 title="Request For Comments">RFC</abbr> 6557). More detail can be
221 found in <a href="theory.html">Theory and pragmatics of the tz code and data</a>.
222 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
223 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
224 </p>
225 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
226 <ul>
227 <li>The article
228 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
229 an encyclopedic summary.</li>
230 <li><a href="tz-how-to.html">How to Read the
231 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
232 database format.</li>
233 <li><a
234 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
235 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
236 database's style.</li>
237 </ul>
238 <h2 id="web">Web sites using recent versions of the
239 <code><abbr>tz</abbr></code> database</h2>
240 <p>
241 These are listed roughly in ascending order of complexity and fanciness.
242 </p>
243 <ul>
244 <li><a href="https://time.is">Time.is</a> shows locations'
245 time and zones.</li>
246 <li><a href="https://www.timejones.com">TimeJones.com</a>,
247 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
248 <a href="http://worldclock.com">The World Clock</a>
249 are time zone converters.</li>
250 <li><a
251 href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
252 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
253 <li><a
254 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
255 Time in 1000 Places</a> uses descriptions of the values.</li>
256 <li><a href="http://www.timezoneconverter.com/cgi-bin/tzc.tzc">Time Zone
257 Converter</a>
258 uses a pulldown menu.</li>
259 <li><a href="http://home.kpn.nl/vanadovv/time/TZworld.html">Complete
260 timezone information for all countries</a> displays tables of DST rules.
261 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
262 Worldwide</a> lets you sort zone names and convert times.</li>
263 <li><a href="https://24timezones.com">24TimeZones</a> has a world
264 time map and a time converter.</li>
265 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
266 calculates the current time difference between locations.</li>
267 <li><a href="http://www.wx-now.com">Weather Now</a> and
268 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
269 </ul>
270 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
271 <ul>
272 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
273 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
274 Distribution Service (tzdist) working group</a> defined <a
275 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
276 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
277 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
278 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
279 transferring time zone data by reference.
280 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
281 mailing list</a> discusses two Internet drafts: <a
282 id="TZDIST-Geolocate"
283 href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
284 Geolocate Extension</a> lets a client determine its timezone
285 from its geographic location using a <a
286 href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>, and
287 <a href="https://tools.ietf.org/html/draft-murchison-tzdist-tzif">The
288 Time Zone Information Format (<abbr>TZif</abbr>)</a> specifies the format of
289 <abbr>TZif</abbr> data.</li>
290 <li>The <a href="https://tools.ietf.org/html/rfc5545">
291 Internet Calendaring and Scheduling Core Object Specification
292 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
293 covers time zone
294 data; see its VTIMEZONE calendar component.
295 The iCalendar format requires specialized parsers and generators; a
296 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
297 (Internet <abbr>RFC</abbr> 6321) uses
298 <a href="https://www.w3.org/XML/"><abbr
299 title="Extensible Markup Language">XML</abbr></a> format, and a variant
300 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
301 (Internet <abbr>RFC</abbr> 7265)
