tz-link.html revision 1.16 1 1.1 christos <!DOCTYPE html>
2 1.1 christos <html lang="en">
3 1.1 christos <head>
4 1.13 christos <title>Time zone and daylight saving time data</title>
5 1.1 christos <meta charset="UTF-8">
6 1.2 christos <style>
7 1.2 christos pre {margin-left: 2em; white-space: pre-wrap;}
8 1.2 christos </style>
9 1.1 christos </head>
10 1.1 christos <body>
11 1.13 christos <h1>Time zone and daylight saving time data</h1>
12 1.1 christos <p>
13 1.1 christos <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
14 1.1 christos <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
15 1.1 christos rules are controlled by individual
16 1.1 christos governments. They are sometimes changed with little notice, and their
17 1.1 christos histories and planned futures are often recorded only fitfully. Here
18 1.1 christos is a summary of attempts to organize and record relevant data in this
19 1.1 christos area.
20 1.1 christos </p>
21 1.6 christos <h3>Outline</h3>
22 1.6 christos <nav>
23 1.6 christos <ul>
24 1.6 christos <li>The <code><abbr>tz</abbr></code> database product and process
25 1.6 christos <ul>
26 1.6 christos <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
27 1.6 christos <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
28 1.6 christos <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
29 1.14 christos <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
30 1.6 christos <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
31 1.6 christos </ul>
32 1.6 christos </li>
33 1.6 christos <li>Uses of the <code><abbr>tz</abbr></code> database
34 1.6 christos <ul>
35 1.6 christos <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
36 1.6 christos <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
37 1.6 christos <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
38 1.6 christos <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
39 1.6 christos <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
40 1.6 christos </ul>
41 1.6 christos </li>
42 1.6 christos <li>Related data
43 1.6 christos <ul>
44 1.6 christos <li><a href="#other-dbs">Other time zone databases</a></li>
45 1.6 christos <li><a href="#maps">Maps</a></li>
46 1.6 christos <li><a href="#boundaries">Time zone boundaries</a></li>
47 1.6 christos </ul>
48 1.6 christos </li>
49 1.6 christos <li>Timekeeping concepts
50 1.6 christos <ul>
51 1.6 christos <li><a href="#civil">Civil time concepts and history</a></li>
52 1.6 christos <li><a href="#national">National histories of legal time</a></li>
53 1.7 christos <li><a href="#costs">Costs and benefits of time shifts</a></li>
54 1.6 christos <li><a href="#precision">Precision timekeeping</a></li>
55 1.6 christos <li><a href="#notation">Time notation</a></li>
56 1.6 christos </ul>
57 1.6 christos </li>
58 1.13 christos <li><a href="#see-also">See also</a></li>
59 1.6 christos </ul>
60 1.6 christos </nav>
61 1.6 christos
62 1.6 christos <section>
63 1.1 christos <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
64 1.1 christos <p>
65 1.1 christos The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
66 1.1 christos time zone database contains code and data
67 1.1 christos that represent the history of local time
68 1.1 christos for many representative locations around the globe.
69 1.1 christos It is updated periodically to reflect changes made by political bodies
70 1.1 christos to time zone boundaries and daylight saving rules.
71 1.1 christos This database (known as <code><abbr>tz</abbr></code>,
72 1.1 christos <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
73 1.1 christos is used by several implementations,
74 1.1 christos including
75 1.1 christos <a href="https://www.gnu.org/software/libc/">the
76 1.1 christos <abbr title="GNU's Not Unix">GNU</abbr>
77 1.1 christos C Library</a> (used in
78 1.1 christos <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
79 1.1 christos <a href="https://www.android.com">Android</a>,
80 1.1 christos <a href="https://www.freebsd.org">Free<abbr
81 1.1 christos title="Berkeley Software Distribution">BSD</abbr></a>,
82 1.1 christos <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
83 1.1 christos <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
84 1.13 christos <a href="https://www.chromium.org/chromium-os/">Chromium OS</a>,
85 1.1 christos <a href="https://cygwin.com">Cygwin</a>,
86 1.7 christos <a href="https://mariadb.org">MariaDB</a>,
87 1.1 christos <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
88 1.1 christos <a href="https://www.mysql.com">MySQL</a>,
89 1.1 christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
90 1.1 christos title="Web Operating System">webOS</abbr></a>,
91 1.7 christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
92 1.1 christos title="Advanced Interactive eXecutive">AIX</abbr></a>,
93 1.6 christos <a href="https://www.apple.com/ios"><abbr
94 1.1 christos title="iPhone OS">iOS</abbr></a>,
95 1.6 christos <a href="https://www.apple.com/macos">macOS</a>,
96 1.1 christos <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
97 1.6 christos <a href="https://www.vmssoftware.com">Open<abbr
98 1.1 christos title="Virtual Memory System">VMS</abbr></a>,
99 1.6 christos <a href="https://www.oracle.com/database/">Oracle Database</a>, and
100 1.1 christos <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
101 1.1 christos <p>
102 1.2 christos Each main entry in the database represents a <dfn>timezone</dfn>
103 1.2 christos for a set of civil-time clocks that have all agreed since 1970.
104 1.2 christos Timezones are typically identified by continent or ocean and then by the
105 1.2 christos name of the largest city within the region containing the clocks.
106 1.1 christos For example, <code>America/New_York</code>
107 1.1 christos represents most of the <abbr title="United States">US</abbr> eastern time zone;
108 1.1 christos <code>America/Phoenix</code> represents most of Arizona, which
109 1.2 christos uses mountain time without daylight saving time (<abbr>DST</abbr>);
110 1.1 christos <code>America/Detroit</code> represents most of Michigan, which uses
111 1.1 christos eastern time but with different <abbr>DST</abbr> rules in 1975;
112 1.1 christos and other entries represent smaller regions like Starke County,
113 1.1 christos Indiana, which switched from central to eastern time in 1991
114 1.1 christos and switched back in 2006.
115 1.1 christos To use the database on an extended <a
116 1.1 christos href="https://en.wikipedia.org/wiki/POSIX"><abbr
117 1.16 christos title="Portable Operating System Interface">POSIX</abbr>.1-2017</a>
118 1.1 christos implementation set the <code><abbr>TZ</abbr></code>
119 1.1 christos environment variable to the location's full name,
120 1.1 christos e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
121 1.1 christos <p>
122 1.2 christos Associated with each timezone is a history of offsets from
123 1.1 christos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
124 1.1 christos Time</a> (<abbr>UT</abbr>), which is <a
125 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
126 1.1 christos Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
127 1.2 christos for timestamps after 1960 this is more precisely <a
128 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
129 1.1 christos Universal Time</a> (<abbr>UTC</abbr>).
130 1.1 christos The database also records when daylight saving time was in use,
131 1.1 christos along with some time zone abbreviations such as <abbr>EST</abbr>
132 1.1 christos for Eastern Standard Time in the <abbr>US</abbr>.</p>
133 1.6 christos </section>
134 1.6 christos
135 1.6 christos <section>
136 1.1 christos <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
137 1.1 christos <p>
138 1.1 christos The following <a
139 1.1 christos href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
140 1.1 christos the latest release's two
141 1.1 christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
142 1.1 christos to a <abbr>GNU</abbr>/Linux or similar host.</p>
143 1.2 christos <pre><code>mkdir tzdb
144 1.1 christos cd tzdb
145 1.1 christos <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
146 1.1 christos wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
147 1.1 christos <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 1.1 christos gzip -dc tzdata-latest.tar.gz | tar -xf -
149 1.1 christos </code></pre>
150 1.1 christos <p>Alternatively, the following shell commands download the same
151 1.1 christos release in a single-tarball format containing extra data
152 1.1 christos useful for regression testing:</p>
153 1.2 christos <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 1.1 christos <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
155 1.1 christos </code></pre>
156 1.1 christos <p>These commands use convenience links to the latest release
157 1.1 christos of the <code><abbr>tz</abbr></code> database hosted by the
158 1.1 christos <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
159 1.1 christos of the <a href="https://www.iana.org">Internet Assigned Numbers
160 1.1 christos Authority (IANA)</a>.
