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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="#costs">Costs and benefits of time shifts</a></li>
53 <li><a href="#precision">Precision timekeeping</a></li>
54 <li><a href="#notation">Time notation</a></li>
55 <li><a href="#see-also">See also</a></li>
56 </ul>
57 </li>
58 </ul>
59 </nav>
60
61 <section>
62 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
63 <p>
64 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
65 time zone database contains code and data
66 that represent the history of local time
67 for many representative locations around the globe.
68 It is updated periodically to reflect changes made by political bodies
69 to time zone boundaries and daylight saving rules.
70 This database (known as <code><abbr>tz</abbr></code>,
71 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
72 is used by several implementations,
73 including
74 <a href="https://www.gnu.org/software/libc/">the
75 <abbr title="GNU's Not Unix">GNU</abbr>
76 C Library</a> (used in
77 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
78 <a href="https://www.android.com">Android</a>,
79 <a href="https://www.freebsd.org">Free<abbr
80 title="Berkeley Software Distribution">BSD</abbr></a>,
81 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
82 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
83 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
84 <a href="https://cygwin.com">Cygwin</a>,
85 <a href="https://mariadb.org">MariaDB</a>,
86 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
87 <a href="https://www.mysql.com">MySQL</a>,
88 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
89 title="Web Operating System">webOS</abbr></a>,
90 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
91 title="Advanced Interactive eXecutive">AIX</abbr></a>,
92 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
93 <a href="https://www.apple.com/ios"><abbr
94 title="iPhone OS">iOS</abbr></a>,
95 <a href="https://www.apple.com/macos">macOS</a>,
96 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
97 <a href="https://www.vmssoftware.com">Open<abbr
98 title="Virtual Memory System">VMS</abbr></a>,
99 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
100 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
101 <p>
102 Each main entry in the database represents a <dfn>timezone</dfn>
103 for a set of civil-time clocks that have all agreed since 1970.
104 Timezones are typically identified by continent or ocean and then by the
105 name of the largest city within the region containing the clocks.
106 For example, <code>America/New_York</code>
107 represents most of the <abbr title="United States">US</abbr> eastern time zone;
108 <code>America/Phoenix</code> represents most of Arizona, which
109 uses mountain time without daylight saving time (<abbr>DST</abbr>);
110 <code>America/Detroit</code> represents most of Michigan, which uses
111 eastern time but with different <abbr>DST</abbr> rules in 1975;
112 and other entries represent smaller regions like Starke County,
113 Indiana, which switched from central to eastern time in 1991
114 and switched back in 2006.
115 To use the database on an extended <a
116 href="https://en.wikipedia.org/wiki/POSIX"><abbr
117 title="Portable Operating System Interface">POSIX</abbr></a>
118 implementation set the <code><abbr>TZ</abbr></code>
119 environment variable to the location's full name,
120 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
121 <p>
122 Associated with each timezone is a history of offsets from
123 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
124 Time</a> (<abbr>UT</abbr>), which is <a
125 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
126 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
127 for timestamps after 1960 this is more precisely <a
128 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
129 Universal Time</a> (<abbr>UTC</abbr>).
130 The database also records when daylight saving time was in use,
131 along with some time zone abbreviations such as <abbr>EST</abbr>
132 for Eastern Standard Time in the <abbr>US</abbr>.</p>
133 </section>
134
135 <section>
136 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
137 <p>
138 The following <a
139 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
140 the latest release's two
141 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
142 to a <abbr>GNU</abbr>/Linux or similar host.</p>
143 <pre><code>mkdir tzdb
144 cd tzdb
145 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
146 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
147 <a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf -
148 gzip -dc tzdata-latest.tar.gz | tar -xf -
149 </code></pre>
150 <p>Alternatively, the following shell commands download the same
151 release in a single-tarball format containing extra data
152 useful for regression testing:</p>
153 <pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
154 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
155 </code></pre>
156 <p>These commands use convenience links to the latest release
157 of the <code><abbr>tz</abbr></code> database hosted by the
158 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
159 of the <a href="https://www.iana.org">Internet Assigned Numbers
160 Authority (IANA)</a>.
161 Older releases are in files named
162 <code>tzcode<var>V</var>.tar.gz</code>,
163 <code>tzdata<var>V</var>.tar.gz</code>, and
164 <code>tzdb-<var>V</var>.tar.lz</code>,
165 where <code><var>V</var></code> is the version.
166 Since 1996, each version has been a four-digit year followed by
167 lower-case letter (<samp>a</samp> through <samp>z</samp>,
168 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
169 through <samp>zzz</samp>, and so on).
