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4 <title>Sources for time zone and daylight saving time data</title>
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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 <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="#precision">Precision timekeeping</a></li>
53 <li><a href="#notation">Time notation</a></li>
54 <li><a href="#see-also">See also</a></li>
55 </ul>
56 </li>
57 </ul>
58 </nav>
59
60 <section>
61 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
62 <p>
63 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
64 time zone database contains code and data
65 that represent the history of local time
66 for many representative locations around the globe.
67 It is updated periodically to reflect changes made by political bodies
68 to time zone boundaries and daylight saving rules.
69 This database (known as <code><abbr>tz</abbr></code>,
70 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
71 is used by several implementations,
72 including
73 <a href="https://www.gnu.org/software/libc/">the
74 <abbr title="GNU's Not Unix">GNU</abbr>
75 C Library</a> (used in
76 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
77 <a href="https://www.android.com">Android</a>,
78 <a href="https://www.freebsd.org">Free<abbr
79 title="Berkeley Software Distribution">BSD</abbr></a>,
80 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
81 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
82 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
83 <a href="https://cygwin.com">Cygwin</a>,
84 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
85 <a href="https://www.mysql.com">MySQL</a>,
86 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
87 title="Web Operating System">webOS</abbr></a>,
88 <a href="https://ibm.com/aix"><abbr
89 title="Advanced Interactive eXecutive">AIX</abbr></a>,
90 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
91 <a href="https://www.apple.com/ios"><abbr
92 title="iPhone OS">iOS</abbr></a>,
93 <a href="https://www.apple.com/macos">macOS</a>,
94 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
95 <a href="https://www.vmssoftware.com">Open<abbr
96 title="Virtual Memory System">VMS</abbr></a>,
97 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
98 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
99 <p>
100 Each main entry in the database represents a <dfn>timezone</dfn>
101 for a set of civil-time clocks that have all agreed since 1970.
102 Timezones are typically identified by continent or ocean and then by the
103 name of the largest city within the region containing the clocks.
104 For example, <code>America/New_York</code>
105 represents most of the <abbr title="United States">US</abbr> eastern time zone;
106 <code>America/Phoenix</code> represents most of Arizona, which
107 uses mountain time without daylight saving time (<abbr>DST</abbr>);
108 <code>America/Detroit</code> represents most of Michigan, which uses
109 eastern time but with different <abbr>DST</abbr> rules in 1975;
110 and other entries represent smaller regions like Starke County,
111 Indiana, which switched from central to eastern time in 1991
112 and switched back in 2006.
113 To use the database on an extended <a
114 href="https://en.wikipedia.org/wiki/POSIX"><abbr
115 title="Portable Operating System Interface">POSIX</abbr></a>
116 implementation set the <code><abbr>TZ</abbr></code>
117 environment variable to the location's full name,
118 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
119 <p>
120 Associated with each timezone is a history of offsets from
121 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
122 Time</a> (<abbr>UT</abbr>), which is <a
123 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
124 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
125 for timestamps after 1960 this is more precisely <a
126 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
127 Universal Time</a> (<abbr>UTC</abbr>).
128 The database also records when daylight saving time was in use,
129 along with some time zone abbreviations such as <abbr>EST</abbr>
130 for Eastern Standard Time in the <abbr>US</abbr>.</p>
131 </section>
132
133 <section>
134 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
135 <p>
136 The following <a
137 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
138 the latest release's two
139 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
140 to a <abbr>GNU</abbr>/Linux or similar host.</p>
141 <pre><code>mkdir tzdb
142 cd tzdb
143 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
144 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
145 <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 -
146 gzip -dc tzdata-latest.tar.gz | tar -xf -
147 </code></pre>
148 <p>Alternatively, the following shell commands download the same
149 release in a single-tarball format containing extra data
150 useful for regression testing:</p>
151 <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>
152 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
153 </code></pre>
154 <p>These commands use convenience links to the latest release
155 of the <code><abbr>tz</abbr></code> database hosted by the
156 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
157 of the <a href="https://www.iana.org">Internet Assigned Numbers
158 Authority (IANA)</a>.
159 Older releases are in files named
160 <code>tzcode<var>V</var>.tar.gz</code>,
161 <code>tzdata<var>V</var>.tar.gz</code>, and
162 <code>tzdb-<var>V</var>.tar.lz</code>,
163 where <code><var>V</var></code> is the version.