302 uses <a href="https://www.json.org"><abbr
303 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
304 </ul>
305 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
306 <ul>
307 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
308 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
309 program that compiles
310 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
311 Vzic is freely
312 available under the <a
313 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
314 General Public License (<abbr
315 title="General Public License">GPL</abbr>)</a>.</li>
316 <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
317 database conversion utility</a> is like Vzic, except for the <a
318 href="https://www.microsoft.com/net">.NET framework</a>
319 and with a <abbr>BSD</abbr>-style license.</li>
320 <li><a
321 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
322 contains a script <code>parse_olson</code> that compiles
323 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
324 modules. It is part of the Perl <a
325 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
326 which is freely
327 available under both the <abbr>GPL</abbr> and the Perl Artistic
328 License. DateTime::TimeZone also contains a script
329 <code>tests_from_zdump</code> that generates test cases for each clock
330 transition in the <code><abbr>tz</abbr></code> database.</li>
331 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
332 Database Parser</a> is a
333 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
334 runtime library that is <a
335 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0355r4.html">moving
336 forward</a> for inclusion in the next iteration of <a
337 href="https://isocpp.org/std/the-standard"><em><abbr
338 title="International Organization for Standardization">ISO</abbr>
339 International Standard ISO/IEC 14882:2017(E) – Programming
340 Language C++</em></a>.
341 It is freely available under the
342 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
343 <li><a id="ICU" href="http://site.icu-project.org">International Components for
344 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
345 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
346 libraries for internationalization that
347 has a compiler from <code><abbr>tz</abbr></code> source
348 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
349 (mentioned <a href="#CLDR">below</a>)
350 into an <abbr>ICU</abbr>-specific format.
351 <abbr>ICU</abbr> is freely available under a
352 <abbr>BSD</abbr>-style license.</li>
353 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
354 the <a href="https://elixir-lang.org">Elixir</a> language downloads
355 and compiles tz source and exposes <abbr
356 title="Application Program Interface">API</abbr>s for use. It is
357 freely available under the <abbr>MIT</abbr> license.</li>
358 <li>Java-based compilers and libraries include:
359 <ul>
360 <li>The <a
361 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
362 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
363 Oracle Java.</li>
364 <li>The <a
365 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
366 SE <code>java.time</code> <abbr>API</abbr></a> in Java 8 and later
367 can be supplemented by <a
368 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
369 which is freely available under a <abbr>BSD</abbr>-style license.</li>
370 <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
371 and time <abbr>API</abbr></a> contains a class
372 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
373 <code><abbr>tz</abbr></code> source into a binary format. It inspired
374 Java 8 <code>java.time</code>, which its users should migrate to once
375 they can assume Java 8 or later. It is available under the <a
376 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
377 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
378 time library for Android</a> and
379 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
380 time and interval library for Java</a> compile
381 <code><abbr>tz</abbr></code> source into a binary format.
382 Time4A is available under the Apache License and Time4J is
383 available under the <a
384 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
385 General Public License (<abbr title="Lesser General Public
386 License">LGPL</abbr>)</a>.</li>
387 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
388 Java-based libraries.</li>
389 </ul>
390 <li><a href="https://nodatime.org">Noda Time – Date and
391 time <abbr>API</abbr> for .NET</a>
392 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
393 It is freely available under the Apache License.</li>
394 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
395 compilers and libraries include:
396 <ul>
397 <li><a
398 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
399 compiles time zone data into a compact form designed for
400 JavaScript. It is freely available under a combination of
401 the <abbr>MIT</abbr> license and the Apache License.</li>
402 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
403 plugin for the <a href="https://momentjs.com">Moment.js</a> date
404 manipulation library. It is freely available under the <abbr>MIT</abbr>
405 license.</li>
406 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
407 <abbr>API</abbr> is upward compatible with standard JavaScript
408 Dates. It is freely available under the Apache License.</li>
409 <li><a href="https://github.com/sproutsocial/walltime-js">Walltime-js</a>
410 translates <abbr>UT</abbr> to local time. It is freely available under
411 the <abbr>MIT</abbr> license.</li>
412 </ul>
413 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
414 compiler from <code><abbr>tz</abbr></code> source into
415 <a href="https://julialang.org/">Julia</a>. It is freely available
416 under the <abbr>MIT</abbr> license.</li>
417 <li><a href="https://github.com/pavkam/tzdb">Delphi/<abbr
418 title="Free Pascal Compiler">FPC</abbr> Time Zone Database</a>
419 compiles from <code><abbr>tz</abbr></code> source into
420 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
421 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
422 and <a
423 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
424 It is freely available under a <abbr>BSD</abbr>-style license.</li>
425 <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
426 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
427 <a href="https://www.python.org">Python</a>.