170 Since version 2016h, each release has contained a text file named
171 "<samp>version</samp>" whose first (and currently only) line is the version.
172 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
173 and are also available in an
174 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
175 title="File Transfer Protocol">FTP</abbr> directory</a> via a
176 less-secure protocol.</p>
177 <p>Alternatively, a development repository of code and data can be
178 retrieved from <a href="https://github.com">GitHub</a> via the shell
179 command:</p>
180 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
181 </code></pre>
182 <p>
183 Since version 2012e, each release has been tagged in development repositories.
184 Untagged commits are less well tested and probably contain
185 more errors.</p>
186 <p>
187 After obtaining the code and data files, see the
188 <code>README</code> file for what to do next.
189 The code lets you compile the <code><abbr>tz</abbr></code> source files into
190 machine-readable binary files, one for each location. The binary files
191 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
192 specified by <a href="https://tools.ietf.org/html/8536">Internet
193 <abbr>RFC</abbr> 8536</a>.
194 The code also lets
195 you read a <abbr>TZif</abbr> file and interpret timestamps for that
196 location.</p>
197 </section>
198
199 <section>
200 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
201 <p>
202 The <code><abbr>tz</abbr></code> code and data
203 are by no means authoritative. If you find errors, please
204 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
205 the time zone mailing list. You can also <a
206 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
207 and browse the <a
208 href="https://mm.icann.org/pipermail/tz/">archive of old
209 messages</a>.
210 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
211 discussions</a> and corresponding data changes can be
212 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
213 </p>
214 <p>
215 If your government plans to change its time zone boundaries or
216 daylight saving rules, inform <code>tz (a] iana.org</code> well in
217 advance, as this will coordinate updates to many cell phones,
218 computers, and other devices around the world. With
219 less than a year's notice there is a good chance that some
220 computer-based clocks will operate incorrectly after the change, due
221 to delays in propagating updates to software and data. The shorter
222 the notice, the more likely clock problems will arise; see "<a
223 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
224 the Timing of Time Zone Changes</a>" for examples.
225 </p>
226 <p>
227 Changes to the <code><abbr>tz</abbr></code> code and data are often
228 propagated to clients via operating system updates, so
229 client <code><abbr>tz</abbr></code> data can often be corrected by
230 applying these updates. With GNU/Linux and similar systems, if your
231 maintenance provider has not yet adopted the
232 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
233 the process by tailoring the generic instructions in
234 the <code><abbr>tz</abbr> README</code> file and installing the latest
235 data yourself. System-specific instructions for installing the
236 latest <code><abbr>tz</abbr></code> data have also been published
237 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
238 <a
239 href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
240 <a
241 href="http://userguide.icu-project.org/datetime/timezone"><abbr
242 title="International Components for Unicode">ICU</abbr></a>,
243 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
244 and <a
245 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
246 Java, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
247 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
248 and <a
249 href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
250 </p>
251 <p>Sources for the <code><abbr>tz</abbr></code> database are
252 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
253 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
254 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
255 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
256 title="linefeed">LF</abbr></a>,
257 which can be modified by common text editors such
258 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
259 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
260 <a href="https://www.vim.org">vim</a>.
261 Specialized source-file editing can be done via the
262 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
263 zoneinfo</a> package for <a
264 href="https://www.sublimetext.com">Sublime Text</a> and the <a
265 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
266 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
267 Studio Code</a>.
268 </p>
269 <p>
270 For further information about updates, please see
271 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
272 Maintaining the Time Zone Database</a> (Internet <abbr
273 title="Request For Comments">RFC</abbr> 6557). More detail can be
274 found in <a href="theory.html">Theory and pragmatics of the
275 <code><abbr>tz</abbr></code> code and data</a>.
276 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
277 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
278 </p>
279 </section>
280
281 <section>
282 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
283 <ul>
284 <li>The article
285 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
286 an encyclopedic summary.</li>
287 <li><a href="tz-how-to.html">How to Read the
288 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
289 database format.</li>
290 <li><a
291 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
292 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
293 database's style.</li>
294 </ul>
295 </section>
296
297 <section>
298 <h2 id="web">Web sites using recent versions of the
299 <code><abbr>tz</abbr></code> database</h2>
300 <p>
301 These are listed roughly in ascending order of complexity and fanciness.
302 </p>
303 <ul>
304 <li><a href="https://time.is">Time.is</a> shows locations'
305 time and zones.</li>
306 <li><a href="https://www.timejones.com">TimeJones.com</a>,
307 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
308 <a href="https://www.worldclock.com">The World Clock</a>
309 are time zone converters.</li>
310 <li><a
311 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
312 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
313 <li><a
314 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
315 Time in 1000 Places</a> uses descriptions of the values.</li>
316 <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
317 uses a pulldown menu.</li>
318 <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
319 timezone information for all countries</a> displays tables of DST rules.