164 Since 1996, each version has been a four-digit year followed by
165 lower-case letter (<samp>a</samp> through <samp>z</samp>,
166 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
167 through <samp>zzz</samp>, and so on).
168 Since version 2016h, each release has contained a text file named
169 "<samp>version</samp>" whose first (and currently only) line is the version.
170 The releases are also available in an
171 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
172 title="File Transfer Protocol">FTP</abbr> directory</a> via a
173 less-secure protocol.</p>
174 <p>Alternatively, a development repository of code and data can be
175 retrieved from <a href="https://github.com">GitHub</a> via the shell
176 command:</p>
177 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
178 </code></pre>
179 <p>
180 Since version 2012e, each release has been tagged in development repositories.
181 Untagged commits are less well tested and probably contain
182 more errors.</p>
183 <p>
184 After obtaining the code and data files, see the
185 <code>README</code> file for what to do next.
186 The code lets you compile the <code><abbr>tz</abbr></code> source files into
187 machine-readable binary files, one for each location. The binary files
188 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
189 specified by <a href="https://tools.ietf.org/html/8536">Internet
190 <abbr>RFC</abbr> 8536</a>.
191 The code also lets
192 you read a <abbr>TZif</abbr> file and interpret timestamps for that
193 location.</p>
194 </section>
195
196 <section>
197 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
198 <p>
199 The <code><abbr>tz</abbr></code> code and data
200 are by no means authoritative. If you find errors, please
201 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
202 the time zone mailing list. You can also <a
203 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
204 and browse the <a
205 href="https://mm.icann.org/pipermail/tz/">archive of old
206 messages</a>.
207 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
208 discussions</a> and corresponding data changes can be
209 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
210 </p>
211 <p>
212 If your government plans to change its time zone boundaries or
213 daylight saving rules, inform <code>tz (a] iana.org</code> well in
214 advance, as this will coordinate updates to many cell phones,
215 computers, and other devices around the world. With
216 less than a year's notice there is a good chance that some
217 computer-based clocks will operate incorrectly after the change, due
218 to delays in propagating updates to software and data. The shorter
219 the notice, the more likely clock problems will arise; see "<a
220 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
221 the Timing of Time Zone Changes</a>" for examples.
222 </p>
223 <p>
224 Changes to the <code><abbr>tz</abbr></code> code and data are often
225 propagated to clients via operating system updates, so
226 client <code><abbr>tz</abbr></code> data can often be corrected by
227 applying these updates. With GNU/Linux and similar systems, if your
228 maintenance provider has not yet adopted the
229 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
230 the process by tailoring the generic instructions in
231 the <code><abbr>tz</abbr> README</code> file and installing the latest
232 data yourself. System-specific instructions for installing the
233 latest <code><abbr>tz</abbr></code> data have also been published
234 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
235 <a
236 href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
237 <a
238 href="http://userguide.icu-project.org/datetime/timezone"><abbr
239 title="International Components for Unicode">ICU</abbr></a>,
240 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
241 and <a
242 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
243 Java, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
244 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
245 and <a
246 href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
247 </p>
248 <p>Sources for the <code><abbr>tz</abbr></code> database are
249 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
250 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
251 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
252 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
253 title="linefeed">LF</abbr></a>,
254 which can be modified by common text editors such
255 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
256 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
257 <a href="https://www.vim.org">vim</a>.
258 Specialized source-file editing can be done via the
259 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
260 zoneinfo</a> package for <a
261 href="https://www.sublimetext.com">Sublime Text</a> and the <a
262 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
263 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
264 Studio Code</a>.
265 </p>
266 <p>
267 For further information about updates, please see
268 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
269 Maintaining the Time Zone Database</a> (Internet <abbr
270 title="Request For Comments">RFC</abbr> 6557). More detail can be
271 found in <a href="theory.html">Theory and pragmatics of the tz code and data</a>.
