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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 <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="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
193 Java, <a href="http://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="http://search.cpan.org/dist/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="http://datetime.perl.org">DateTime Project</a>, which is freely
326 available under both the <abbr>GPL</abbr> and the Perl Artistic
327 License. DateTime::TimeZone also contains a script
328 <code>tests_from_zdump</code> that generates test cases for each clock
329 transition in the <code><abbr>tz</abbr></code> database.</li>
330 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
331 Database Parser</a> is a
332 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
333 runtime library that is <a
334 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0355r4.html">moving
335 forward</a> for inclusion in the next iteration of <a
336 href="https://isocpp.org/std/the-standard"><em><abbr
337 title="International Organization for Standardization">ISO</abbr>
338 International Standard ISO/IEC 14882:2017(E) – Programming
339 Language C++</em></a>.
340 It is freely available under the
341 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
342 <li><a id="ICU" href="http://site.icu-project.org">International Components for
343 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
344 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
345 libraries for internationalization that
346 has a compiler from <code><abbr>tz</abbr></code> source
347 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
348 (mentioned <a href="#CLDR">below</a>)
349 into an <abbr>ICU</abbr>-specific format.
350 <abbr>ICU</abbr> is freely available under a
351 <abbr>BSD</abbr>-style license.</li>
352 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
353 the <a href="https://elixir-lang.org">Elixir</a> language downloads
354 and compiles tz source and exposes <abbr
355 title="Application Program Interface">API</abbr>s for use. It is
356 freely available under the <abbr>MIT</abbr> license.</li>
357 <li>Java-based compilers and libraries include:
358 <ul>
359 <li>The <a
360 href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
361 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
362 Oracle Java.</li>
363 <li>The <a
364 href="http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
365 8 <code>java.time</code> <abbr>API</abbr></a> can be supplemented by <a
366 href="http://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
367 which is freely available under a <abbr>BSD</abbr>-style license.</li>
368 <li><a href="http://www.joda.org/joda-time/">Joda-Time – Java date
369 and time <abbr>API</abbr></a> contains a class
370 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
371 <code><abbr>tz</abbr></code> source into a binary format. It inspired
372 Java 8 <code>java.time</code>, which its users should migrate to once
373 they can assume Java 8 or later. It is available under the <a
374 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
375 <li><a href="https://github.com/MenoData/Time4J/">Time4J –
376 Advanced date, time and interval library for Java</a> contains a class
377 <code>net.time4j.tool.TimezoneRepositoryCompiler</code> that compiles
378 <code><abbr>tz</abbr></code> source into a binary format. Time4J is
379 available under the <a
380 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
381 General Public License (<abbr title="Lesser General Public
382 License">LGPL</abbr>)</a>.</li>
383 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
384 Java-based libraries.</li>
385 </ul>
386 <li><a href="https://nodatime.org">Noda Time – Date and
387 time <abbr>API</abbr> for .NET</a>
388 and <a href="http://www.babiej.demon.nl/Tz4Net/main.htm">TZ4Net</a>
389 are similar to Joda-Time and Time4J, but for the .NET framework instead of
390 Java. They are freely available under the
391 Apache License
392 and a <abbr>BSD</abbr>-style license, respectively.</li>
393 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
394 compilers and libraries include:
395 <ul>
396 <li><a
397 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
398 compiles time zone data into a compact form designed for
399 JavaScript. It is freely available under a combination of
400 the <abbr>MIT</abbr> license and the Apache License.</li>
401 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
402 plugin for the <a href="https://momentjs.com">Moment.js</a> date
403 manipulation library. It is freely available under the <abbr>MIT</abbr>
404 license.</li>
405 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
406 <abbr>API</abbr> is upward compatible with standard JavaScript
407 Dates. It is freely available under the Apache License.</li>
408 <li><a href="https://github.com/sproutsocial/walltime-js">Walltime-js</a>
409 translates <abbr>UT</abbr> to local time. It is freely available under
410 the <abbr>MIT</abbr> license.</li>
411 </ul>
412 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
413 compiler from <code><abbr>tz</abbr></code> source into
414 <a href="https://julialang.org/">Julia</a>. It is freely available
415 under the <abbr>MIT</abbr> license.</li>
416 <li><a href="https://github.com/pavkam/tzdb">Delphi/<abbr
417 title="Free Pascal Compiler">FPC</abbr> Time Zone Database</a>
418 compiles from <code><abbr>tz</abbr></code> source into
419 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
420 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
421 and <a
422 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
423 It is freely available under a <abbr>BSD</abbr>-style license.</li>
424 <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
425 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
426 <a href="https://www.python.org">Python</a>.