428 It is freely available under a <abbr>BSD</abbr>-style license.</li>
429 <li><a href="https://tzinfo.github.io">TZInfo –
430 Ruby Timezone Library</a>
431 compiles <code><abbr>tz</abbr></code> source into
432 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
433 It is freely available under the <abbr>MIT</abbr> license.</li>
434 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
435 Library</a> is
436 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
437 library that compiles <code><abbr>tz</abbr></code> source into a time
438 zone repository whose format
439 is either proprietary or an <abbr>XML</abbr>-encoded
440 representation.</li>
441 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
442 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
443 source into text files, along with a runtime that can read those
444 files. Tcl is freely available under a <abbr>BSD</abbr>-style
445 license.</li>
446 </ul>
447 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
448 <ul>
449 <li>The <a
450 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
451 Library</a>
452 has an independent, thread-safe implementation of
453 a <abbr>TZif</abbr> file reader.
454 This library is freely available under the LGPL
455 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
456 <li><a href="https://www.gnome.org">GNOME</a>'s
457 <a href="https://developer.gnome.org/glib/">GLib</a> has
458 a <abbr>TZif</abbr> file reader written in C that
459 creates a <code>GTimeZone</code> object representing sets
460 of <abbr>UT</abbr> offsets.
461 It is freely available under the <abbr>LGPL</abbr>.</li>
462 <li>The
463 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
464 <code>baltzo::TimeZoneUtil</code> component contains a C++
465 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
466 the Apache License.</li>
467 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
468 library that translates between <abbr>UT</abbr> and civil time and
469 can read <abbr>TZif</abbr> files. It is freely available under the Apache
470 License.</li>
471 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
472 is a <abbr>TZif</abbr> file reader written in Java.
473 It is freely available under the <abbr>LGPL</abbr>.</li>
474 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
475 library that reads <abbr>TZif</abbr> files and converts
476 timestamps from one time zone or format to another.
477 It is used by <a href="https://secure.php.net"><abbr
478 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
479 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
480 and <a href="https://www.mongodb.com">MongoDB</a>.
481 It is freely available under the <abbr>MIT</abbr> license.</li>
482 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
483 JavaScript library that supports date arithmetic that is time zone
484 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
485 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
486 <abbr>TZif</abbr> file reader.</li>
487 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
488 DateTime::TimeZone::Tzfile</a>
489 is a <abbr>TZif</abbr> file reader written in Perl.
490 It is freely available under the same terms as Perl
491 (dual <abbr>GPL</abbr> and Artistic license).</li>
492 <li>The
493 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
494 library contains a Python tool that
495 converts <abbr>TZif</abbr> data into
496 <abbr>JSON</abbr>-format data suitable for use
497 in its JavaScript library for time zone conversion. Dates before 1970
498 are not supported.</li>
499 <li>The <a
500 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
501 package contains <a href="https://www.haskell.org">Haskell</a> code that
502 parses and uses <abbr>TZif</abbr> data. It is freely
503 available under a <abbr>BSD</abbr>-style license.</li>
504 </ul>
505 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
506 <ul>
507 <li><a href="https://foxclocks.org">FoxClocks</a>
508 is an extension for <a href="https://www.google.com/chrome/">Google
509 Chrome</a> and for <a
510 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
511 Toolkit</a> applications like <a
512 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
513 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
514 It displays multiple clocks in the application window, and has a mapping
515 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
516 It is freely available under the <abbr>GPL</abbr>.</li>
517 <li><a href="https://golang.org">Go programming language</a>
518 implementations contain a copy of a 32-bit subset of a recent
519 <code><abbr>tz</abbr></code> database in a
520 Go-specific format.</li>
521 <li><a
522 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
523 clock (intclock)</a> is a clock that displays multiple time zones on
524 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
525 under the <abbr>GPL</abbr>.</li>
526 <li>Microsoft Windows 8.1
527 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
528 data (mentioned <a href="#CLDR">below</a>) used by the
529 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
530 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
531 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
532 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
533 <a id="System.TimeZoneInfo"
534 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
535 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
536 the older, proprietary method of Microsoft Windows 2000 and later,
537 which stores time zone data in the
538 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
539 <a
540 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
541 Tzid table</a> or <a
542 href="https://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
543 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
544 to <code><abbr>tz</abbr></code> names.