320 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
321 Worldwide</a> lets you sort zone names and convert times.</li>
322 <li><a href="https://24timezones.com">24TimeZones</a> has a world
323 time map and a time converter.</li>
324 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
325 calculates the current time difference between locations.</li>
326 <li><a href="http://www.wx-now.com">Weather Now</a> and
327 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
328 </ul>
329 </section>
330
331 <section>
332 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
333 <ul>
334 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
335 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
336 Distribution Service (tzdist) working group</a> defined <a
337 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
338 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
339 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
340 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
341 transferring time zone data by reference.
342 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
343 mailing list</a> discussed the Internet draft <a
344 id="TZDIST-Geolocate"
345 href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
346 Geolocate Extension</a> (now expired), which let a client determine its timezone
347 from its geographic location using a <a
348 href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li>
349 <li>The <a href="https://tools.ietf.org/html/rfc5545">
350 Internet Calendaring and Scheduling Core Object Specification
351 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
352 covers time zone
353 data; see its VTIMEZONE calendar component.
354 The iCalendar format requires specialized parsers and generators; a
355 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
356 (Internet <abbr>RFC</abbr> 6321) uses
357 <a href="https://www.w3.org/XML/"><abbr
358 title="Extensible Markup Language">XML</abbr></a> format, and a variant
359 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
360 (Internet <abbr>RFC</abbr> 7265)
361 uses <a href="https://www.json.org"><abbr
362 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
363 </ul>
364 </section>
365
366 <section>
367 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
368 <p>Although some of these do not fully support
369 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
370 distributions you can generally work around compatibility problems by
371 running the command <code>make rearguard_tarballs</code> and compiling
372 from the resulting tarballs instead.</p>
373 <ul>
374 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
375 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
376 program that compiles
377 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
378 Vzic is freely
379 available under the <a
380 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
381 General Public License (<abbr
382 title="General Public License">GPL</abbr>)</a>.</li>
383 <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
384 database conversion utility</a> is like Vzic, except for the <a
385 href="https://dotnet.microsoft.com">.NET framework</a>
386 and with a <abbr>BSD</abbr>-style license.</li>
387 <li><a
388 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
389 contains a script <code>parse_olson</code> that compiles
390 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
391 modules. It is part of the Perl <a
392 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
393 which is freely
394 available under both the <abbr>GPL</abbr> and the Perl Artistic
395 License. DateTime::TimeZone also contains a script
396 <code>tests_from_zdump</code> that generates test cases for each clock
397 transition in the <code><abbr>tz</abbr></code> database.</li>
398 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
399 Database Parser</a> is a
400 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
401 runtime library with API <a
402 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">adopted
403 into the draft standard</a> for
404 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
405 the next iteration of the C++ standard.
406 It is freely available under the
407 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
408 <li><a id="ICU" href="http://site.icu-project.org">International Components for
409 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
410 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
411 libraries for internationalization that
412 has a compiler from <code><abbr>tz</abbr></code> source
413 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
414 (mentioned <a href="#CLDR">below</a>)
415 into an <abbr>ICU</abbr>-specific format.
416 <abbr>ICU</abbr> is freely available under a
417 <abbr>BSD</abbr>-style license.</li>
418 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
419 the <a href="https://elixir-lang.org">Elixir</a> language downloads
420 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
421 title="Application Program Interface">API</abbr>s for use. It is
422 freely available under the <abbr>MIT</abbr> license.</li>
423 <li>Java-based compilers and libraries include:
424 <ul>
425 <li>The <a
426 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
427 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
428 Oracle Java.</li>
429 <li>The <a
430 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
431 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
432 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
433 which is freely available under a <abbr>BSD</abbr>-style license.</li>
434 <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
435 and time <abbr>API</abbr></a> contains a class
436 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
437 <code><abbr>tz</abbr></code> source into a binary format. It inspired
438 Java 8 <code>java.time</code>, which its users should migrate to once
439 they can assume Java 8 or later. It is available under the <a
440 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
441 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a>,
442 <a href="https://github.com/akashche/tzdbgen">tzdbgen</a>, and <a
443 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
444 are other alternatives to TZUpdater. IANA Updater's license is unclear;
445 the others are licensed under the <abbr>GPL</abbr>.</li>
446 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
447 time library for Android</a> and
448 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
449 time and interval library for Java</a> compile
450 <code><abbr>tz</abbr></code> source into a binary format.