427 It is freely available under a <abbr>BSD</abbr>-style license.</li>
428 <li><a href="https://tzinfo.github.io">TZInfo –
429 Ruby Timezone Library</a>
430 compiles <code><abbr>tz</abbr></code> source into
431 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
432 It is freely available under the <abbr>MIT</abbr> license.</li>
433 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
434 Library</a> is
435 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
436 library that compiles <code><abbr>tz</abbr></code> source into a time
437 zone repository whose format
438 is either proprietary or an <abbr>XML</abbr>-encoded
439 representation.</li>
440 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
441 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
442 source into text files, along with a runtime that can read those
443 files. Tcl is freely available under a <abbr>BSD</abbr>-style
444 license.</li>
445 </ul>
446 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
447 <ul>
448 <li>The <a
449 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
450 Library</a>
451 has an independent, thread-safe implementation of
452 a <abbr>TZif</abbr> file reader.
453 This library is freely available under the LGPL
454 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
455 <li><a href="https://www.gnome.org">GNOME</a>'s
456 <a href="https://developer.gnome.org/glib/">GLib</a> has
457 a <abbr>TZif</abbr> file reader written in C that
458 creates a <code>GTimeZone</code> object representing sets
459 of <abbr>UT</abbr> offsets.
460 It is freely available under the <abbr>LGPL</abbr>.</li>
461 <li>The
462 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
463 <code>baltzo::TimeZoneUtil</code> component contains a C++
464 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
465 the Apache License.</li>
466 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
467 library that translates between <abbr>UT</abbr> and civil time and
468 can read <abbr>TZif</abbr> files. It is freely available under the Apache
469 License.</li>
470 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
471 is a <abbr>TZif</abbr> file reader written in Java.
472 It is freely available under the <abbr>LGPL</abbr>.</li>
473 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
474 library that reads <abbr>TZif</abbr> files and converts
475 timestamps from one time zone or format to another.
476 It is used by <a href="https://secure.php.net"><abbr
477 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
478 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
479 and <a href="https://www.mongodb.com">MongoDB</a>.
480 It is freely available under the <abbr>MIT</abbr> license.</li>
481 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
482 JavaScript library that supports date arithmetic that is time zone
483 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
484 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
485 <abbr>TZif</abbr> file reader.</li>
486 <li><a href="http://search.cpan.org/perldoc?DateTime::TimeZone::Tzfile">
487 DateTime::TimeZone::Tzfile</a>
488 is a <abbr>TZif</abbr> file reader written in Perl.
489 It is freely available under the same terms as Perl
490 (dual <abbr>GPL</abbr> and Artistic license).</li>
491 <li>The
492 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
493 library contains a Python tool that
494 converts <abbr>TZif</abbr> data into
495 <abbr>JSON</abbr>-format data suitable for use
496 in its JavaScript library for time zone conversion. Dates before 1970
497 are not supported.</li>
498 <li>The <a
499 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
500 package contains <a href="https://www.haskell.org">Haskell</a> code that
501 parses and uses <abbr>TZif</abbr> data. It is freely
502 available under a <abbr>BSD</abbr>-style license.</li>
503 </ul>
504 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
505 <ul>
506 <li><a href="https://foxclocks.org">FoxClocks</a>
507 is an extension for <a href="https://www.google.com/chrome/">Google
508 Chrome</a> and for <a
509 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
510 Toolkit</a> applications like <a
511 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
512 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
513 It displays multiple clocks in the application window, and has a mapping
514 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
515 It is freely available under the <abbr>GPL</abbr>.</li>
516 <li><a href="https://golang.org">Go programming language</a>
517 implementations contain a copy of a 32-bit subset of a recent
518 <code><abbr>tz</abbr></code> database in a
519 Go-specific format.</li>
520 <li><a
521 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
522 clock (intclock)</a> is a clock that displays multiple time zones on
523 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
524 under the <abbr>GPL</abbr>.</li>
525 <li>Microsoft Windows 8.1
526 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
527 data (mentioned <a href="#CLDR">below</a>) used by the
528 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
529 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
530 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
531 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
532 <a id="System.TimeZoneInfo"
533 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
534 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
535 the older, proprietary method of Microsoft Windows 2000 and later,
536 which stores time zone data in the
537 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
538 <a
539 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
540 Tzid table</a> or <a
541 href="https://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
542 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
543 to <code><abbr>tz</abbr></code> names.