161 1.1 christos Older releases are in files named
162 1.1 christos <code>tzcode<var>V</var>.tar.gz</code>,
163 1.1 christos <code>tzdata<var>V</var>.tar.gz</code>, and
164 1.1 christos <code>tzdb-<var>V</var>.tar.lz</code>,
165 1.1 christos where <code><var>V</var></code> is the version.
166 1.1 christos Since 1996, each version has been a four-digit year followed by
167 1.1 christos lower-case letter (<samp>a</samp> through <samp>z</samp>,
168 1.1 christos then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
169 1.1 christos through <samp>zzz</samp>, and so on).
170 1.13 christos Since version 2022a, each release has been distributed in
171 1.10 christos <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
172 1.10 christos ustar interchange format</a>, compressed as described above;
173 1.14 christos older releases use a nearly compatible format.
174 1.1 christos Since version 2016h, each release has contained a text file named
175 1.16 christos "<code>version</code>" whose first (and currently only) line is the version.
176 1.7 christos Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
177 1.7 christos and are also available in an
178 1.1 christos <a href="ftp://ftp.iana.org/tz/releases/"><abbr
179 1.1 christos title="File Transfer Protocol">FTP</abbr> directory</a> via a
180 1.14 christos less secure protocol.</p>
181 1.1 christos <p>Alternatively, a development repository of code and data can be
182 1.1 christos retrieved from <a href="https://github.com">GitHub</a> via the shell
183 1.1 christos command:</p>
184 1.2 christos <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
185 1.1 christos </code></pre>
186 1.1 christos <p>
187 1.1 christos Since version 2012e, each release has been tagged in development repositories.
188 1.1 christos Untagged commits are less well tested and probably contain
189 1.1 christos more errors.</p>
190 1.1 christos <p>
191 1.1 christos After obtaining the code and data files, see the
192 1.1 christos <code>README</code> file for what to do next.
193 1.1 christos The code lets you compile the <code><abbr>tz</abbr></code> source files into
194 1.2 christos machine-readable binary files, one for each location. The binary files
195 1.5 christos are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
196 1.13 christos specified by <a href="https://datatracker.ietf.org/doc/html/8536">Internet
197 1.5 christos <abbr>RFC</abbr> 8536</a>.
198 1.2 christos The code also lets
199 1.2 christos you read a <abbr>TZif</abbr> file and interpret timestamps for that
200 1.1 christos location.</p>
201 1.6 christos </section>
202 1.6 christos
203 1.6 christos <section>
204 1.1 christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
205 1.1 christos <p>
206 1.1 christos The <code><abbr>tz</abbr></code> code and data
207 1.1 christos are by no means authoritative. If you find errors, please
208 1.1 christos send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
209 1.1 christos the time zone mailing list. You can also <a
210 1.1 christos href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
211 1.1 christos and browse the <a
212 1.1 christos href="https://mm.icann.org/pipermail/tz/">archive of old
213 1.5 christos messages</a>.
214 1.5 christos <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
215 1.5 christos discussions</a> and corresponding data changes can be
216 1.5 christos generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
217 1.5 christos </p>
218 1.1 christos <p>
219 1.1 christos Changes to the <code><abbr>tz</abbr></code> code and data are often
220 1.1 christos propagated to clients via operating system updates, so
221 1.1 christos client <code><abbr>tz</abbr></code> data can often be corrected by
222 1.1 christos applying these updates. With GNU/Linux and similar systems, if your
223 1.1 christos maintenance provider has not yet adopted the
224 1.1 christos latest <code><abbr>tz</abbr></code> data, you can often short-circuit
225 1.1 christos the process by tailoring the generic instructions in
226 1.1 christos the <code><abbr>tz</abbr> README</code> file and installing the latest
227 1.1 christos data yourself. System-specific instructions for installing the
228 1.1 christos latest <code><abbr>tz</abbr></code> data have also been published
229 1.11 christos for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
230 1.1 christos <a
231 1.10 christos href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
232 1.1 christos <a
233 1.11 christos href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
234 1.1 christos title="International Components for Unicode">ICU</abbr></a>,
235 1.11 christos <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
236 1.9 christos JDK</a>,
237 1.9 christos <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
238 1.1 christos href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
239 1.9 christos <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
240 1.11 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
241 1.1 christos </p>
242 1.12 christos <p>Since version 2013a,
243 1.12 christos sources for the <code><abbr>tz</abbr></code> database have been
244 1.1 christos <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
245 1.1 christos title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
246 1.1 christos <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
247 1.1 christos with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
248 1.1 christos title="linefeed">LF</abbr></a>,
249 1.1 christos which can be modified by common text editors such
250 1.1 christos as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
251 1.1 christos <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
252 1.1 christos <a href="https://www.vim.org">vim</a>.
253 1.1 christos Specialized source-file editing can be done via the
254 1.1 christos <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
255 1.1 christos zoneinfo</a> package for <a
256 1.1 christos href="https://www.sublimetext.com">Sublime Text</a> and the <a
257 1.1 christos href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
258 1.1 christos zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
259 1.1 christos Studio Code</a>.
260 1.1 christos </p>
261 1.1 christos <p>
262 1.1 christos For further information about updates, please see
263 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc6557">Procedures for
264 1.1 christos Maintaining the Time Zone Database</a> (Internet <abbr
265 1.1 christos title="Request For Comments">RFC</abbr> 6557). More detail can be
266 1.7 christos found in <a href="theory.html">Theory and pragmatics of the
267 1.7 christos <code><abbr>tz</abbr></code> code and data</a>.
268 1.2 christos <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
269 1.2 christos displays changes between recent <code><abbr>tzdb</abbr></code> versions.
270 1.1 christos </p>
271 1.6 christos </section>
272 1.6 christos
273 1.6 christos <section>
274 1.14 christos <h2 id="coordinating">Coordinating with governments and distributors</h2>
275 1.14 christos <p>
276 1.14 christos As discussed in
277 1.14 christos "<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
278 1.14 christos Time Zones Are Coordinated</a>", the time zone database relies on
279 1.14 christos collaboration among governments, the time zone database volunteer
280 1.14 christos community, and data distributors downstream.
281 1.14 christos <p>
282 1.14 christos If your government plans to change its time zone boundaries or
283 1.14 christos daylight saving rules, please send email to <a
284 1.14 christos href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a> well in advance,
285 1.14 christos as this will lessen confusion and will coordinate updates to many cell phones,
286 1.14 christos computers, and other devices around the world.
287 1.14 christos In your email, please cite the legislation or regulation that specifies
288 1.14 christos the change, so that it can be checked for details such as the exact times
289 1.14 christos when clock transitions occur.
290 1.14 christos It is OK if a rule change is planned to affect clocks
291 1.14 christos far into the future, as a long-planned change can easily be reverted
292 1.14 christos or otherwise altered with a year's notice before the change would have
293 1.14 christos affected clocks.</p>
294 1.14 christos <p>
295 1.14 christos There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
296 1.14 christos However, typically a release occurs every few months.
297 1.14 christos Many downstream timezone data distributors wait for
298 1.14 christos a <code><abbr>tzdb</abbr></code> release before they produce an update
299 1.14 christos to time zone behavior in consumer devices and software products.