272 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
273 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
274 </p>
275 </section>
276
277 <section>
278 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
279 <ul>
280 <li>The article
281 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
282 an encyclopedic summary.</li>
283 <li><a href="tz-how-to.html">How to Read the
284 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
285 database format.</li>
286 <li><a
287 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
288 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
289 database's style.</li>
290 </ul>
291 </section>
292
293 <section>
294 <h2 id="web">Web sites using recent versions of the
295 <code><abbr>tz</abbr></code> database</h2>
296 <p>
297 These are listed roughly in ascending order of complexity and fanciness.
298 </p>
299 <ul>
300 <li><a href="https://time.is">Time.is</a> shows locations'
301 time and zones.</li>
302 <li><a href="https://www.timejones.com">TimeJones.com</a>,
303 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
304 <a href="https://worldclock.com">The World Clock</a>
305 are time zone converters.</li>
306 <li><a
307 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
308 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
309 <li><a
310 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
311 Time in 1000 Places</a> uses descriptions of the values.</li>
312 <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
313 uses a pulldown menu.</li>
314 <li><a href="http://home.kpn.nl/vanadovv/time/TZworld.html">Complete
315 timezone information for all countries</a> displays tables of DST rules.
316 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
317 Worldwide</a> lets you sort zone names and convert times.</li>
318 <li><a href="https://24timezones.com">24TimeZones</a> has a world
319 time map and a time converter.</li>
320 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
321 calculates the current time difference between locations.</li>
322 <li><a href="http://www.wx-now.com">Weather Now</a> and
323 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
324 </ul>
325 </section>
326
327 <section>
328 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
329 <ul>
330 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
331 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
332 Distribution Service (tzdist) working group</a> defined <a
333 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
334 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
335 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
336 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
337 transferring time zone data by reference.
338 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
339 mailing list</a> discussed the Internet draft <a
340 id="TZDIST-Geolocate"
341 href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
342 Geolocate Extension</a> (now expired), which let a client determine its timezone
343 from its geographic location using a <a
344 href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li>
345 <li>The <a href="https://tools.ietf.org/html/rfc5545">
346 Internet Calendaring and Scheduling Core Object Specification
347 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
348 covers time zone
349 data; see its VTIMEZONE calendar component.
350 The iCalendar format requires specialized parsers and generators; a
351 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
352 (Internet <abbr>RFC</abbr> 6321) uses
353 <a href="https://www.w3.org/XML/"><abbr
354 title="Extensible Markup Language">XML</abbr></a> format, and a variant
355 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
356 (Internet <abbr>RFC</abbr> 7265)
357 uses <a href="https://www.json.org"><abbr
358 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
359 </ul>
360 </section>
361
362 <section>
363 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
364 <ul>
365 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
366 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
367 program that compiles
368 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
369 Vzic is freely
370 available under the <a
371 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
372 General Public License (<abbr
373 title="General Public License">GPL</abbr>)</a>.</li>
374 <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
375 database conversion utility</a> is like Vzic, except for the <a
376 href="https://dotnet.microsoft.com">.NET framework</a>
377 and with a <abbr>BSD</abbr>-style license.</li>
378 <li><a
379 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
380 contains a script <code>parse_olson</code> that compiles
381 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
382 modules. It is part of the Perl <a
383 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
384 which is freely
385 available under both the <abbr>GPL</abbr> and the Perl Artistic
386 License. DateTime::TimeZone also contains a script
387 <code>tests_from_zdump</code> that generates test cases for each clock
388 transition in the <code><abbr>tz</abbr></code> database.</li>
389 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
390 Database Parser</a> is a
391 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
392 runtime library that is <a
393 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0355r4.html">moving
394 forward</a> for inclusion in
395 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
396 the next iteration of the C++ standard.
397 It is freely available under the
398 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
399 <li><a id="ICU" href="http://site.icu-project.org">International Components for
400 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
401 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
402 libraries for internationalization that
403 has a compiler from <code><abbr>tz</abbr></code> source
404 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
405 (mentioned <a href="#CLDR">below</a>)
406 into an <abbr>ICU</abbr>-specific format.
407 <abbr>ICU</abbr> is freely available under a
408 <abbr>BSD</abbr>-style license.</li>
409 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
410 the <a href="https://elixir-lang.org">Elixir</a> language downloads
411 and compiles tz source and exposes <abbr
412 title="Application Program Interface">API</abbr>s for use. It is
413 freely available under the <abbr>MIT</abbr> license.</li>
414 <li>Java-based compilers and libraries include:
415 <ul>
416 <li>The <a
417 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
418 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
419 Oracle Java.</li>
420 <li>The <a
421 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
422 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
423 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
424 which is freely available under a <abbr>BSD</abbr>-style license.</li>
425 <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
426 and time <abbr>API</abbr></a> contains a class
427 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
428 <code><abbr>tz</abbr></code> source into a binary format. It inspired
429 Java 8 <code>java.time</code>, which its users should migrate to once
430 they can assume Java 8 or later. It is available under the <a
431 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
432 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
433 time library for Android</a> and
434 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
435 time and interval library for Java</a> compile
436 <code><abbr>tz</abbr></code> source into a binary format.