544 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
545 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
546 <li><a
547 href="https://www.oracle.com/java/index.html">Oracle
548 Java</a> contains a copy of a subset of a recent
549 <code><abbr>tz</abbr></code> database in a
550 Java-specific format.</li>
551 <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
552 Master</a> is a Microsoft Windows clock program that can automatically
553 download, compile and use <code>tz</code> releases. The Basic version
554 is free.</li>
555 <li><a
556 href="http://veladg.com/velaterra.html">VelaTerra</a> is
557 a macOS program. Its developers
558 <a href="http://veladg.com/tzoffer.html">offer free
559 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
560 </ul>
561 <h2 id="other-dbs">Other time zone databases</h2>
562 <ul>
563 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
564 is Astrodienst's Web version of Shanks and Pottenger's
565 time zone history atlases also published in <a
566 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
567 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
568 These atlases are extensive but unreliable, as Shanks appears to have
569 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
570 sources and do not indicate which entries are guesswork.</li>
571 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
572 its own <code>tztab</code>(4) format.</li>
573 <li>Microsoft Windows has proprietary data mentioned
574 <a href="#System.TimeZoneInfo">above</a>.</li>
575 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
576 is another time zone database.</li>
577 <li>The <a
578 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
579 Schedules Information Manual</a> of the
580 International Air Transport Association
581 gives current time zone rules for airports served by commercial aviation.</li>
582 </ul>
583 <h2 id="maps">Maps</h2>
584 <ul>
585 <li>The <a href="https://www.cia.gov/index.html">United States Central
586 Intelligence Agency (<abbr
587 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
588 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
589 zone map</a>; the
590 <a
591 href="https://www.lib.utexas.edu/maps/world.html">Perry–Castañeda
592 Library Map Collection</a>
593 of the University of Texas at Austin has copies of
594 recent editions.
595 The pictorial quality is good,
596 but the maps do not indicate daylight saving time,
597 and parts of the data are a few years out of date.</li>
598 <li><a href="https://www.worldtimezone.com">Current time around the world
599 and standard time zones map of the world</a>
600 has several fancy time zone maps; it covers Russia particularly well.
601 The maps' pictorial quality is not quite as good as the
602 <abbr>CIA</abbr>'s
603 but the maps are more up to date.</li>
604 <li><a
605 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
606 much is time wrong around the world?</a> maps the difference between
607 mean solar and standard time, highlighting areas such as western China
608 where the two differ greatly. It's a bit out of date, unfortunately.</li>
609 </ul>
610 <h2 id="boundaries">Time zone boundaries</h2>
611 <p>Geographical boundaries between timezones are available
612 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
613 services and other sources.</p>
614 <ul>
615 <li>Databases of time zone boundaries include:
616 <ul>
617 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
618 Boundary Builder</a> extracts
619 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
620 boundaries of <code><abbr>tzdb</abbr></code> timezones.
621 Its code is freely available under the <abbr>MIT</abbr> license, and
622 its data entries are freely available under the
623 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
624 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
625 <li><a href="http://efele.net/maps/tz/"><abbr>TZ</abbr> timezones
626 maps</a> contains <a
627 href="https://en.wikipedia.org/wiki/Shapefile">shapefiles</a> of
628 sets of <code><abbr>tzdb</abbr></code> timezones. This includes
629 <a href="http://efele.net/maps/tz/world/">tz_world</a>, a shapefile
630 for all the world's timezones. These maps are no longer maintained and
631 are superseded by the Timezone Boundary Builder.</li>
632 <li><a
633 href="https://github.com/straup/whereonearth-timezone">Whereonearth-timezone</a>
634 is in <a href="https://tools.ietf.org/html/rfc7946">GeoJSON</a> format
635 (Internet <abbr>RFC</abbr> 7946), and combines the
636 the tz_world shapefiles with the
637 <a href="https://developer.yahoo.com/geo/geoplanet/">GeoPlanet</a>
638 dataset.</li>
639 </ul></li>
640 <li>Programmatic interfaces that map geographical coordinates via tz_world to
641 <code><abbr>tzdb</abbr></code> timezones include:
642 <ul>
643 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
644 written in <a
645 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
646 and is freely available under the <abbr>MIT</abbr> license.</li>
647 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
648 is written in Go and is freely available under the Apache License.</li>
649 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
650 in both Java and
651 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
652 form, is freely available under the MIT license.</li>
653 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
654 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
655 is freely available under the MIT license, and
656 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
657 is in the public domain.</li>
658 <li>The <a
659 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
660 library for Python is freely available under the MIT license.