545 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
546 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
547 <li><a
548 href="https://www.oracle.com/java/index.html">Oracle
549 Java</a> contains a copy of a subset of a recent
550 <code><abbr>tz</abbr></code> database in a
551 Java-specific format.</li>
552 <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
553 Master</a> is a Microsoft Windows clock program that can automatically
554 download, compile and use <code><abbr>tz</abbr></code> releases.
555 The Basic version is free.</li>
556 <li><a
557 href="http://veladg.com/velaterra.html">VelaTerra</a> is
558 a macOS program. Its developers
559 <a href="http://veladg.com/tzoffer.html">offer free
560 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
561 </ul>
562 <h2 id="other-dbs">Other time zone databases</h2>
563 <ul>
564 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
565 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
566 time zone history atlases
567 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
568 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>,
569 now published in <a
570 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
571 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
572 Although these extensive atlases
573 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
574 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
575 they are unreliable as Shanks appears to have
576 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
577 sources and do not indicate which entries are guesswork.</li>
578 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
579 its own <code>tztab</code>(4) format.</li>
580 <li>Microsoft Windows has proprietary data mentioned
581 <a href="#System.TimeZoneInfo">above</a>.</li>
582 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
583 is another time zone database.</li>
584 <li>The <a
585 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
586 Schedules Information Manual</a> of the
587 International Air Transport Association
588 gives current time zone rules for airports served by commercial aviation.</li>
589 </ul>
590 <h2 id="maps">Maps</h2>
591 <ul>
592 <li>The <a href="https://www.cia.gov/index.html">United States Central
593 Intelligence Agency (<abbr
594 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
595 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
596 zone map</a>; the
597 <a
598 href="https://www.lib.utexas.edu/maps/world.html">Perry–Castañeda
599 Library Map Collection</a>
600 of the University of Texas at Austin has copies of
601 recent editions.
602 The pictorial quality is good,
603 but the maps do not indicate daylight saving time,
604 and parts of the data are a few years out of date.</li>
605 <li><a href="https://www.worldtimezone.com">Current time around the world
606 and standard time zones map of the world</a>
607 has several fancy time zone maps; it covers Russia particularly well.
608 The maps' pictorial quality is not quite as good as the
609 <abbr>CIA</abbr>'s
610 but the maps are more up to date.</li>
611 <li><a
612 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
613 much is time wrong around the world?</a> maps the difference between
614 mean solar and standard time, highlighting areas such as western China
615 where the two differ greatly. It's a bit out of date, unfortunately.</li>
616 </ul>
617 <h2 id="boundaries">Time zone boundaries</h2>
618 <p>Geographical boundaries between timezones are available
619 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
620 services and other sources.</p>
621 <ul>
622 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
623 Boundary Builder</a> extracts
624 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
625 boundaries of <code><abbr>tzdb</abbr></code> timezones.
626 Its code is freely available under the <abbr>MIT</abbr> license, and
627 its data entries are freely available under the
628 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
629 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
630 <li>Programmatic interfaces that map geographical coordinates via tz_world to
631 <code><abbr>tzdb</abbr></code> timezones include:
632 <ul>
633 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
634 written in <a
635 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
636 and is freely available under the <abbr>MIT</abbr> license.</li>
637 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
638 is written in Go and is freely available under the Apache License.</li>
639 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
640 in both Java and
641 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
642 form, is freely available under the MIT license.</li>
643 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
644 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
645 is freely available under the MIT license, and
646 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
647 is in the public domain.</li>
648 <li>The <a
649 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
650 library for Python is freely available under the MIT license.
651 <li>The <a
652 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
653 library for Ruby is freely available under the MIT license.</li>
654 </ul></li>
655 <li>Free access via a network API, if you register a key, is provided by
656 the <a
657 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
658 Timezone web service</a>, the <a
659 href="https://developers.google.com/maps/documentation/timezone/intro">Google
660 Maps Time Zone API</a>, and
661 the <a href="https://timezonedb.com">Time Zone Database & API</a>.