300 1.14 christos After a release, various parties must integrate, test,
301 1.14 christos and roll out an update before <a
302 1.14 christos href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
303 1.14 christos These updates can be expensive, for both the <a
304 1.14 christos href="https://en.wikipedia.org/wiki/Quality_assurance">quality
305 1.14 christos assurance</a> process and the overall cost of shipping and installing
306 1.14 christos updates to each device's copy of <code><abbr>tzdb</abbr></code>.
307 1.14 christos Updates may be batched with other updates and may take substantial
308 1.14 christos time to reach end users after a release.
309 1.14 christos Older devices may no longer be supported and thus may never be updated,
310 1.14 christos which means they will continue to use out-of-date rules.</p>
311 1.14 christos <p>
312 1.14 christos For these reasons any rule change should be promulgated at least a
313 1.14 christos year before it affects how clocks operate; otherwise, there is a good
314 1.14 christos chance that many clocks will be wrong due to delays in propagating updates,
315 1.14 christos and that residents will be confused or even actively resist the change.
316 1.14 christos The shorter the notice, the more likely clock problems will arise; see "<a
317 1.14 christos href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
318 1.14 christos the Timing of Time Zone Changes</a>" for examples.
319 1.14 christos </p>
320 1.14 christos </section>
321 1.14 christos
322 1.14 christos <section>
323 1.1 christos <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
324 1.1 christos <ul>
325 1.1 christos <li>The article
326 1.1 christos <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
327 1.1 christos an encyclopedic summary.</li>
328 1.1 christos <li><a href="tz-how-to.html">How to Read the
329 1.1 christos tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
330 1.1 christos database format.</li>
331 1.1 christos <li><a
332 1.1 christos href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
333 1.1 christos literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
334 1.1 christos database's style.</li>
335 1.10 christos <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
336 1.10 christos managing time in the internet</a> analyzes the database longitudinally.</li>
337 1.1 christos </ul>
338 1.6 christos </section>
339 1.6 christos
340 1.6 christos <section>
341 1.1 christos <h2 id="web">Web sites using recent versions of the
342 1.1 christos <code><abbr>tz</abbr></code> database</h2>
343 1.1 christos <p>
344 1.1 christos These are listed roughly in ascending order of complexity and fanciness.
345 1.1 christos </p>
346 1.1 christos <ul>
347 1.1 christos <li><a href="https://time.is">Time.is</a> shows locations'
348 1.1 christos time and zones.</li>
349 1.1 christos <li><a href="https://www.timejones.com">TimeJones.com</a>,
350 1.1 christos <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
351 1.7 christos <a href="https://www.worldclock.com">The World Clock</a>
352 1.1 christos are time zone converters.</li>
353 1.8 christos <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
354 1.8 christos publishes <code><abbr>tzdb</abbr></code>-derived data in
355 1.8 christos <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
356 1.8 christos title="comma-separated values">CSV</abbr></a> and
357 1.8 christos in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
358 1.8 christos title="Structured Query Language">SQL</abbr></a> form.</li>
359 1.1 christos <li><a
360 1.6 christos href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
361 1.1 christos lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
362 1.1 christos <li><a
363 1.13 christos href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
364 1.1 christos Time in 1000 Places</a> uses descriptions of the values.</li>
365 1.1 christos <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
366 1.1 christos Worldwide</a> lets you sort zone names and convert times.</li>
367 1.1 christos <li><a href="https://24timezones.com">24TimeZones</a> has a world
368 1.1 christos time map and a time converter.</li>
369 1.1 christos <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
370 1.1 christos calculates the current time difference between locations.</li>
371 1.13 christos <li><a href="https://www.wx-now.com">Weather Now</a> and
372 1.13 christos <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
373 1.1 christos </ul>
374 1.6 christos </section>
375 1.6 christos
376 1.6 christos <section>
377 1.1 christos <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
378 1.1 christos <ul>
379 1.1 christos <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
380 1.1 christos <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
381 1.1 christos Distribution Service (tzdist) working group</a> defined <a
382 1.13 christos href="https://datatracker.ietf.org/doc/html/rfc7808">TZDIST</a>
383 1.1 christos (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
384 1.13 christos along with <a href="https://datatracker.ietf.org/doc/html/rfc7809">CalDAV</a>
385 1.1 christos (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
386 1.2 christos transferring time zone data by reference.
387 1.8 christos <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
388 1.8 christos implementations</a> are available.
389 1.2 christos The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
390 1.9 christos mailing list</a> discusses possible extensions.</li>
391 1.13 christos <li>The <a href="https://datatracker.ietf.org/doc/html/rfc5545">
392 1.1 christos Internet Calendaring and Scheduling Core Object Specification
393 1.1 christos (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
394 1.1 christos covers time zone
395 1.1 christos data; see its VTIMEZONE calendar component.
396 1.1 christos The iCalendar format requires specialized parsers and generators; a
397 1.13 christos variant <a href="https://datatracker.ietf.org/doc/html/rfc6321">xCal</a>
398 1.1 christos (Internet <abbr>RFC</abbr> 6321) uses
399 1.1 christos <a href="https://www.w3.org/XML/"><abbr
400 1.1 christos title="Extensible Markup Language">XML</abbr></a> format, and a variant
401 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc7265">jCal</a>
402 1.1 christos (Internet <abbr>RFC</abbr> 7265)
403 1.1 christos uses <a href="https://www.json.org"><abbr
404 1.1 christos title="JavaScript Object Notation">JSON</abbr></a> format.</li>
405 1.1 christos </ul>
406 1.6 christos </section>
407 1.6 christos
408 1.6 christos <section>
409 1.1 christos <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
410 1.7 christos <p>Although some of these do not fully support
411 1.7 christos <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
412 1.7 christos distributions you can generally work around compatibility problems by
413 1.7 christos running the command <code>make rearguard_tarballs</code> and compiling
414 1.7 christos from the resulting tarballs instead.</p>
415 1.1 christos <ul>
416 1.1 christos <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
417 1.14 christos href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
418 1.1 christos program that compiles
419 1.1 christos <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
420 1.1 christos Vzic is freely
421 1.1 christos available under the <a
422 1.1 christos href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
423 1.1 christos General Public License (<abbr
424 1.1 christos title="General Public License">GPL</abbr>)</a>.</li>
425 1.1 christos <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
426 1.1 christos database conversion utility</a> is like Vzic, except for the <a
427 1.6 christos href="https://dotnet.microsoft.com">.NET framework</a>
428 1.1 christos and with a <abbr>BSD</abbr>-style license.</li>
429 1.1 christos <li><a
430 1.4 christos href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
431 1.1 christos contains a script <code>parse_olson</code> that compiles
432 1.1 christos <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
433 1.1 christos modules. It is part of the Perl <a
434 1.4 christos href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
435 1.4 christos which is freely
436 1.1 christos available under both the <abbr>GPL</abbr> and the Perl Artistic
437 1.1 christos License. DateTime::TimeZone also contains a script
438 1.1 christos <code>tests_from_zdump</code> that generates test cases for each clock
439 1.1 christos transition in the <code><abbr>tz</abbr></code> database.</li>
440 1.1 christos <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
441 1.1 christos Database Parser</a> is a
442 1.14 christos <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
443 1.9 christos runtime library with <a
444 1.13 christos href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
445 1.9 christos adopted by
446 1.6 christos <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
447 1.9 christos the current iteration of the C++ standard.