437 Time4A is available under the Apache License and Time4J is
438 available under the <a
439 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
440 General Public License (<abbr title="Lesser General Public
441 License">LGPL</abbr>)</a>.</li>
442 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
443 Java-based libraries.</li>
444 </ul>
445 <li><a href="https://nodatime.org">Noda Time – Date and
446 time <abbr>API</abbr> for .NET</a>
447 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
448 It is freely available under the Apache License.</li>
449 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
450 compilers and libraries include:
451 <ul>
452 <li><a
453 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
454 compiles time zone data into a compact form designed for
455 JavaScript. It is freely available under a combination of
456 the <abbr>MIT</abbr> license and the Apache License.</li>
457 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
458 plugin for the <a href="https://momentjs.com">Moment.js</a> date
459 manipulation library. It is freely available under the <abbr>MIT</abbr>
460 license.</li>
461 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
462 <abbr>API</abbr> is upward compatible with standard JavaScript
463 Dates. It is freely available under the Apache License.</li>
464 </ul>
465 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
466 compiler from <code><abbr>tz</abbr></code> source into
467 <a href="https://julialang.org/">Julia</a>. It is freely available
468 under the <abbr>MIT</abbr> license.</li>
469 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
470 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
471 title="Free Pascal Compiler">FPC</abbr></a>
472 compiles from <code><abbr>tz</abbr></code> source into
473 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
474 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
475 and <a
476 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
477 It is freely available under a <abbr>BSD</abbr>-style license.</li>
478 <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
479 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
480 <a href="https://www.python.org">Python</a>.
481 It is freely available under a <abbr>BSD</abbr>-style license.</li>
482 <li><a href="https://tzinfo.github.io">TZInfo –
483 Ruby Timezone Library</a>
484 compiles <code><abbr>tz</abbr></code> source into
485 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
486 It is freely available under the <abbr>MIT</abbr> license.</li>
487 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
488 Library</a> is
489 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
490 library that compiles <code><abbr>tz</abbr></code> source into a time
491 zone repository whose format
492 is either proprietary or an <abbr>XML</abbr>-encoded
493 representation.</li>
494 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
495 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
496 source into text files, along with a runtime that can read those
497 files. Tcl is freely available under a <abbr>BSD</abbr>-style
498 license.</li>
499 </ul>
500 </section>
501
502 <section>
503 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
504 <ul>
505 <li>The <a
506 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
507 Library</a>
508 has an independent, thread-safe implementation of
509 a <abbr>TZif</abbr> file reader.
510 This library is freely available under the LGPL
511 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
512 <li><a href="https://www.gnome.org">GNOME</a>'s
513 <a href="https://developer.gnome.org/glib/">GLib</a> has
514 a <abbr>TZif</abbr> file reader written in C that
515 creates a <code>GTimeZone</code> object representing sets
516 of <abbr>UT</abbr> offsets.
517 It is freely available under the <abbr>LGPL</abbr>.</li>
518 <li>The
519 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
520 <code>baltzo::TimeZoneUtil</code> component contains a C++
521 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
522 the Apache License.</li>
523 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
524 library that translates between <abbr>UT</abbr> and civil time and
525 can read <abbr>TZif</abbr> files. It is freely available under the Apache
526 License.</li>
527 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
528 is a <abbr>TZif</abbr> file reader written in Java.
529 It is freely available under the <abbr>LGPL</abbr>.</li>
530 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
531 library that reads <abbr>TZif</abbr> files and converts
532 timestamps from one time zone or format to another.
533 It is used by <a href="https://secure.php.net"><abbr
534 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
535 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
536 and <a href="https://www.mongodb.com">MongoDB</a>.
537 It is freely available under the <abbr>MIT</abbr> license.</li>
538 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
539 JavaScript library that supports date arithmetic that is time zone
540 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
541 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
542 <abbr>TZif</abbr> file reader.</li>
543 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
544 DateTime::TimeZone::Tzfile</a>
545 is a <abbr>TZif</abbr> file reader written in Perl.