662 Commercial network API access is provided
663 by <a href="https://askgeo.com">AskGeo</a>
664 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
665 </li>
666 <li>"<a
667 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
668 to get a time zone from a location using latitude and longitude
669 coordinates?</a>" discusses other geolocation possibilities.</li>
670 <li><a href="http://statoids.com/statoids.html">Administrative
671 Divisions of Countries ("Statoids")</a> lists
672 political subdivision data related to time zones.</li>
673 <li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time
674 zone boundaries for multizone countries</a> summarizes legal
675 boundaries between time zones within countries.</li>
676 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
677 – GIS and Database Tools</a> includes a Manifold-format map of
678 world time zone boundaries distributed under the
679 <abbr>GPL</abbr>.</li>
680 <li>A ship within the <a
681 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
682 waters</a> of any nation uses that nation's time. In international
683 waters, time zone boundaries are meridians 15° apart, except that
684 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
685 wide and are separated by
686 the 180° meridian (not by the International Date Line, which is
687 for land and territorial waters only). A captain can change ship's
688 clocks any time after entering a new time zone; midnight changes are
689 common.</li>
690 </ul>
691 <h2 id="civil">Civil time concepts and history</h2>
692 <ul>
693 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
694 Walk through Time</a>
695 surveys the evolution of timekeeping.</li>
696 <li>The history of daylight saving time is surveyed in <a
697 href="http://www.webexhibits.org/daylightsaving/">About Daylight
698 Saving Time – History, rationale, laws & dates</a> and summarized in
699 <a href="http://seizethedaylight.com/dst/">A Brief
700 History of Daylight Saving Time</a>.</li>
701 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
702 Lords</a> discusses how authoritarians manipulate civil time.</li>
703 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
704 contains guidelines and best practices for software applications that
705 deal with civil time.</li>
706 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
707 the International Date Line</a> tells the story of the most important
708 time zone boundary.</li>
709 <li><a href="http://statoids.com/tconcept.html">Basic Time
710 Zone Concepts</a> discusses terminological issues behind time zones.</li>
711 </ul>
712 <h2 id="national">National histories of legal time</h2>
713 <dl>
714 <dt>Australia</dt>
715 <dd>The Parliamentary Library has commissioned a <a
716 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
717 paper on daylight saving time in Australia</a>.
718 The Bureau of Meteorology publishes a list of <a
719 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
720 Dates of Daylight Savings Time within Australia</a>.</dd>
721 <dt>Belgium</dt>
722 <dd>The Royal Observatory of Belgium maintains a table of <a
723 href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
724 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
725 <dt>Brazil</dt>
726 <dd>The Time Service Department of the National Observatory
727 records <a href="http://pcdsh01.on.br/DecHV.html"
728 hreflang="pt-BR">Brazil's daylight saving time decrees (in
729 Portuguese)</a>.</dd>
730 <dt>Canada</dt>
731 <dd>National Research Council Canada publishes current
732 and some older information about <a
733 href="https://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time
734 zones & daylight saving time</a>.</dd>
735 <dt>Chile</dt>
736 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
737 <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
738 Chile's official time (in Spanish)</a>.</dd>
739 <dt>China</dt>
740 <dd>The Hong Kong Observatory maintains a
741 <a href="https://www.hko.gov.hk/gts/time/Summertime.htm">history of
742 summer time in Hong Kong</a>,
743 and Macau's Meteorological and Geophysical Bureau maintains a <a
744 href="http://www.smg.gov.mo/smg/geophysics/e_t_Summer%20Time.htm">similar
745 history for Macau</a>.