448 1.1 christos It is freely available under the
449 1.1 christos <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
450 1.13 christos <li><a id="ICU" href="https://icu.unicode.org">International Components for
451 1.1 christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
452 1.14 christos href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
453 1.1 christos libraries for internationalization that
454 1.1 christos has a compiler from <code><abbr>tz</abbr></code> source
455 1.1 christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
456 1.1 christos (mentioned <a href="#CLDR">below</a>)
457 1.1 christos into an <abbr>ICU</abbr>-specific format.
458 1.1 christos <abbr>ICU</abbr> is freely available under a
459 1.1 christos <abbr>BSD</abbr>-style license.</li>
460 1.1 christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
461 1.1 christos the <a href="https://elixir-lang.org">Elixir</a> language downloads
462 1.7 christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
463 1.1 christos title="Application Program Interface">API</abbr>s for use. It is
464 1.1 christos freely available under the <abbr>MIT</abbr> license.</li>
465 1.1 christos <li>Java-based compilers and libraries include:
466 1.1 christos <ul>
467 1.1 christos <li>The <a
468 1.9 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
469 1.1 christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
470 1.9 christos <a href="https://openjdk.java.net/">OpenJDK</a> and
471 1.9 christos <a href="https://jdk.java.net/">Oracle JDK</a>.
472 1.9 christos Although its source code is proprietary, its executable is available under the
473 1.9 christos <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
474 1.9 christos Timezone Updater License Agreement</a>.</li>
475 1.1 christos <li>The <a
476 1.4 christos href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
477 1.6 christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
478 1.4 christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
479 1.1 christos which is freely available under a <abbr>BSD</abbr>-style license.</li>
480 1.4 christos <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
481 1.1 christos and time <abbr>API</abbr></a> contains a class
482 1.1 christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
483 1.1 christos <code><abbr>tz</abbr></code> source into a binary format. It inspired
484 1.1 christos Java 8 <code>java.time</code>, which its users should migrate to once
485 1.1 christos they can assume Java 8 or later. It is available under the <a
486 1.1 christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
487 1.9 christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
488 1.7 christos href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
489 1.9 christos are alternatives to TZUpdater. IANA Updater's license is unclear;
490 1.9 christos ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
491 1.4 christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
492 1.4 christos time library for Android</a> and
493 1.4 christos <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
494 1.4 christos time and interval library for Java</a> compile
495 1.4 christos <code><abbr>tz</abbr></code> source into a binary format.
496 1.4 christos Time4A is available under the Apache License and Time4J is
497 1.1 christos available under the <a
498 1.1 christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
499 1.1 christos General Public License (<abbr title="Lesser General Public
500 1.1 christos License">LGPL</abbr>)</a>.</li>
501 1.1 christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
502 1.1 christos Java-based libraries.</li>
503 1.1 christos </ul>
504 1.1 christos <li><a href="https://nodatime.org">Noda Time – Date and
505 1.1 christos time <abbr>API</abbr> for .NET</a>
506 1.4 christos is like Joda-Time and Time4J, but for the .NET framework instead of Java.
507 1.4 christos It is freely available under the Apache License.</li>
508 1.8 christos <li>Many modern
509 1.8 christos <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
510 1.8 christos runtimes support <code><abbr>tz</abbr></code> natively via the
511 1.16 christos <code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>.
512 1.10 christos This can be used as-is or with most of the following libraries,
513 1.16 christos many of which also support runtimes lacking the <code>timeZone</code> option.
514 1.1 christos <ul>
515 1.8 christos <li>The <a
516 1.16 christos href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
517 1.8 christos timezone polyfill</a>
518 1.8 christos is freely available under a <abbr>BSD</abbr>-style license.</li>
519 1.10 christos <li>The <a href="https://date-fns.org/">date-fns</a>
520 1.10 christos library manipulates timezone-aware timestamps in browsers and
521 1.10 christos in <a href="https://nodejs.org/en/">Node.js</a>.
522 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
523 1.10 christos <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
524 1.10 christos minimalist replacement for the date and time API of
525 1.10 christos the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
526 1.10 christos manipulation library.
527 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
528 1.10 christos <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
529 1.16 christos timezone support for the <code>Intl</code> API.
530 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
531 1.1 christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
532 1.10 christos Moment.js plugin.
533 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
534 1.8 christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
535 1.8 christos JavaScript library that supports date arithmetic that is time zone
536 1.8 christos aware. It is freely available under the <abbr>MIT</abbr> license.</li>
537 1.10 christos <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
538 1.10 christos supports live <code><abbr>tzdb</abbr></code> updates,
539 1.10 christos astronomical and atomic time, a command-line interface,
540 1.10 christos and full <a
541 1.10 christos href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
542 1.10 christos Its companion <a
543 1.10 christos href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
544 1.10 christos can generate <abbr>TZif</abbr> and other files, and a companion website
545 1.10 christos <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
546 1.10 christos convert timestamps, view transition histories, and download code and data.
547 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
548 1.1 christos </ul>
549 1.10 christos The proposed <a
550 1.16 christos href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
551 1.10 christos objects</a> let programs access an abstract view of
552 1.10 christos <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
553 1.10 christos href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
554 1.16 christos problematic <code>Date</code> objects</a> when working with dates and times.
555 1.1 christos <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
556 1.1 christos compiler from <code><abbr>tz</abbr></code> source into
557 1.1 christos <a href="https://julialang.org/">Julia</a>. It is freely available
558 1.1 christos under the <abbr>MIT</abbr> license.</li>
559 1.6 christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
560 1.6 christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
561 1.6 christos title="Free Pascal Compiler">FPC</abbr></a>
562 1.2 christos compiles from <code><abbr>tz</abbr></code> source into
563 1.2 christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
564 1.2 christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
565 1.2 christos and <a
566 1.2 christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
567 1.2 christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
568 1.1 christos <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
569 1.1 christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
570 1.1 christos <a href="https://www.python.org">Python</a>.
571 1.9 christos It is freely available under a <abbr>BSD</abbr>-style license.
572 1.11 christos In code that can assume Python 3.6 or later it is largely superseded; see <a
573 1.11 christos href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
574 1.11 christos The Fastest Footgun in the West</a>.</li>
575 1.1 christos <li><a href="https://tzinfo.github.io">TZInfo –
576 1.1 christos Ruby Timezone Library</a>
577 1.1 christos compiles <code><abbr>tz</abbr></code> source into
578 1.1 christos <a href="https://www.ruby-lang.org/en/">Ruby</a>.
579 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
580 1.13 christos <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
581 1.1 christos Library</a> is
582 1.1 christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
583 1.1 christos library that compiles <code><abbr>tz</abbr></code> source into a time
584 1.1 christos zone repository whose format
585 1.1 christos is either proprietary or an <abbr>XML</abbr>-encoded
586 1.1 christos representation.</li>
587 1.1 christos <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
588 1.1 christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
589 1.1 christos source into text files, along with a runtime that can read those
590 1.1 christos files. Tcl is freely available under a <abbr>BSD</abbr>-style
591 1.1 christos license.</li>
592 1.1 christos </ul>
593 1.6 christos </section>
594 1.6 christos
595 1.6 christos <section>
596 1.2 christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
597 1.1 christos <ul>
598 1.1 christos <li>The <a
599 1.1 christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
600 1.1 christos Library</a>
601 1.1 christos has an independent, thread-safe implementation of
602 1.2 christos a <abbr>TZif</abbr> file reader.
603 1.1 christos This library is freely available under the LGPL
604 1.1 christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
605 1.1 christos <li><a href="https://www.gnome.org">GNOME</a>'s
606 1.1 christos <a href="https://developer.gnome.org/glib/">GLib</a> has
607 1.2 christos a <abbr>TZif</abbr> file reader written in C that
608 1.1 christos creates a <code>GTimeZone</code> object representing sets
609 1.1 christos of <abbr>UT</abbr> offsets.