546 It is freely available under the same terms as Perl
547 (dual <abbr>GPL</abbr> and Artistic license).</li>
548 <li>The
549 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
550 library contains a Python tool that
551 converts <abbr>TZif</abbr> data into
552 <abbr>JSON</abbr>-format data suitable for use
553 in its JavaScript library for time zone conversion. Dates before 1970
554 are not supported.</li>
555 <li>The <a
556 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
557 package contains <a href="https://www.haskell.org">Haskell</a> code that
558 parses and uses <abbr>TZif</abbr> data. It is freely
559 available under a <abbr>BSD</abbr>-style license.</li>
560 </ul>
561 </section>
562
563 <section>
564 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
565 <ul>
566 <li><a href="https://foxclocks.org">FoxClocks</a>
567 is an extension for <a href="https://www.google.com/chrome/">Google
568 Chrome</a> and for <a
569 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
570 Toolkit</a> applications like <a
571 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
572 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
573 It displays multiple clocks in the application window, and has a mapping
574 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
575 It is freely available under the <abbr>GPL</abbr>.</li>
576 <li><a href="https://golang.org">Go programming language</a>
577 implementations contain a copy of a 32-bit subset of a recent
578 <code><abbr>tz</abbr></code> database in a
579 Go-specific format.</li>
580 <li><a
581 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
582 clock (intclock)</a> is a clock that displays multiple time zones on
583 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
584 under the <abbr>GPL</abbr>.</li>
585 <li>Microsoft Windows 8.1
586 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
587 data (mentioned <a href="#CLDR">below</a>) used by the
588 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
589 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
590 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
591 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
592 <a id="System.TimeZoneInfo"
593 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
594 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
595 the older, proprietary method of Microsoft Windows 2000 and later,
596 which stores time zone data in the
597 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
598 <a
599 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
600 Tzid table</a> or <a
601 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
602 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
603 to <code><abbr>tz</abbr></code> names.
604 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
605 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
606 <li><a
607 href="https://www.oracle.com/java/index.html">Oracle
608 Java</a> contains a copy of a subset of a recent
609 <code><abbr>tz</abbr></code> database in a
610 Java-specific format.</li>
611 <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
612 Master</a> is a Microsoft Windows clock program that can automatically
613 download, compile and use <code><abbr>tz</abbr></code> releases.
614 The Basic version is free.</li>
615 <li><a
616 href="http://veladg.com/velaterra.html">VelaTerra</a> is
617 a macOS program. Its developers
618 <a href="http://veladg.com/tzoffer.html">offer free
619 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
620 </ul>
621 </section>
622
623 <section>
624 <h2 id="other-dbs">Other time zone databases</h2>
625 <ul>
626 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
627 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
628 time zone history atlases
629 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
630 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>,
631 now published in <a
632 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
633 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
634 Although these extensive atlases
635 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
636 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
637 they are unreliable as Shanks appears to have
638 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
639 sources and do not indicate which entries are guesswork.</li>
640 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
641 its own <code>tztab</code>(4) format.</li>
642 <li>Microsoft Windows has proprietary data mentioned
643 <a href="#System.TimeZoneInfo">above</a>.</li>
644 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
645 is another time zone database.</li>
646 <li>The <a
647 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
648 Schedules Information Manual</a> of the
649 International Air Transport Association
650 gives current time zone rules for airports served by commercial aviation.</li>
651 </ul>
652 </section>
653
654 <section>
655 <h2 id="maps">Maps</h2>
656 <ul>
657 <li>The <a href="https://www.cia.gov/index.html">United States Central
658 Intelligence Agency (<abbr
659 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
660 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
661 zone map</a>; the
662 <a
663 href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
664 Library Map Collection</a>
665 of the University of Texas at Austin has copies of
666 recent editions.
667 The pictorial quality is good,
668 but the maps do not indicate daylight saving time,
669 and parts of the data are a few years out of date.</li>
670 <li><a href="https://www.worldtimezone.com">World Time Zone Map
671 with current time</a>
672 has several fancy time zone maps; it covers Russia particularly well.
673 The maps' pictorial quality is not quite as good as the
674 <abbr>CIA</abbr>'s
675 but the maps are more up to date.</li>
676 <li><a
677 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
678 much is time wrong around the world?</a> maps the difference between
679 mean solar and standard time, highlighting areas such as western China
680 where the two differ greatly. It's a bit out of date, unfortunately.</li>
681 </ul>
682 </section>
683
684 <section>
685 <h2 id="boundaries">Time zone boundaries</h2>
686 <p>Geographical boundaries between timezones are available
687 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
688 services and other sources.</p>
689 <ul>
690 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
691 Boundary Builder</a> extracts
692 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
693 boundaries of <code><abbr>tzdb</abbr></code> timezones.