451 Time4A is available under the Apache License and Time4J is
452 available under the <a
453 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
454 General Public License (<abbr title="Lesser General Public
455 License">LGPL</abbr>)</a>.</li>
456 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
457 Java-based libraries.</li>
458 </ul>
459 <li><a href="https://nodatime.org">Noda Time – Date and
460 time <abbr>API</abbr> for .NET</a>
461 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
462 It is freely available under the Apache License.</li>
463 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
464 compilers and libraries include:
465 <ul>
466 <li><a
467 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
468 compiles time zone data into a compact form designed for
469 JavaScript. It is freely available under a combination of
470 the <abbr>MIT</abbr> license and the Apache License.</li>
471 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
472 plugin for the <a href="https://momentjs.com">Moment.js</a> date
473 manipulation library. It is freely available under the <abbr>MIT</abbr>
474 license.</li>
475 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
476 <abbr>API</abbr> is upward compatible with standard JavaScript
477 Dates. It is freely available under the Apache License.</li>
478 </ul>
479 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
480 compiler from <code><abbr>tz</abbr></code> source into
481 <a href="https://julialang.org/">Julia</a>. It is freely available
482 under the <abbr>MIT</abbr> license.</li>
483 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
484 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
485 title="Free Pascal Compiler">FPC</abbr></a>
486 compiles from <code><abbr>tz</abbr></code> source into
487 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
488 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
489 and <a
490 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
491 It is freely available under a <abbr>BSD</abbr>-style license.</li>
492 <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
493 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
494 <a href="https://www.python.org">Python</a>.
495 It is freely available under a <abbr>BSD</abbr>-style license.</li>
496 <li><a href="https://tzinfo.github.io">TZInfo –
497 Ruby Timezone Library</a>
498 compiles <code><abbr>tz</abbr></code> source into
499 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
500 It is freely available under the <abbr>MIT</abbr> license.</li>
501 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
502 Library</a> is
503 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
504 library that compiles <code><abbr>tz</abbr></code> source into a time
505 zone repository whose format
506 is either proprietary or an <abbr>XML</abbr>-encoded
507 representation.</li>
508 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
509 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
510 source into text files, along with a runtime that can read those
511 files. Tcl is freely available under a <abbr>BSD</abbr>-style
512 license.</li>
513 </ul>
514 </section>
515
516 <section>
517 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
518 <ul>
519 <li>The <a
520 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
521 Library</a>
522 has an independent, thread-safe implementation of
523 a <abbr>TZif</abbr> file reader.
524 This library is freely available under the LGPL
525 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
526 <li><a href="https://www.gnome.org">GNOME</a>'s
527 <a href="https://developer.gnome.org/glib/">GLib</a> has
528 a <abbr>TZif</abbr> file reader written in C that
529 creates a <code>GTimeZone</code> object representing sets
530 of <abbr>UT</abbr> offsets.
531 It is freely available under the <abbr>LGPL</abbr>.</li>
532 <li>The
533 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
534 <code>baltzo::TimeZoneUtil</code> component contains a C++
535 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
536 the Apache License.</li>
537 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
538 library that translates between <abbr>UT</abbr> and civil time and
539 can read <abbr>TZif</abbr> files. It is freely available under the Apache
540 License.</li>
541 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
542 is a <abbr>TZif</abbr> file reader written in Java.
543 It is freely available under the <abbr>LGPL</abbr>.</li>
544 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
545 library that reads <abbr>TZif</abbr> files and converts
546 timestamps from one time zone or format to another.
547 It is used by <a href="https://secure.php.net"><abbr
548 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
549 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
550 and <a href="https://www.mongodb.com">MongoDB</a>.
551 It is freely available under the <abbr>MIT</abbr> license.</li>
552 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
553 JavaScript library that supports date arithmetic that is time zone
554 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
555 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
556 <abbr>TZif</abbr> file reader.</li>
557 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
558 DateTime::TimeZone::Tzfile</a>
559 is a <abbr>TZif</abbr> file reader written in Perl.