610 1.1 christos It is freely available under the <abbr>LGPL</abbr>.</li>
611 1.1 christos <li>The
612 1.1 christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
613 1.1 christos <code>baltzo::TimeZoneUtil</code> component contains a C++
614 1.2 christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under
615 1.1 christos the Apache License.</li>
616 1.1 christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
617 1.1 christos library that translates between <abbr>UT</abbr> and civil time and
618 1.2 christos can read <abbr>TZif</abbr> files. It is freely available under the Apache
619 1.1 christos License.</li>
620 1.16 christos <li>The
621 1.16 christos <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
622 1.16 christos package</a> contains Python code
623 1.16 christos to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
624 1.16 christos <code><abbr>tz</abbr></code> settings to POSIX.1-2017-like approximations.
625 1.16 christos For example, it maps <code>"Africa/Cairo"</code>
626 1.16 christos to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
627 1.16 christos an approximation valid for Cairo timestamps from 2023 on.
628 1.16 christos This can help porting to platforms that support only POSIX.1-2017.
629 1.16 christos The package is freely available under the MIT license.</li>
630 1.1 christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
631 1.2 christos library that reads <abbr>TZif</abbr> files and converts
632 1.2 christos timestamps from one time zone or format to another.
633 1.1 christos It is used by <a href="https://secure.php.net"><abbr
634 1.1 christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
635 1.1 christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
636 1.1 christos and <a href="https://www.mongodb.com">MongoDB</a>.
637 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
638 1.1 christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
639 1.2 christos <abbr>TZif</abbr> file reader.</li>
640 1.4 christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
641 1.1 christos DateTime::TimeZone::Tzfile</a>
642 1.2 christos is a <abbr>TZif</abbr> file reader written in Perl.
643 1.1 christos It is freely available under the same terms as Perl
644 1.1 christos (dual <abbr>GPL</abbr> and Artistic license).</li>
645 1.9 christos <li>Python has a <a id="python-zoneinfo"
646 1.11 christos href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
647 1.8 christos class</a> that reads <abbr>TZif</abbr> data and creates objects
648 1.8 christos that represent <code><abbr>tzdb</abbr></code> timezones.
649 1.8 christos Python is freely available under the
650 1.11 christos <a href="https://docs.python.org/3/license.html">Python Software Foundation
651 1.8 christos License</a>.
652 1.9 christos A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
653 1.8 christos <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
654 1.8 christos supplies TZif data if the underlying system data cannot be found;
655 1.8 christos it is freely available under the Apache License.</li>
656 1.1 christos <li>The
657 1.1 christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
658 1.1 christos library contains a Python tool that
659 1.2 christos converts <abbr>TZif</abbr> data into
660 1.1 christos <abbr>JSON</abbr>-format data suitable for use
661 1.1 christos in its JavaScript library for time zone conversion. Dates before 1970
662 1.1 christos are not supported.</li>
663 1.1 christos <li>The <a
664 1.1 christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
665 1.1 christos package contains <a href="https://www.haskell.org">Haskell</a> code that
666 1.2 christos parses and uses <abbr>TZif</abbr> data. It is freely
667 1.1 christos available under a <abbr>BSD</abbr>-style license.</li>
668 1.1 christos </ul>
669 1.6 christos </section>
670 1.6 christos
671 1.6 christos <section>
672 1.1 christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
673 1.1 christos <ul>
674 1.1 christos <li><a href="https://foxclocks.org">FoxClocks</a>
675 1.1 christos is an extension for <a href="https://www.google.com/chrome/">Google
676 1.16 christos Chrome</a>, <a
677 1.1 christos href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
678 1.1 christos href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
679 1.1 christos It displays multiple clocks in the application window, and has a mapping
680 1.1 christos interface to <a href="https://www.google.com/earth/">Google Earth</a>.
681 1.1 christos It is freely available under the <abbr>GPL</abbr>.</li>
682 1.1 christos <li><a href="https://golang.org">Go programming language</a>
683 1.1 christos implementations contain a copy of a 32-bit subset of a recent
684 1.1 christos <code><abbr>tz</abbr></code> database in a
685 1.1 christos Go-specific format.</li>
686 1.1 christos <li>Microsoft Windows 8.1
687 1.1 christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
688 1.3 christos data (mentioned <a href="#CLDR">below</a>) used by the
689 1.3 christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
690 1.3 christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
691 1.3 christos <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
692 1.3 christos <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
693 1.1 christos <a id="System.TimeZoneInfo"
694 1.1 christos href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
695 1.1 christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
696 1.1 christos the older, proprietary method of Microsoft Windows 2000 and later,
697 1.1 christos which stores time zone data in the
698 1.1 christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
699 1.1 christos <a
700 1.1 christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
701 1.1 christos Tzid table</a> or <a
702 1.6 christos href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
703 1.1 christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
704 1.1 christos to <code><abbr>tz</abbr></code> names.
705 1.3 christos These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
706 1.3 christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
707 1.1 christos <li><a
708 1.1 christos href="https://www.oracle.com/java/index.html">Oracle
709 1.1 christos Java</a> contains a copy of a subset of a recent
710 1.1 christos <code><abbr>tz</abbr></code> database in a
711 1.1 christos Java-specific format.</li>
712 1.1 christos </ul>
713 1.6 christos </section>
714 1.6 christos
715 1.6 christos <section>
716 1.1 christos <h2 id="other-dbs">Other time zone databases</h2>
717 1.1 christos <ul>
718 1.1 christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
719 1.4 christos is Astrodienst's Web version of Shanks and Pottenger's out-of-print
720 1.4 christos time zone history atlases
721 1.4 christos <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
722 1.9 christos <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
723 1.4 christos Although these extensive atlases
724 1.4 christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
725 1.4 christos sources for much of the older <code><abbr>tz</abbr></code> data</a>,
726 1.4 christos they are unreliable as Shanks appears to have
727 1.1 christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
728 1.1 christos sources and do not indicate which entries are guesswork.</li>
729 1.1 christos <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
730 1.1 christos its own <code>tztab</code>(4) format.</li>
731 1.1 christos <li>Microsoft Windows has proprietary data mentioned
732 1.1 christos <a href="#System.TimeZoneInfo">above</a>.</li>
733 1.1 christos <li><a href="https://www.worldtimeserver.com">World Time Server</a>
734 1.1 christos is another time zone database.</li>
735 1.1 christos <li>The <a
736 1.1 christos href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
737 1.1 christos Schedules Information Manual</a> of the
738 1.1 christos International Air Transport Association
739 1.1 christos gives current time zone rules for airports served by commercial aviation.</li>
740 1.1 christos </ul>
741 1.6 christos </section>
742 1.6 christos
743 1.6 christos <section>
744 1.1 christos <h2 id="maps">Maps</h2>
745 1.1 christos <ul>
746 1.9 christos <li>The <a
747 1.9 christos href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
748 1.9 christos and Regional Maps section</a> of <em>The World Factbook</em>, published by the
749 1.9 christos <a href="https://www.cia.gov">US Central Intelligence
750 1.9 christos Agency (<abbr
751 1.9 christos title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
752 1.9 christos zone map; the
753 1.1 christos <a
754 1.6 christos href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
755 1.1 christos Library Map Collection</a>
756 1.1 christos of the University of Texas at Austin has copies of
757 1.1 christos recent editions.