746 Unfortunately the latter is incomplete and has errors.</dd>
747 <dt>Czech Republic</dt>
748 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
749 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
750 summarizes and cites historical DST regulations.</dd>
751 <dt>Germany</dt>
752 <dd>The National Institute for Science and Technology maintains the <a
753 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
754 of Legal Time in Germany</a>.</dd>
755 <dt>Israel</dt>
756 <dd>The Interior Ministry periodically issues <a
757 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
758 hreflang="he">announcements (in Hebrew)</a>.</dd>
759 <dt>Italy</dt>
760 <dd>The National Institute of Metrological Research maintains a
761 <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
762 of civil time (in Italian)</a>.</dd>
763 <dt>Malaysia</dt>
764 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
765 <dt>Mexico</dt>
766 <dd>The Investigation and Analysis Service of the Mexican Library of
767 Congress has published a <a
768 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
769 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
770 <dt>Netherlands</dt>
771 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
772 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
773 covers the history of local time in the Netherlands from ancient times.</dd>
774 <dt>New Zealand</dt>
775 <dd>The Department of Internal Affairs maintains a brief <a
776 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
777 Daylight Saving</a>. The privately-maintained <a
778 href="http://astrologyschool.com/nztime.html">History of New Zealand
779 time</a> has more details.</dd>
780 <dt>Singapore</dt>
781 <dd><a id="Singapore"
782 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
783 is Singapore in the "Wrong" Time Zone?</a> details the
784 history of legal time in Singapore and Malaysia.</dd>
785 <dt>United Kingdom</dt>
786 <dd><a
787 href="https://www.polyomino.org.uk/british-time/">History of
788 legal time in Britain</a> discusses in detail the country
789 with perhaps the best-documented history of clock adjustments.
790 The National Physical Laboratory also maintains an <a
791 href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates">Archive
792 of Summer time dates</a>.</dd>
793 <dt>United States</dt>
794 <dd>The Department of Transportation's <a
795 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
796 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
797 <dt>Uruguay</dt>
798 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
799 Navy (SOHMA) publishes an annual <a
800 href="http://www.armada.mil.uy/Pagina/institucion/dimat/sohma/almanaque.html" hreflang="es">almanac
801 (in Spanish)</a>.</dd>
802 </dl>
803 <h2 id="precision">Precision timekeeping</h2>
804 <ul>
805 <li><a
806 href="http://leapsecond.com/hpan/an1289.pdf">The
807 Science of Timekeeping</a> is a thorough introduction
808 to the theory and practice of precision timekeeping.</li>
809 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
810 Time 2016</a> contains several freely-readable papers.</li>
811 <li><a href="http://www.ntp.org"><abbr
812 title="Network Time Protocol">NTP</abbr>: The Network
813 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
814 discusses how to synchronize clocks of
815 Internet hosts.</li>
816 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
817 family of software algorithms can achieve accuracy to a few tens of
818 nanoseconds in scalable server farms without special hardware.</li>
819 <li>The <a
820 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
821 Time Protocol</a> (<abbr
822 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
823 can achieve submicrosecond clock accuracy on a local area network
824 with special-purpose hardware.</li>
825 <li><a
826 href="https://tools.ietf.org/html/rfc4833">Timezone
827 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
828 (Internet <abbr>RFC</abbr> 4833)
829 specifies a <a
830 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
831 option for a server to configure
832 a client's time zone and daylight saving settings automatically.</li>
833 <li><a
834 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
835 Times</a> explains more abstruse astronomical time scales like
836 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
837 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
838 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
839 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
840 Scales</a> goes into more detail, particularly for historical variants.</li>
841 <li>The <a href="https://www.iau.org"><abbr
842 title="International Astronomical Union">IAU</abbr></a>'s <a
843 href="http://www.iausofa.org"><abbr
844 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
845 collection contains C and <a
846 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
847 code for converting among time scales like
848 <abbr title="International Atomic Time">TAI</abbr>,
849 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
850 <abbr>UTC</abbr>.</li>
851 <li><a
852 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
853 – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
854 diverse local time
855 scales used by each landed mission on Mars.</li>
856 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
857 dedicated not only to leap seconds but to precise time and frequency
858 in general. It covers the state of the art in amateur timekeeping, and
859 how the art has progressed over the past few decades.</li>
860 <li><a
861 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
862 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
863 Bulletins</a> contains official publications of the International
864 Earth Rotation and Reference Systems Service, which decides when leap
865 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
866 via an optional "<code>right</code>" configuration, as opposed to the
867 default "<code>posix</code>" configuration.</li>
868 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
869 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
870 leap second so that they disagree with <abbr>UTC</abbr> by at most a
871 half second, even though every <abbr>POSIX</abbr> minute has exactly
872 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
873 "<code>posix</code>" configuration, is <a
874 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
875 the <abbr>NTP</abbr> reference implementation, and is used by major
876 cloud service providers.</li>
877 <li>The <a
878 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
879 Second Discussion List</a> covers <a
880 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
881 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
882 discussed further in
883 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
884 leap second: its history and possible future</a>.