560 It is freely available under the same terms as Perl
561 (dual <abbr>GPL</abbr> and Artistic license).</li>
562 <li>The
563 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
564 library contains a Python tool that
565 converts <abbr>TZif</abbr> data into
566 <abbr>JSON</abbr>-format data suitable for use
567 in its JavaScript library for time zone conversion. Dates before 1970
568 are not supported.</li>
569 <li>The <a
570 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
571 package contains <a href="https://www.haskell.org">Haskell</a> code that
572 parses and uses <abbr>TZif</abbr> data. It is freely
573 available under a <abbr>BSD</abbr>-style license.</li>
574 </ul>
575 </section>
576
577 <section>
578 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
579 <ul>
580 <li><a href="https://foxclocks.org">FoxClocks</a>
581 is an extension for <a href="https://www.google.com/chrome/">Google
582 Chrome</a> and for <a
583 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
584 Toolkit</a> applications like <a
585 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
586 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
587 It displays multiple clocks in the application window, and has a mapping
588 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
589 It is freely available under the <abbr>GPL</abbr>.</li>
590 <li><a href="https://golang.org">Go programming language</a>
591 implementations contain a copy of a 32-bit subset of a recent
592 <code><abbr>tz</abbr></code> database in a
593 Go-specific format.</li>
594 <li><a
595 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
596 clock (intclock)</a> is a clock that displays multiple time zones on
597 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
598 under the <abbr>GPL</abbr>.</li>
599 <li>Microsoft Windows 8.1
600 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
601 data (mentioned <a href="#CLDR">below</a>) used by the
602 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
603 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
604 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
605 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
606 <a id="System.TimeZoneInfo"
607 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
608 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
609 the older, proprietary method of Microsoft Windows 2000 and later,
610 which stores time zone data in the
611 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
612 <a
613 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
614 Tzid table</a> or <a
615 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
616 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
617 to <code><abbr>tz</abbr></code> names.
618 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
619 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
620 <li><a
621 href="https://www.oracle.com/java/index.html">Oracle
622 Java</a> contains a copy of a subset of a recent
623 <code><abbr>tz</abbr></code> database in a
624 Java-specific format.</li>
625 <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
626 Master</a> is a Microsoft Windows clock program that can automatically
627 download, compile and use <code><abbr>tz</abbr></code> releases.
628 The Basic version is free.</li>
629 <li><a
630 href="http://veladg.com/velaterra.html">VelaTerra</a> is
631 a macOS program. Its developers
632 <a href="http://veladg.com/tzoffer.html">offer free
633 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
634 </ul>
635 </section>
636
637 <section>
638 <h2 id="other-dbs">Other time zone databases</h2>
639 <ul>
640 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
641 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
642 time zone history atlases
643 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
644 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>,
645 now published in <a
646 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
647 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
648 Although these extensive atlases
649 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
650 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
651 they are unreliable as Shanks appears to have
652 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
653 sources and do not indicate which entries are guesswork.</li>
654 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
655 its own <code>tztab</code>(4) format.</li>
656 <li>Microsoft Windows has proprietary data mentioned
657 <a href="#System.TimeZoneInfo">above</a>.</li>
658 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
659 is another time zone database.</li>
660 <li>The <a
661 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
662 Schedules Information Manual</a> of the
663 International Air Transport Association
664 gives current time zone rules for airports served by commercial aviation.</li>
665 </ul>
666 </section>
667
668 <section>
669 <h2 id="maps">Maps</h2>
670 <ul>
671 <li>The <a href="https://www.cia.gov/index.html">United States Central
672 Intelligence Agency (<abbr
673 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
674 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
675 zone map</a>; the
676 <a
677 href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
678 Library Map Collection</a>
679 of the University of Texas at Austin has copies of
680 recent editions.
681 The pictorial quality is good,
682 but the maps do not indicate daylight saving time,
683 and parts of the data are a few years out of date.</li>
684 <li><a href="https://www.worldtimezone.com">World Time Zone Map
685 with current time</a>
686 has several fancy time zone maps; it covers Russia particularly well.
687 The maps' pictorial quality is not quite as good as the
688 <abbr>CIA</abbr>'s
689 but the maps are more up to date.</li>
690 <li><a
691 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
692 much is time wrong around the world?</a> maps the difference between
693 mean solar and standard time, highlighting areas such as western China
694 where the two differ greatly. It's a bit out of date, unfortunately.</li>
695 </ul>
696 </section>
697
698 <section>
699 <h2 id="boundaries">Time zone boundaries</h2>
700 <p>Geographical boundaries between timezones are available
701 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
702 services and other sources.</p>
703 <ul>
704 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
705 Boundary Builder</a> extracts
706 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
707 boundaries of <code><abbr>tzdb</abbr></code> timezones.
708 Its code is freely available under the <abbr>MIT</abbr> license, and
709 its data entries are freely available under the
710 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
711 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
712 <li>Programmatic interfaces that map geographical coordinates via tz_world to
713 <code><abbr>tzdb</abbr></code> timezones include:
714 <ul>
715 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
716 written in <a
717 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
718 and is freely available under the <abbr>MIT</abbr> license.</li>
719 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
720 is written in Go and is freely available under the Apache License.</li>
721 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
722 in both Java and
723 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
724 form, is freely available under the MIT license.</li>
725 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
726 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
727 is freely available under the MIT license, and
728 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
729 is in the public domain.</li>
730 <li>The <a
731 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
732 library for Python is freely available under the MIT license.