758 1.1 christos The pictorial quality is good,
759 1.1 christos but the maps do not indicate daylight saving time,
760 1.1 christos and parts of the data are a few years out of date.</li>
761 1.6 christos <li><a href="https://www.worldtimezone.com">World Time Zone Map
762 1.6 christos with current time</a>
763 1.1 christos has several fancy time zone maps; it covers Russia particularly well.
764 1.1 christos The maps' pictorial quality is not quite as good as the
765 1.1 christos <abbr>CIA</abbr>'s
766 1.1 christos but the maps are more up to date.</li>
767 1.1 christos <li><a
768 1.1 christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
769 1.1 christos much is time wrong around the world?</a> maps the difference between
770 1.1 christos mean solar and standard time, highlighting areas such as western China
771 1.1 christos where the two differ greatly. It's a bit out of date, unfortunately.</li>
772 1.1 christos </ul>
773 1.6 christos </section>
774 1.6 christos
775 1.6 christos <section>
776 1.1 christos <h2 id="boundaries">Time zone boundaries</h2>
777 1.2 christos <p>Geographical boundaries between timezones are available
778 1.9 christos from several <a
779 1.9 christos href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
780 1.9 christos geolocation</a>
781 1.1 christos services and other sources.</p>
782 1.1 christos <ul>
783 1.1 christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
784 1.1 christos Boundary Builder</a> extracts
785 1.1 christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
786 1.2 christos boundaries of <code><abbr>tzdb</abbr></code> timezones.
787 1.1 christos Its code is freely available under the <abbr>MIT</abbr> license, and
788 1.1 christos its data entries are freely available under the
789 1.1 christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
790 1.16 christos Open Database License</a>. The borders appear to be quite accurate.
791 1.16 christos Its main web page lists more than twenty libraries
792 1.16 christos for looking up a timezone name from a GPS coordinate.</li>
793 1.1 christos <li>Free access via a network API, if you register a key, is provided by
794 1.4 christos the <a
795 1.4 christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
796 1.4 christos Timezone web service</a>, the <a
797 1.4 christos href="https://developers.google.com/maps/documentation/timezone/intro">Google
798 1.4 christos Maps Time Zone API</a>, and
799 1.8 christos the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
800 1.1 christos Commercial network API access is provided
801 1.1 christos by <a href="https://askgeo.com">AskGeo</a>
802 1.1 christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
803 1.1 christos </li>
804 1.1 christos <li>"<a
805 1.1 christos href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
806 1.1 christos to get a time zone from a location using latitude and longitude
807 1.1 christos coordinates?</a>" discusses other geolocation possibilities.</li>
808 1.1 christos <li><a href="http://statoids.com/statoids.html">Administrative
809 1.1 christos Divisions of Countries ("Statoids")</a> lists
810 1.1 christos political subdivision data related to time zones.</li>
811 1.13 christos <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
812 1.1 christos – GIS and Database Tools</a> includes a Manifold-format map of
813 1.16 christos world time zone boundaries circa 2007, distributed under the
814 1.1 christos <abbr>GPL</abbr>.</li>
815 1.1 christos <li>A ship within the <a
816 1.1 christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
817 1.1 christos waters</a> of any nation uses that nation's time. In international
818 1.1 christos waters, time zone boundaries are meridians 15° apart, except that
819 1.1 christos <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
820 1.1 christos wide and are separated by
821 1.1 christos the 180° meridian (not by the International Date Line, which is
822 1.1 christos for land and territorial waters only). A captain can change ship's
823 1.1 christos clocks any time after entering a new time zone; midnight changes are
824 1.1 christos common.</li>
825 1.1 christos </ul>
826 1.6 christos </section>
827 1.6 christos
828 1.6 christos <section>
829 1.1 christos <h2 id="civil">Civil time concepts and history</h2>
830 1.1 christos <ul>
831 1.1 christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
832 1.1 christos Walk through Time</a>
833 1.1 christos surveys the evolution of timekeeping.</li>
834 1.4 christos <li>The history of daylight saving time is surveyed in <a
835 1.4 christos href="http://www.webexhibits.org/daylightsaving/">About Daylight
836 1.4 christos Saving Time – History, rationale, laws & dates</a> and summarized in
837 1.4 christos <a href="http://seizethedaylight.com/dst/">A Brief
838 1.4 christos History of Daylight Saving Time</a>.</li>
839 1.4 christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
840 1.4 christos Lords</a> discusses how authoritarians manipulate civil time.</li>
841 1.1 christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
842 1.1 christos contains guidelines and best practices for software applications that
843 1.1 christos deal with civil time.</li>
844 1.16 christos <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
845 1.1 christos the International Date Line</a> tells the story of the most important
846 1.1 christos time zone boundary.</li>
847 1.1 christos <li><a href="http://statoids.com/tconcept.html">Basic Time
848 1.1 christos Zone Concepts</a> discusses terminological issues behind time zones.</li>
849 1.1 christos </ul>
850 1.6 christos </section>
851 1.6 christos
852 1.6 christos <section>
853 1.1 christos <h2 id="national">National histories of legal time</h2>
854 1.1 christos <dl>
855 1.1 christos <dt>Australia</dt>
856 1.6 christos <dd>The Parliamentary Library commissioned a <a
857 1.16 christos href="https://parlinfo.aph.gov.au/parlInfo/download/library/prspub/359V6/upload_binary/359v60.pdf">research
858 1.1 christos paper on daylight saving time in Australia</a>.
859 1.1 christos The Bureau of Meteorology publishes a list of <a
860 1.1 christos href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
861 1.1 christos Dates of Daylight Savings Time within Australia</a>.</dd>
862 1.1 christos <dt>Belgium</dt>
863 1.7 christos <dd>The Royal Observatory of Belgium maintains a table of time in
864 1.7 christos Belgium (in
865 1.16 christos <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
866 1.7 christos hreflang="nl">Dutch</a> and <a
867 1.16 christos href="https://robinfo.oma.be/fr/astro-info/heure/"
868 1.7 christos hreflang="fr">French</a>).</dd>
869 1.1 christos <dt>Brazil</dt>
870 1.1 christos <dd>The Time Service Department of the National Observatory
871 1.1 christos records <a href="http://pcdsh01.on.br/DecHV.html"
872 1.1 christos hreflang="pt-BR">Brazil's daylight saving time decrees (in
873 1.1 christos Portuguese)</a>.</dd>
874 1.1 christos <dt>Canada</dt>
875 1.1 christos <dd>National Research Council Canada publishes current
876 1.1 christos and some older information about <a
877 1.6 christos href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
878 1.6 christos zones and daylight saving time</a>.</dd>
879 1.1 christos <dt>Chile</dt>
880 1.1 christos <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
881 1.11 christos <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
882 1.1 christos Chile's official time (in Spanish)</a>.</dd>
883 1.3 christos <dt>China</dt>
884 1.3 christos <dd>The Hong Kong Observatory maintains a
885 1.7 christos <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
886 1.3 christos summer time in Hong Kong</a>,
887 1.3 christos and Macau's Meteorological and Geophysical Bureau maintains a <a
888 1.7 christos href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
889 1.3 christos history for Macau</a>.