661 <li>The <a
662 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
663 library for Ruby is freely available under the MIT license.</li>
664 <li><a href="https://derickrethans.nl/what-time-is-it.html">What Time
665 is It Here?</a> applies MongoDB
666 geospatial query operators to shapefiles' data.</li>
667 </ul></li>
668 <li>Free access via a network API, if you register a key, is provided by
669 the <a href="http://www.geonames.org/export/web-services.html#timezone">GeoNames Timezone web service</a>,
670 the <a href="https://developers.google.com/maps/documentation/timezone/intro">Google Maps Time Zone API</a>, and
671 the <a href="https://timezonedb.com">Time Zone Database & API</a>.
672 Commercial network API access is provided
673 by <a href="https://askgeo.com">AskGeo</a>
674 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
675 </li>
676 <li>"<a
677 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
678 to get a time zone from a location using latitude and longitude
679 coordinates?</a>" discusses other geolocation possibilities.</li>
680 <li><a href="http://statoids.com/statoids.html">Administrative
681 Divisions of Countries ("Statoids")</a> lists
682 political subdivision data related to time zones.</li>
683 <li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time
684 zone boundaries for multizone countries</a> summarizes legal
685 boundaries between time zones within countries.</li>
686 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
687 – GIS and Database Tools</a> includes a Manifold-format map of
688 world time zone boundaries distributed under the
689 <abbr>GPL</abbr>.</li>
690 <li>A ship within the <a
691 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
692 waters</a> of any nation uses that nation's time. In international
693 waters, time zone boundaries are meridians 15° apart, except that
694 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
695 wide and are separated by
696 the 180° meridian (not by the International Date Line, which is
697 for land and territorial waters only). A captain can change ship's
698 clocks any time after entering a new time zone; midnight changes are
699 common.</li>
700 </ul>
701 <h2 id="civil">Civil time concepts and history</h2>
702 <ul>
703 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
704 Walk through Time</a>
705 surveys the evolution of timekeeping.</li>
706 <li><a href="http://www.webexhibits.org/daylightsaving/">About Daylight
707 Saving Time – History, rationale, laws & dates</a>
708 is an overall history of <abbr>DST</abbr>.</li>
709 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
710 contains guidelines and best practices for software applications that
711 deal with civil time.</li>
712 <li><a href="http://seizethedaylight.com/dst/">A Brief
713 History of Daylight Saving Time</a> summarizes some of the contentious
714 history of <abbr>DST</abbr>.</li>
715 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
716 the International Date Line</a> tells the story of the most important
717 time zone boundary.</li>
718 <li><a href="http://statoids.com/tconcept.html">Basic Time
719 Zone Concepts</a> discusses terminological issues behind time zones.</li>
720 </ul>
721 <h2 id="national">National histories of legal time</h2>
722 <dl>
723 <dt>Australia</dt>
724 <dd>The Parliamentary Library has commissioned a <a
725 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
726 paper on daylight saving time in Australia</a>.
727 The Bureau of Meteorology publishes a list of <a
728 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
729 Dates of Daylight Savings Time within Australia</a>.</dd>
730 <dt>Belgium</dt>
731 <dd>The Royal Observatory of Belgium maintains a table of <a
732 href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
733 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
734 <dt>Brazil</dt>
735 <dd>The Time Service Department of the National Observatory
736 records <a href="http://pcdsh01.on.br/DecHV.html"
737 hreflang="pt-BR">Brazil's daylight saving time decrees (in
738 Portuguese)</a>.</dd>
739 <dt>Canada</dt>
740 <dd>National Research Council Canada publishes current
741 and some older information about <a
742 href="https://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time
743 zones & daylight saving time</a>.</dd>
744 <dt>Chile</dt>
745 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
746 <a href="http://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
747 Chile's official time (in Spanish)</a>.</dd>
748 <dt>China</dt>
749 <dd>The Hong Kong Observatory maintains a
750 <a href="https://www.hko.gov.hk/gts/time/Summertime.htm">history of
751 summer time in Hong Kong</a>,
752 and Macau's Meteorological and Geophysical Bureau maintains a <a
753 href="http://www.smg.gov.mo/smg/geophysics/e_t_Summer%20Time.htm">similar
754 history for Macau</a>.