885 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
886 might be redefined
887 without Leap Seconds</a> gives pointers on this
888 contentious issue, which was active until 2015 and could become active
889 again.</li>
890 </ul>
891 <h2 id="notation">Time notation</h2>
892 <ul>
893 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
894 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
895 zone names, abbreviations, identifiers, and formats. For example, it
896 contains French translations for "Eastern European Summer Time",
897 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
898 "Bucharest". Its
899 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
900 charts</a> show these values for many locales. Data values are available in
901 both <abbr title="Locale Data Markup Language">LDML</abbr>
902 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
903 <li>
904 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
905 the international standard date and time notation</a> is a good
906 summary of
907 <a
908 href="https://www.iso.org/standard/40874.html"><em><abbr>ISO</abbr>
909 8601:2004 – Data elements and interchange formats – Information
910 interchange – Representation of dates and times</em></a>.</li>
911 <li>
912 <a href="https://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
913 Schema: Datatypes – dateTime</a> specifies a format inspired by
914 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
915 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of
916 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
917 specifies the time notation used in email and <a
918 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
919 headers.</li>
920 <li>
921 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
922 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
923 specifies an <abbr>ISO</abbr> 8601
924 profile for use in new Internet
925 protocols.</li>
926 <li>
927 <a href="https://www.hackcraft.net/web/datetime/">Date & Time
928 Formats on the Web</a> surveys web- and Internet-oriented date and time
929 formats.</li>
930 <li>Alphabetic time zone abbreviations should not be used as unique
931 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
932 practice. For example, in English-speaking North America
933 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
934 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
935 and French-speaking North Americans prefer
936 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
937 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
938 database contains English abbreviations for many timestamps;
939 unfortunately some of these abbreviations were merely the database maintainers'
940 inventions, and these have been removed when possible.</li>
941 <li>Numeric time zone abbreviations typically count hours east of
942 <abbr>UT</abbr>, e.g., +09 for Japan and
943 −10 for Hawaii. However, the <abbr>POSIX</abbr>
944 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
945 For example, one might use <code><abbr>TZ</abbr>="<abbr
946 title="Japan Standard Time">JST</abbr>-9"</code> and
947 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
948 for Japan and Hawaii, respectively. If the
949 <code><abbr>tz</abbr></code> database is available, it is usually better to use
950 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
951 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
952 confusion, handle old timestamps better, and insulate you better from
953 any future changes to the rules. One should never set
954 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
955 <code>"GMT-9"</code>, though, since this would incorrectly imply that
956 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
957 is called "<abbr>GMT</abbr>".</li>
958 </ul>
959 <h2 id="see-also">See also</h2>
960 <ul>
961 <li><a href="theory.html">Theory and pragmatics of the tz code and data</a></li>
962 <li><a href="tz-art.html">Time and the Arts</a></li>
963 </ul>
964 <hr>
965 <address>
966 This web page is in the public domain, so clarified as of
967 2009-05-17 by Arthur David Olson.
968 <br>
969 Please send corrections to this web page to the
970 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
971 </address>
972 </body>
973 </html>
974