890 1.3 christos Unfortunately the latter is incomplete and has errors.</dd>
891 1.1 christos <dt>Czech Republic</dt>
892 1.1 christos <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
893 1.1 christos hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
894 1.10 christos summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
895 1.1 christos <dt>Germany</dt>
896 1.1 christos <dd>The National Institute for Science and Technology maintains the <a
897 1.1 christos href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
898 1.1 christos of Legal Time in Germany</a>.</dd>
899 1.1 christos <dt>Israel</dt>
900 1.1 christos <dd>The Interior Ministry periodically issues <a
901 1.1 christos href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
902 1.1 christos hreflang="he">announcements (in Hebrew)</a>.</dd>
903 1.2 christos <dt>Malaysia</dt>
904 1.2 christos <dd>See Singapore <a href="#Singapore">below</a>.</dd>
905 1.1 christos <dt>Mexico</dt>
906 1.1 christos <dd>The Investigation and Analysis Service of the Mexican Library of
907 1.1 christos Congress has published a <a
908 1.13 christos href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
909 1.1 christos hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
910 1.1 christos <dt>Netherlands</dt>
911 1.16 christos <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
912 1.1 christos hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
913 1.1 christos covers the history of local time in the Netherlands from ancient times.</dd>
914 1.1 christos <dt>New Zealand</dt>
915 1.1 christos <dd>The Department of Internal Affairs maintains a brief <a
916 1.1 christos href="https://www.dia.govt.nz/Daylight-Saving-History">History of
917 1.6 christos Daylight Saving</a>.</dd>
918 1.14 christos <dt>Palestine</dt>
919 1.14 christos <dd>The Ministry of Telecom and IT publishes a <a
920 1.16 christos href="https://mtit.pna.ps/home/TimeZone"
921 1.14 christos hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
922 1.10 christos <dt>Portugal</dt>
923 1.10 christos <dd>The Lisbon Astronomical Observatory publishes a
924 1.10 christos <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
925 1.10 christos legal time (in Portuguese)</a>.</dd>
926 1.1 christos <dt>Singapore</dt>
927 1.1 christos <dd><a id="Singapore"
928 1.7 christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
929 1.1 christos is Singapore in the "Wrong" Time Zone?</a> details the
930 1.1 christos history of legal time in Singapore and Malaysia.</dd>
931 1.1 christos <dt>United Kingdom</dt>
932 1.1 christos <dd><a
933 1.1 christos href="https://www.polyomino.org.uk/british-time/">History of
934 1.1 christos legal time in Britain</a> discusses in detail the country
935 1.6 christos with perhaps the best-documented history of clock adjustments.</dd>
936 1.1 christos <dt>United States</dt>
937 1.1 christos <dd>The Department of Transportation's <a
938 1.1 christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
939 1.1 christos Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
940 1.1 christos <dt>Uruguay</dt>
941 1.1 christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
942 1.1 christos Navy (SOHMA) publishes an annual <a
943 1.13 christos href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
944 1.1 christos (in Spanish)</a>.</dd>
945 1.1 christos </dl>
946 1.6 christos </section>
947 1.6 christos
948 1.6 christos <section>
949 1.7 christos <h2 id="costs">Costs and benefits of time shifts</h2>
950 1.7 christos <p>Various sources argue for and against daylight saving time and time
951 1.7 christos zone shifts, and many scientific studies have been conducted. This
952 1.8 christos section summarizes reviews and position statements based on
953 1.8 christos scientific literature in the area.</p>
954 1.7 christos <ul>
955 1.15 christos <li>In 2022 the American Medical Association issued a
956 1.15 christos <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
957 1.15 christos supporting permanent standard time</a> on health grounds.</li>
958 1.7 christos <li>Carey RN, Sarma KM.
959 1.7 christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
960 1.7 christos daylight saving time on road traffic collision risk: a systematic
961 1.7 christos review</a>.
962 1.7 christos <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>.
963 1.7 christos This reviews research literature and concludes that the evidence
964 1.7 christos neither supports nor refutes road safety benefits from
965 1.7 christos shifts in time zones.</li>
966 1.7 christos <li>Havranek T, Herman D, Irsova D.
967 1.13 christos <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
968 1.16 christos daylight saving save electricity? A meta-analysis</a>.
969 1.13 christos <em>Energy J.</em> 2018;39(2):35–61.
970 1.7 christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
971 1.7 christos This analyzes research literature and concludes, "Electricity savings
972 1.7 christos are larger for countries farther away from the equator, while
973 1.10 christos subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
974 1.16 christos <li>Malow BA. <a
975 1.16 christos href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
976 1.16 christos to abolish the clock change and adopt permanent
977 1.16 christos standard time in the United States:
978 1.16 christos a Sleep Research Society position statement</a>.
979 1.15 christos <em>Sleep.</em> 2022;45(12):zsac236.
980 1.15 christos doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
981 1.15 christos After reviewing the scientific literature, the Sleep Research Society
982 1.15 christos advocates permanent standard time due to its health benefits.
983 1.15 christos <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
984 1.15 christos <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
985 1.15 christos is the optimal choice for health and safety:
986 1.8 christos an American Academy of Sleep Medicine position statement</a>.
987 1.16 christos <em>J Clin Sleep Med.</em> 2024;20(1):121–125.
988 1.15 christos doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
989 1.15 christos The AASM argues for permanent standard time due to health and safety risks
990 1.15 christos and economic costs of both <abbr>DST</abbr> transitions and
991 1.15 christos permanent <abbr>DST</abbr>.</li>
992 1.13 christos <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
993 1.16 christos <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
994 1.13 christos should we abolish Daylight Saving Time?</a>
995 1.15 christos <em>J Biol Rhythms.</em> 2019;34(3):227–230.
996 1.13 christos doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
997 1.15 christos The Society for Research on Biological Rhythms
998 1.13 christos opposes DST changes and permanent DST, and advocates that governments adopt
999 1.13 christos "permanent Standard Time for the health and safety of their citizens".</li>
1000 1.7 christos </ul>
1001 1.7 christos </section>
1002 1.7 christos
1003 1.7 christos <section>
1004 1.1 christos <h2 id="precision">Precision timekeeping</h2>
1005 1.1 christos <ul>
1006 1.1 christos <li><a
1007 1.1 christos href="http://leapsecond.com/hpan/an1289.pdf">The
1008 1.1 christos Science of Timekeeping</a> is a thorough introduction
1009 1.1 christos to the theory and practice of precision timekeeping.</li>
1010 1.1 christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
1011 1.14 christos Time 2016</a> contains several freely readable papers.</li>
1012 1.13 christos <li><a href="https://www.ntp.org"><abbr
1013 1.1 christos title="Network Time Protocol">NTP</abbr>: The Network
1014 1.1 christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
1015 1.1 christos discusses how to synchronize clocks of
1016 1.1 christos Internet hosts.</li>
1017 1.2 christos <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
1018 1.2 christos family of software algorithms can achieve accuracy to a few tens of
1019 1.2 christos nanoseconds in scalable server farms without special hardware.</li>
1020 1.1 christos <li>The <a
1021 1.1 christos href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
1022 1.1 christos Time Protocol</a> (<abbr
1023 1.1 christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
1024 1.2 christos can achieve submicrosecond clock accuracy on a local area network
1025 1.2 christos with special-purpose hardware.</li>
1026 1.1 christos <li><a
1027 1.13 christos href="https://datatracker.ietf.org/doc/html/rfc4833">Timezone
1028 1.1 christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1029 1.1 christos (Internet <abbr>RFC</abbr> 4833)
1030 1.1 christos specifies a <a
1031 1.1 christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1032 1.1 christos option for a server to configure
1033 1.1 christos a client's time zone and daylight saving settings automatically.</li>
1034 1.11 christos <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1035 1.11 christos Scales</a> describes astronomical time scales like
1036 1.1 christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1037 1.1 christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1038 1.1 christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1039 1.1 christos <li>The <a href="https://www.iau.org"><abbr
1040 1.1 christos title="International Astronomical Union">IAU</abbr></a>'s <a
1041 1.13 christos href="https://www.iausofa.org"><abbr
1042 1.1 christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1043 1.1 christos collection contains C and <a
1044 1.1 christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1045 1.1 christos code for converting among time scales like
1046 1.1 christos <abbr title="International Atomic Time">TAI</abbr>,
1047 1.1 christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1048 1.13 christos <abbr>UTC</abbr>. It is freely available under the
1049 1.13 christos <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
1050 1.1 christos <li><a
1051 1.1 christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1052 1.1 christos – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1053 1.1 christos diverse local time
1054 1.1 christos scales used by each landed mission on Mars.</li>
1055 1.1 christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1056 1.1 christos dedicated not only to leap seconds but to precise time and frequency
1057 1.1 christos in general. It covers the state of the art in amateur timekeeping, and
1058 1.1 christos how the art has progressed over the past few decades.</li>
1059 1.7 christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1060 1.7 christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1061 1.7 christos and time-signal emissions</a>, International Telecommunication Union –
1062 1.7 christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1063 1.1 christos <li><a
1064 1.1 christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1065 1.1 christos title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1066 1.1 christos Bulletins</a> contains official publications of the International
1067 1.1 christos Earth Rotation and Reference Systems Service, which decides when leap
1068 1.4 christos seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1069 1.9 christos via an optional "<code>right</code>" configuration where a computer's internal
1070 1.9 christos <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1071 1.9 christos as opposed to the default "<code>posix</code>" configuration
1072 1.9 christos where the internal clock ignores leap seconds.