694 Its code is freely available under the <abbr>MIT</abbr> license, and
695 its data entries are freely available under the
696 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
697 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
698 <li>Programmatic interfaces that map geographical coordinates via tz_world to
699 <code><abbr>tzdb</abbr></code> timezones include:
700 <ul>
701 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
702 written in <a
703 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
704 and is freely available under the <abbr>MIT</abbr> license.</li>
705 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
706 is written in Go and is freely available under the Apache License.</li>
707 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
708 in both Java and
709 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
710 form, is freely available under the MIT license.</li>
711 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
712 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
713 is freely available under the MIT license, and
714 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
715 is in the public domain.</li>
716 <li>The <a
717 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
718 library for Python is freely available under the MIT license.
719 <li>The <a
720 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
721 library for Ruby is freely available under the MIT license.</li>
722 </ul></li>
723 <li>Free access via a network API, if you register a key, is provided by
724 the <a
725 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
726 Timezone web service</a>, the <a
727 href="https://developers.google.com/maps/documentation/timezone/intro">Google
728 Maps Time Zone API</a>, and
729 the <a href="https://timezonedb.com">Time Zone Database & API</a>.
730 Commercial network API access is provided
731 by <a href="https://askgeo.com">AskGeo</a>
732 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
733 </li>
734 <li>"<a
735 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
736 to get a time zone from a location using latitude and longitude
737 coordinates?</a>" discusses other geolocation possibilities.</li>
738 <li><a href="http://statoids.com/statoids.html">Administrative
739 Divisions of Countries ("Statoids")</a> lists
740 political subdivision data related to time zones.</li>
741 <li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time
742 zone boundaries for multizone countries</a> summarizes legal
743 boundaries between time zones within countries.</li>
744 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
745 – GIS and Database Tools</a> includes a Manifold-format map of
746 world time zone boundaries distributed under the
747 <abbr>GPL</abbr>.</li>
748 <li>A ship within the <a
749 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
750 waters</a> of any nation uses that nation's time. In international
751 waters, time zone boundaries are meridians 15° apart, except that
752 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
753 wide and are separated by
754 the 180° meridian (not by the International Date Line, which is
755 for land and territorial waters only). A captain can change ship's
756 clocks any time after entering a new time zone; midnight changes are
757 common.</li>
758 </ul>
759 </section>
760
761 <section>
762 <h2 id="civil">Civil time concepts and history</h2>
763 <ul>
764 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
765 Walk through Time</a>
766 surveys the evolution of timekeeping.</li>
767 <li>The history of daylight saving time is surveyed in <a
768 href="http://www.webexhibits.org/daylightsaving/">About Daylight
769 Saving Time – History, rationale, laws & dates</a> and summarized in
770 <a href="http://seizethedaylight.com/dst/">A Brief
771 History of Daylight Saving Time</a>.</li>
772 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
773 Lords</a> discusses how authoritarians manipulate civil time.</li>
774 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
775 contains guidelines and best practices for software applications that
776 deal with civil time.</li>
777 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
778 the International Date Line</a> tells the story of the most important
779 time zone boundary.</li>
780 <li><a href="http://statoids.com/tconcept.html">Basic Time
781 Zone Concepts</a> discusses terminological issues behind time zones.</li>
782 </ul>
783 </section>
784
785 <section>
786 <h2 id="national">National histories of legal time</h2>
787 <dl>
788 <dt>Australia</dt>
789 <dd>The Parliamentary Library commissioned a <a
790 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
791 paper on daylight saving time in Australia</a>.