733 <li>The <a
734 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
735 library for Ruby is freely available under the MIT license.</li>
736 </ul></li>
737 <li>Free access via a network API, if you register a key, is provided by
738 the <a
739 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
740 Timezone web service</a>, the <a
741 href="https://developers.google.com/maps/documentation/timezone/intro">Google
742 Maps Time Zone API</a>, and
743 the <a href="https://timezonedb.com">Time Zone Database & API</a>.
744 Commercial network API access is provided
745 by <a href="https://askgeo.com">AskGeo</a>
746 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
747 </li>
748 <li>"<a
749 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
750 to get a time zone from a location using latitude and longitude
751 coordinates?</a>" discusses other geolocation possibilities.</li>
752 <li><a href="http://statoids.com/statoids.html">Administrative
753 Divisions of Countries ("Statoids")</a> lists
754 political subdivision data related to time zones.</li>
755 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
756 zone boundaries for multizone countries</a> summarizes legal
757 boundaries between time zones within countries.</li>
758 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
759 – GIS and Database Tools</a> includes a Manifold-format map of
760 world time zone boundaries distributed under the
761 <abbr>GPL</abbr>.</li>
762 <li>A ship within the <a
763 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
764 waters</a> of any nation uses that nation's time. In international
765 waters, time zone boundaries are meridians 15° apart, except that
766 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
767 wide and are separated by
768 the 180° meridian (not by the International Date Line, which is
769 for land and territorial waters only). A captain can change ship's
770 clocks any time after entering a new time zone; midnight changes are
771 common.</li>
772 </ul>
773 </section>
774
775 <section>
776 <h2 id="civil">Civil time concepts and history</h2>
777 <ul>
778 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
779 Walk through Time</a>
780 surveys the evolution of timekeeping.</li>
781 <li>The history of daylight saving time is surveyed in <a
782 href="http://www.webexhibits.org/daylightsaving/">About Daylight
783 Saving Time – History, rationale, laws & dates</a> and summarized in
784 <a href="http://seizethedaylight.com/dst/">A Brief
785 History of Daylight Saving Time</a>.</li>
786 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
787 Lords</a> discusses how authoritarians manipulate civil time.</li>
788 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
789 contains guidelines and best practices for software applications that
790 deal with civil time.</li>
791 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
792 the International Date Line</a> tells the story of the most important
793 time zone boundary.</li>
794 <li><a href="http://statoids.com/tconcept.html">Basic Time
795 Zone Concepts</a> discusses terminological issues behind time zones.</li>
796 </ul>
797 </section>
798
799 <section>
800 <h2 id="national">National histories of legal time</h2>
801 <dl>
802 <dt>Australia</dt>
803 <dd>The Parliamentary Library commissioned a <a
804 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
805 paper on daylight saving time in Australia</a>.
806 The Bureau of Meteorology publishes a list of <a
807 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
808 Dates of Daylight Savings Time within Australia</a>.</dd>
809 <dt>Belgium</dt>
810 <dd>The Royal Observatory of Belgium maintains a table of time in
811 Belgium (in
812 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
813 hreflang="nl">Dutch</a> and <a
814 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
815 hreflang="fr">French</a>).</dd>
816 <dt>Brazil</dt>
817 <dd>The Time Service Department of the National Observatory
818 records <a href="http://pcdsh01.on.br/DecHV.html"
819 hreflang="pt-BR">Brazil's daylight saving time decrees (in
820 Portuguese)</a>.</dd>
821 <dt>Canada</dt>
822 <dd>National Research Council Canada publishes current
823 and some older information about <a
824 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
825 zones and daylight saving time</a>.</dd>
826 <dt>Chile</dt>
827 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
828 <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
829 Chile's official time (in Spanish)</a>.</dd>
830 <dt>China</dt>
831 <dd>The Hong Kong Observatory maintains a
832 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
833 summer time in Hong Kong</a>,
834 and Macau's Meteorological and Geophysical Bureau maintains a <a
835 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
836 history for Macau</a>.