755 Unfortunately the latter is incomplete and has errors.</dd>
756 <dt>Czech Republic</dt>
757 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
758 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
759 summarizes and cites historical DST regulations.</dd>
760 <dt>Germany</dt>
761 <dd>The National Institute for Science and Technology maintains the <a
762 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
763 of Legal Time in Germany</a>.</dd>
764 <dt>Israel</dt>
765 <dd>The Interior Ministry periodically issues <a
766 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
767 hreflang="he">announcements (in Hebrew)</a>.</dd>
768 <dt>Italy</dt>
769 <dd>The National Institute of Metrological Research maintains a
770 <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml">table of civil time
771 (in Italian)</a>.</dd>
772 <dt>Malaysia</dt>
773 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
774 <dt>Mexico</dt>
775 <dd>The Investigation and Analysis Service of the Mexican Library of
776 Congress has published a <a
777 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
778 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
779 <dt>Netherlands</dt>
780 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
781 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
782 covers the history of local time in the Netherlands from ancient times.</dd>
783 <dt>New Zealand</dt>
784 <dd>The Department of Internal Affairs maintains a brief <a
785 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
786 Daylight Saving</a>. The privately-maintained <a
787 href="http://astrologyschool.com/nztime.html">History of New Zealand
788 time</a> has more details.</dd>
789 <dt>Singapore</dt>
790 <dd><a id="Singapore"
791 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
792 is Singapore in the "Wrong" Time Zone?</a> details the
793 history of legal time in Singapore and Malaysia.</dd>
794 <dt>United Kingdom</dt>
795 <dd><a
796 href="https://www.polyomino.org.uk/british-time/">History of
797 legal time in Britain</a> discusses in detail the country
798 with perhaps the best-documented history of clock adjustments.
799 The National Physical Laboratory also maintains an <a
800 href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates">Archive
801 of Summer time dates</a>.</dd>
802 <dt>United States</dt>
803 <dd>The Department of Transportation's <a
804 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
805 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
806 <dt>Uruguay</dt>
807 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
808 Navy (SOHMA) publishes an annual <a
809 href="http://www.armada.mil.uy/Pagina/institucion/dimat/sohma/almanaque.html" hreflang="es">almanac
810 (in Spanish)</a>.</dd>
811 </dl>
812 <h2 id="precision">Precision timekeeping</h2>
813 <ul>
814 <li><a
815 href="http://leapsecond.com/hpan/an1289.pdf">The
816 Science of Timekeeping</a> is a thorough introduction
817 to the theory and practice of precision timekeeping.</li>
818 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
819 Time 2016</a> contains several freely-readable papers.</li>
820 <li><a href="http://www.ntp.org"><abbr
821 title="Network Time Protocol">NTP</abbr>: The Network
822 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
823 discusses how to synchronize clocks of
824 Internet hosts.</li>
825 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
826 family of software algorithms can achieve accuracy to a few tens of
827 nanoseconds in scalable server farms without special hardware.</li>
828 <li>The <a
829 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
830 Time Protocol</a> (<abbr
831 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
832 can achieve submicrosecond clock accuracy on a local area network
833 with special-purpose hardware.</li>
834 <li><a
835 href="https://tools.ietf.org/html/rfc4833">Timezone
836 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
837 (Internet <abbr>RFC</abbr> 4833)
838 specifies a <a
839 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
840 option for a server to configure
841 a client's time zone and daylight saving settings automatically.</li>
842 <li><a
843 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
844 Times</a> explains more abstruse astronomical time scales like
845 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
846 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
847 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
848 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
849 Scales</a> goes into more detail, particularly for historical variants.</li>
850 <li>The <a href="https://www.iau.org"><abbr
851 title="International Astronomical Union">IAU</abbr></a>'s <a
852 href="http://www.iausofa.org"><abbr
853 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
854 collection contains C and <a
855 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
856 code for converting among time scales like
857 <abbr title="International Atomic Time">TAI</abbr>,
858 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
859 <abbr>UTC</abbr>.</li>
860 <li><a
861 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
862 – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
863 diverse local time
864 scales used by each landed mission on Mars.</li>
865 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
866 dedicated not only to leap seconds but to precise time and frequency
867 in general. It covers the state of the art in amateur timekeeping, and
868 how the art has progressed over the past few decades.</li>
869 <li><a
870 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
871 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
872 Bulletins</a> contains official publications of the International
873 Earth Rotation and Reference Systems Service, which decides when leap
874 seconds occur. The <code>tz</code> code and data support leap seconds
875 via an optional "<code>right</code>" configuration, as opposed to the
876 default "<code>posix</code>" configuration.</li>
877 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
878 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
879 leap second so that they disagree with <abbr>UTC</abbr> by at most a
880 half second, even though every <abbr>POSIX</abbr> minute has exactly
881 sixty seconds. This approach works with the default <code>tz</code>
882 "<code>posix</code>" configuration, is <a
883 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
884 the <abbr>NTP</abbr> reference implementation, and is used by major
885 cloud service providers.</li>
886 <li>The <a
887 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
888 Second Discussion List</a> covers <a
889 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
890 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
891 discussed further in
892 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
893 leap second: its history and possible future</a>.