792 The Bureau of Meteorology publishes a list of <a
793 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
794 Dates of Daylight Savings Time within Australia</a>.</dd>
795 <dt>Belgium</dt>
796 <dd>The Royal Observatory of Belgium maintains a table of <a
797 href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
798 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
799 <dt>Brazil</dt>
800 <dd>The Time Service Department of the National Observatory
801 records <a href="http://pcdsh01.on.br/DecHV.html"
802 hreflang="pt-BR">Brazil's daylight saving time decrees (in
803 Portuguese)</a>.</dd>
804 <dt>Canada</dt>
805 <dd>National Research Council Canada publishes current
806 and some older information about <a
807 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
808 zones and daylight saving time</a>.</dd>
809 <dt>Chile</dt>
810 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
811 <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
812 Chile's official time (in Spanish)</a>.</dd>
813 <dt>China</dt>
814 <dd>The Hong Kong Observatory maintains a
815 <a href="https://www.hko.gov.hk/gts/time/Summertime.htm">history of
816 summer time in Hong Kong</a>,
817 and Macau's Meteorological and Geophysical Bureau maintains a <a
818 href="http://www.smg.gov.mo/smg/geophysics/e_t_Summer%20Time.htm">similar
819 history for Macau</a>.
820 Unfortunately the latter is incomplete and has errors.</dd>
821 <dt>Czech Republic</dt>
822 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
823 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
824 summarizes and cites historical DST regulations.</dd>
825 <dt>Germany</dt>
826 <dd>The National Institute for Science and Technology maintains the <a
827 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
828 of Legal Time in Germany</a>.</dd>
829 <dt>Israel</dt>
830 <dd>The Interior Ministry periodically issues <a
831 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
832 hreflang="he">announcements (in Hebrew)</a>.</dd>
833 <dt>Italy</dt>
834 <dd>The National Institute of Metrological Research publishes a
835 <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
836 of civil time (in Italian)</a>.</dd>
837 <dt>Malaysia</dt>
838 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
839 <dt>Mexico</dt>
840 <dd>The Investigation and Analysis Service of the Mexican Library of
841 Congress has published a <a
842 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
843 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
844 <dt>Netherlands</dt>
845 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
846 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
847 covers the history of local time in the Netherlands from ancient times.</dd>
848 <dt>New Zealand</dt>
849 <dd>The Department of Internal Affairs maintains a brief <a
850 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
851 Daylight Saving</a>.</dd>
852 <dt>Singapore</dt>
853 <dd><a id="Singapore"
854 href="http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
855 is Singapore in the "Wrong" Time Zone?</a> details the
856 history of legal time in Singapore and Malaysia.</dd>
857 <dt>United Kingdom</dt>
858 <dd><a
859 href="https://www.polyomino.org.uk/british-time/">History of
860 legal time in Britain</a> discusses in detail the country
861 with perhaps the best-documented history of clock adjustments.</dd>
862 <dt>United States</dt>
863 <dd>The Department of Transportation's <a
864 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
865 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
866 <dt>Uruguay</dt>
867 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
868 Navy (SOHMA) publishes an annual <a
869 href="http://www.armada.gub.uy/sohma/index.php/servicios/datos-astronomicos" hreflang="es">almanac
870 (in Spanish)</a>.</dd>
871 </dl>
872 </section>
873
874 <section>
875 <h2 id="precision">Precision timekeeping</h2>
876 <ul>
877 <li><a
878 href="http://leapsecond.com/hpan/an1289.pdf">The
879 Science of Timekeeping</a> is a thorough introduction
880 to the theory and practice of precision timekeeping.</li>
881 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
882 Time 2016</a> contains several freely-readable papers.</li>
883 <li><a href="http://www.ntp.org"><abbr
884 title="Network Time Protocol">NTP</abbr>: The Network
885 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
886 discusses how to synchronize clocks of
887 Internet hosts.</li>
888 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
889 family of software algorithms can achieve accuracy to a few tens of
890 nanoseconds in scalable server farms without special hardware.</li>
891 <li>The <a
892 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
893 Time Protocol</a> (<abbr
894 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
895 can achieve submicrosecond clock accuracy on a local area network
896 with special-purpose hardware.</li>
897 <li><a
898 href="https://tools.ietf.org/html/rfc4833">Timezone
899 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
900 (Internet <abbr>RFC</abbr> 4833)
901 specifies a <a
902 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
903 option for a server to configure
904 a client's time zone and daylight saving settings automatically.</li>
905 <li><a
906 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
907 Times</a> explains more abstruse astronomical time scales like
908 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
909 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
910 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
911 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
912 Scales</a> goes into more detail, particularly for historical variants.</li>
913 <li>The <a href="https://www.iau.org"><abbr
914 title="International Astronomical Union">IAU</abbr></a>'s <a
915 href="http://www.iausofa.org"><abbr
916 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
917 collection contains C and <a
918 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
919 code for converting among time scales like
920 <abbr title="International Atomic Time">TAI</abbr>,
921 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
922 <abbr>UTC</abbr>.</li>
923 <li><a
924 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
925 – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
926 diverse local time
927 scales used by each landed mission on Mars.</li>
928 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
929 dedicated not only to leap seconds but to precise time and frequency
930 in general. It covers the state of the art in amateur timekeeping, and
931 how the art has progressed over the past few decades.</li>
932 <li><a
933 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
934 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
935 Bulletins</a> contains official publications of the International
936 Earth Rotation and Reference Systems Service, which decides when leap
937 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
938 via an optional "<code>right</code>" configuration, as opposed to the
939 default "<code>posix</code>" configuration.</li>
940 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
941 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
942 leap second so that they disagree with <abbr>UTC</abbr> by at most a
943 half second, even though every <abbr>POSIX</abbr> minute has exactly
944 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
945 "<code>posix</code>" configuration, is <a
946 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
947 the <abbr>NTP</abbr> reference implementation, and is used by major
948 cloud service providers.</li>
949 <li>The <a
950 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
951 Second Discussion List</a> covers <a
952 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
953 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
954 discussed further in
955 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
956 leap second: its history and possible future</a>.