837 Unfortunately the latter is incomplete and has errors.</dd>
838 <dt>Czech Republic</dt>
839 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
840 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
841 summarizes and cites historical DST regulations.</dd>
842 <dt>Germany</dt>
843 <dd>The National Institute for Science and Technology maintains the <a
844 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
845 of Legal Time in Germany</a>.</dd>
846 <dt>Israel</dt>
847 <dd>The Interior Ministry periodically issues <a
848 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
849 hreflang="he">announcements (in Hebrew)</a>.</dd>
850 <dt>Italy</dt>
851 <dd>The National Institute of Metrological Research publishes a
852 <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
853 of civil time (in Italian)</a>.</dd>
854 <dt>Malaysia</dt>
855 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
856 <dt>Mexico</dt>
857 <dd>The Investigation and Analysis Service of the Mexican Library of
858 Congress has published a <a
859 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
860 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
861 <dt>Netherlands</dt>
862 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
863 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
864 covers the history of local time in the Netherlands from ancient times.</dd>
865 <dt>New Zealand</dt>
866 <dd>The Department of Internal Affairs maintains a brief <a
867 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
868 Daylight Saving</a>.</dd>
869 <dt>Singapore</dt>
870 <dd><a id="Singapore"
871 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
872 is Singapore in the "Wrong" Time Zone?</a> details the
873 history of legal time in Singapore and Malaysia.</dd>
874 <dt>United Kingdom</dt>
875 <dd><a
876 href="https://www.polyomino.org.uk/british-time/">History of
877 legal time in Britain</a> discusses in detail the country
878 with perhaps the best-documented history of clock adjustments.</dd>
879 <dt>United States</dt>
880 <dd>The Department of Transportation's <a
881 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
882 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
883 <dt>Uruguay</dt>
884 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
885 Navy (SOHMA) publishes an annual <a
886 href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
887 (in Spanish)</a>.</dd>
888 </dl>
889 </section>
890
891 <section>
892 <h2 id="costs">Costs and benefits of time shifts</h2>
893 <p>Various sources argue for and against daylight saving time and time
894 zone shifts, and many scientific studies have been conducted. This
895 section summarizes reviews of scientific literature in the area.</p>
896 <ul>
897 <li>Carey RN, Sarma KM.
898 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
899 daylight saving time on road traffic collision risk: a systematic
900 review</a>.
901 <em>BMJ Open.</em> 2017;7(6):e014319. doi:<a href="https://doi.org/10.1136/bmjopen-2016-014319">10.1136/bmjopen-2016-014319</a>.
902 This reviews research literature and concludes that the evidence
903 neither supports nor refutes road safety benefits from
904 shifts in time zones.</li>
905 <li>Havranek T, Herman D, Irsova D.
906 <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
907 saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
908 2018;39(2).
909 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
910 This analyzes research literature and concludes, "Electricity savings
911 are larger for countries farther away from the equator, while
912 subtropical regions consume more electricity because of DST."</li>
913 <li>Roenneberg T, Winnebeck EC, Klerman EB.
914 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
915 saving time and artificial time zones – a battle between
916 biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
917 doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
918 This reviews evidence about the health effects of DST and concludes,
919 "In summary, the scientific literature strongly argues against the
920 switching between DST and Standard Time and even more so against
921 adopting DST permanently."</li>
922 </ul>
923 </section>
924
925 <section>
926 <h2 id="precision">Precision timekeeping</h2>
927 <ul>
928 <li><a
929 href="http://leapsecond.com/hpan/an1289.pdf">The
930 Science of Timekeeping</a> is a thorough introduction
931 to the theory and practice of precision timekeeping.</li>
932 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
933 Time 2016</a> contains several freely-readable papers.</li>
934 <li><a href="http://www.ntp.org"><abbr
935 title="Network Time Protocol">NTP</abbr>: The Network
936 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
937 discusses how to synchronize clocks of
938 Internet hosts.</li>
939 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
940 family of software algorithms can achieve accuracy to a few tens of
941 nanoseconds in scalable server farms without special hardware.</li>
942 <li>The <a
943 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
944 Time Protocol</a> (<abbr
945 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
946 can achieve submicrosecond clock accuracy on a local area network
947 with special-purpose hardware.</li>
948 <li><a
949 href="https://tools.ietf.org/html/rfc4833">Timezone
950 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
951 (Internet <abbr>RFC</abbr> 4833)
952 specifies a <a
953 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
954 option for a server to configure
955 a client's time zone and daylight saving settings automatically.</li>
956 <li><a
957 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
958 Times</a> explains more abstruse astronomical time scales like
959 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
960 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
961 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
962 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
963 Scales</a> goes into more detail, particularly for historical variants.</li>
964 <li>The <a href="https://www.iau.org"><abbr
965 title="International Astronomical Union">IAU</abbr></a>'s <a
966 href="http://www.iausofa.org"><abbr
967 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
968 collection contains C and <a
969 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
970 code for converting among time scales like
971 <abbr title="International Atomic Time">TAI</abbr>,
972 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
973 <abbr>UTC</abbr>.</li>
974 <li><a
975 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
976 – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