894 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
895 might be redefined
896 without Leap Seconds</a> gives pointers on this
897 contentious issue, which was active until 2015 and could become active
898 again.</li>
899 </ul>
900 <h2 id="notation">Time notation</h2>
901 <ul>
902 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
903 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
904 zone names, abbreviations, identifiers, and formats. For example, it
905 contains French translations for "Eastern European Summer Time",
906 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
907 "Bucharest". Its
908 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
909 charts</a> show these values for many locales. Data values are available in
910 both <abbr title="Locale Data Markup Language">LDML</abbr>
911 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
912 <li>
913 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
914 the international standard date and time notation</a> is a good
915 summary of
916 <a
917 href="https://www.iso.org/standard/40874.html"><em><abbr>ISO</abbr>
918 8601:2004 – Data elements and interchange formats – Information
919 interchange – Representation of dates and times</em></a>.</li>
920 <li>
921 <a href="https://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
922 Schema: Datatypes – dateTime</a> specifies a format inspired by
923 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
924 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of
925 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
926 specifies the time notation used in email and <a
927 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
928 headers.</li>
929 <li>
930 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
931 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
932 specifies an <abbr>ISO</abbr> 8601
933 profile for use in new Internet
934 protocols.</li>
935 <li>
936 <a href="https://www.hackcraft.net/web/datetime/">Date & Time
937 Formats on the Web</a> surveys web- and Internet-oriented date and time
938 formats.</li>
939 <li>Alphabetic time zone abbreviations should not be used as unique
940 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
941 practice. For example, in English-speaking North America
942 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
943 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
944 and French-speaking North Americans prefer
945 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
946 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
947 database contains English abbreviations for many timestamps;
948 unfortunately some of these abbreviations were merely the database maintainers'
949 inventions, and these have been removed when possible.</li>
950 <li>Numeric time zone abbreviations typically count hours east of
951 <abbr>UT</abbr>, e.g., +09 for Japan and
952 −10 for Hawaii. However, the <abbr>POSIX</abbr>
953 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
954 For example, one might use <code><abbr>TZ</abbr>="<abbr
955 title="Japan Standard Time">JST</abbr>-9"</code> and
956 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
957 for Japan and Hawaii, respectively. If the
958 <code><abbr>tz</abbr></code> database is available, it is usually better to use
959 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
960 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
961 confusion, handle old timestamps better, and insulate you better from
962 any future changes to the rules. One should never set
963 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
964 <code>"GMT-9"</code>, though, since this would incorrectly imply that
965 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
966 is called "<abbr>GMT</abbr>".</li>
967 </ul>
968 <h2 id="see-also">See also</h2>
969 <ul>
970 <li><a href="theory.html">Theory and pragmatics of the tz code and data</a></li>
971 <li><a href="tz-art.html">Time and the Arts</a></li>
972 </ul>
973 <hr>
974 <address>
975 This web page is in the public domain, so clarified as of
976 2009-05-17 by Arthur David Olson.
977 <br>
978 Please send corrections to this web page to the
979 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
980 </address>
981 </body>
982 </html>
983