957 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
958 might be redefined
959 without Leap Seconds</a> gives pointers on this
960 contentious issue, which was active until 2015 and could become active
961 again.</li>
962 </ul>
963 </section>
964
965 <section>
966 <h2 id="notation">Time notation</h2>
967 <ul>
968 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
969 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
970 zone names, abbreviations, identifiers, and formats. For example, it
971 contains French translations for "Eastern European Summer Time",
972 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
973 "Bucharest". Its
974 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
975 charts</a> show these values for many locales. Data values are available in
976 both <abbr title="Locale Data Markup Language">LDML</abbr>
977 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
978 <li>
979 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
980 the international standard date and time notation</a> covers
981 <a
982 href="https://www.iso.org/standard/70907.html"><em><abbr
983 title="International Organization for Standardization">ISO</abbr>
984 8601-1:2019 – Date and time – Representations for information
985 interchange – Part 1: Basic rules</em></a>.</li>
986 <li>
987 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
988 Schema: Datatypes – dateTime</a> specifies a format inspired by
989 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
990 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of
991 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
992 specifies the time notation used in email and <a
993 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
994 headers.</li>
995 <li>
996 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
997 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
998 specifies an <abbr>ISO</abbr> 8601
999 profile for use in new Internet
1000 protocols.</li>
1001 <li>
1002 <a href="https://www.hackcraft.net/web/datetime/">Date & Time
1003 Formats on the Web</a> surveys web- and Internet-oriented date and time
1004 formats.</li>
1005 <li>Alphabetic time zone abbreviations should not be used as unique
1006 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1007 practice. For example, in English-speaking North America
1008 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1009 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1010 and French-speaking North Americans prefer
1011 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1012 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1013 database contains English abbreviations for many timestamps;
1014 unfortunately some of these abbreviations were merely the database maintainers'
1015 inventions, and these have been removed when possible.</li>
1016 <li>Numeric time zone abbreviations typically count hours east of
1017 <abbr>UT</abbr>, e.g., +09 for Japan and
1018 −10 for Hawaii. However, the <abbr>POSIX</abbr>
1019 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1020 For example, one might use <code><abbr>TZ</abbr>="<abbr
1021 title="Japan Standard Time">JST</abbr>-9"</code> and
1022 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1023 for Japan and Hawaii, respectively. If the
1024 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1025 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1026 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1027 confusion, handle old timestamps better, and insulate you better from
1028 any future changes to the rules. One should never set
1029 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1030 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1031 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1032 is called "<abbr>GMT</abbr>".</li>
1033 </ul>
1034 </section>
1035
1036 <section>
1037 <h2 id="see-also">See also</h2>
1038 <ul>
1039 <li><a href="theory.html">Theory and pragmatics of the tz code and data</a></li>
1040 <li><a href="tz-art.html">Time and the Arts</a></li>
1041 </ul>
1042 </section>
1043
1044 <footer>
1045 <hr>
1046 This web page is in the public domain, so clarified as of
1047 2009-05-17 by Arthur David Olson.
1048 <br>
1049 Please send corrections to this web page to the
1050 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1051 </footer>
1052 </body>
1053 </html>
1054