977 diverse local time
978 scales used by each landed mission on Mars.</li>
979 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
980 dedicated not only to leap seconds but to precise time and frequency
981 in general. It covers the state of the art in amateur timekeeping, and
982 how the art has progressed over the past few decades.</li>
983 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
984 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
985 and time-signal emissions</a>, International Telecommunication Union –
986 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
987 <li><a
988 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
989 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
990 Bulletins</a> contains official publications of the International
991 Earth Rotation and Reference Systems Service, which decides when leap
992 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
993 via an optional "<code>right</code>" configuration, as opposed to the
994 default "<code>posix</code>" configuration.</li>
995 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
996 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
997 leap second so that they disagree with <abbr>UTC</abbr> by at most a
998 half second, even though every <abbr>POSIX</abbr> minute has exactly
999 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1000 "<code>posix</code>" configuration, is <a
1001 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1002 the <abbr>NTP</abbr> reference implementation, and is used by major
1003 cloud service providers. However, according to
1004 <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of
1005 Network Time Protocol Best Current Practices</a>
1006 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1007 applications requiring accurate UTC or civil time,
1008 and is intended for use only in single, well-controlled environments.</li>
1009 <li>The <a
1010 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1011 Second Discussion List</a> covers <a
1012 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1013 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1014 discussed further in
1015 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1016 leap second: its history and possible future</a>.
1017 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1018 might be redefined
1019 without Leap Seconds</a> gives pointers on this
1020 contentious issue, which was active until 2015 and could become active
1021 again.</li>
1022 </ul>
1023 </section>
1024
1025 <section>
1026 <h2 id="notation">Time notation</h2>
1027 <ul>
1028 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
1029 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1030 zone names, abbreviations, identifiers, and formats. For example, it
1031 contains French translations for "Eastern European Summer Time",
1032 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1033 "Bucharest". Its
1034 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1035 charts</a> show these values for many locales. Data values are available in
1036 both <abbr title="Locale Data Markup Language">LDML</abbr>
1037 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1038 <li>
1039 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1040 the international standard date and time notation</a> covers
1041 <a
1042 href="https://www.iso.org/standard/70907.html"><em><abbr
1043 title="International Organization for Standardization">ISO</abbr>
1044 8601-1:2019 – Date and time – Representations for information
1045 interchange – Part 1: Basic rules</em></a>.</li>
1046 <li>
1047 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1048 Schema: Datatypes – dateTime</a> specifies a format inspired by
1049 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1050 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of
1051 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1052 specifies the time notation used in email and <a
1053 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1054 headers.</li>
1055 <li>
1056 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
1057 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1058 specifies an <abbr>ISO</abbr> 8601
1059 profile for use in new Internet
1060 protocols.</li>
1061 <li>
1062 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1063 Formats on the Web</a> surveys web- and Internet-oriented date and time
1064 formats.</li>
1065 <li>Alphabetic time zone abbreviations should not be used as unique
1066 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1067 practice. For example, in English-speaking North America
1068 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1069 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1070 and French-speaking North Americans prefer
1071 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1072 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1073 database contains English abbreviations for many timestamps;
1074 unfortunately some of these abbreviations were merely the database maintainers'
1075 inventions, and these have been removed when possible.</li>
1076 <li>Numeric time zone abbreviations typically count hours east of
1077 <abbr>UT</abbr>, e.g., +09 for Japan and
1078 −10 for Hawaii. However, the <abbr>POSIX</abbr>
1079 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1080 For example, one might use <code><abbr>TZ</abbr>="<abbr
1081 title="Japan Standard Time">JST</abbr>-9"</code> and
1082 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1083 for Japan and Hawaii, respectively. If the
1084 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1085 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1086 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1087 confusion, handle old timestamps better, and insulate you better from
1088 any future changes to the rules. One should never set
1089 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1090 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1091 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1092 is called "<abbr>GMT</abbr>".</li>
1093 </ul>
1094 </section>
1095
1096 <section>
1097 <h2 id="see-also">See also</h2>
1098 <ul>
1099 <li><a href="theory.html">Theory and pragmatics of the
1100 <code><abbr>tz</abbr></code> code and data</a></li>
1101 <li><a href="tz-art.html">Time and the Arts</a></li>
1102 </ul>
1103 </section>
1104
1105 <footer>
1106 <hr>
1107 This web page is in the public domain, so clarified as of
1108 2009-05-17 by Arthur David Olson.
1109 <br>
1110 Please send corrections to this web page to the
1111 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1112 </footer>
1113 </body>
1114 </html>
1115