1073 1.9 christos The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1074 1.9 christos <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1075 1.9 christos timestamps starting with <code>time_t</code> 78 796 800,
1076 1.9 christos which corresponds to the first leap second
1077 1.9 christos 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1078 1.9 christos and to
1079 1.9 christos 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1080 1.9 christos In practice the two configurations also agree for timestamps before
1081 1.9 christos 1972 even though the historical situation is messy, partly because
1082 1.9 christos neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1083 1.14 christos is well-defined for sufficiently old timestamps.</li>
1084 1.1 christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1085 1.1 christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1086 1.1 christos leap second so that they disagree with <abbr>UTC</abbr> by at most a
1087 1.1 christos half second, even though every <abbr>POSIX</abbr> minute has exactly
1088 1.4 christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1089 1.1 christos "<code>posix</code>" configuration, is <a
1090 1.1 christos href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1091 1.12 christos the <abbr>NTP</abbr> reference implementation, <a
1092 1.12 christos href="https://github.com/google/unsmear">supports</a> conversion between
1093 1.12 christos <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1094 1.7 christos cloud service providers. However, according to
1095 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">§3.7.1 of
1096 1.7 christos Network Time Protocol Best Current Practices</a>
1097 1.7 christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1098 1.9 christos applications requiring accurate <abbr>UTC</abbr> or civil time,
1099 1.7 christos and is intended for use only in single, well-controlled environments.</li>
1100 1.1 christos <li>The <a
1101 1.1 christos href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1102 1.1 christos Second Discussion List</a> covers <a
1103 1.1 christos href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1104 1.1 christos and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1105 1.1 christos discussed further in
1106 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1107 1.1 christos leap second: its history and possible future</a>.
1108 1.1 christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1109 1.1 christos might be redefined
1110 1.1 christos without Leap Seconds</a> gives pointers on this
1111 1.14 christos contentious issue.
1112 1.14 christos The General Conference on Weights and Measures
1113 1.16 christos <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
1114 1.14 christos to discontinue the use of leap seconds by 2035, replacing them with an
1115 1.14 christos as-yet-undetermined scheme some time after the year 2135.
1116 1.16 christos The World Radiocommunication Conference <a
1117 1.16 christos href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
1118 1.16 christos in 2023</a> to cooperate with this process.
1119 1.14 christos </li>
1120 1.1 christos </ul>
1121 1.6 christos </section>
1122 1.6 christos
1123 1.6 christos <section>
1124 1.1 christos <h2 id="notation">Time notation</h2>
1125 1.1 christos <ul>
1126 1.13 christos <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1127 1.1 christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1128 1.1 christos zone names, abbreviations, identifiers, and formats. For example, it
1129 1.1 christos contains French translations for "Eastern European Summer Time",
1130 1.1 christos "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1131 1.1 christos "Bucharest". Its
1132 1.1 christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1133 1.1 christos charts</a> show these values for many locales. Data values are available in
1134 1.1 christos both <abbr title="Locale Data Markup Language">LDML</abbr>
1135 1.1 christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1136 1.1 christos <li>
1137 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1138 1.6 christos the international standard date and time notation</a> covers
1139 1.1 christos <a
1140 1.6 christos href="https://www.iso.org/standard/70907.html"><em><abbr
1141 1.6 christos title="International Organization for Standardization">ISO</abbr>
1142 1.6 christos 8601-1:2019 – Date and time – Representations for information
1143 1.6 christos interchange – Part 1: Basic rules</em></a>.</li>
1144 1.1 christos <li>
1145 1.6 christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1146 1.1 christos Schema: Datatypes – dateTime</a> specifies a format inspired by
1147 1.1 christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1148 1.13 christos <li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">§3.3 of
1149 1.1 christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1150 1.1 christos specifies the time notation used in email and <a
1151 1.1 christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1152 1.1 christos headers.</li>
1153 1.1 christos <li>
1154 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time
1155 1.1 christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1156 1.1 christos specifies an <abbr>ISO</abbr> 8601
1157 1.1 christos profile for use in new Internet
1158 1.1 christos protocols.</li>
1159 1.1 christos <li>
1160 1.7 christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1161 1.1 christos Formats on the Web</a> surveys web- and Internet-oriented date and time
1162 1.1 christos formats.</li>
1163 1.1 christos <li>Alphabetic time zone abbreviations should not be used as unique
1164 1.1 christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1165 1.1 christos practice. For example, in English-speaking North America
1166 1.1 christos "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1167 1.1 christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1168 1.1 christos and French-speaking North Americans prefer
1169 1.1 christos "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1170 1.1 christos "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1171 1.2 christos database contains English abbreviations for many timestamps;
1172 1.1 christos unfortunately some of these abbreviations were merely the database maintainers'
1173 1.1 christos inventions, and these have been removed when possible.</li>
1174 1.1 christos <li>Numeric time zone abbreviations typically count hours east of
1175 1.1 christos <abbr>UT</abbr>, e.g., +09 for Japan and
1176 1.1 christos −10 for Hawaii. However, the <abbr>POSIX</abbr>
1177 1.1 christos <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1178 1.1 christos For example, one might use <code><abbr>TZ</abbr>="<abbr
1179 1.1 christos title="Japan Standard Time">JST</abbr>-9"</code> and
1180 1.1 christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1181 1.1 christos for Japan and Hawaii, respectively. If the
1182 1.1 christos <code><abbr>tz</abbr></code> database is available, it is usually better to use
1183 1.1 christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1184 1.1 christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1185 1.2 christos confusion, handle old timestamps better, and insulate you better from
1186 1.1 christos any future changes to the rules. One should never set
1187 1.1 christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1188 1.1 christos <code>"GMT-9"</code>, though, since this would incorrectly imply that
1189 1.1 christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1190 1.1 christos is called "<abbr>GMT</abbr>".</li>
1191 1.1 christos </ul>
1192 1.6 christos </section>
1193 1.6 christos
1194 1.6 christos <section>
1195 1.1 christos <h2 id="see-also">See also</h2>
1196 1.1 christos <ul>
1197 1.7 christos <li><a href="theory.html">Theory and pragmatics of the
1198 1.7 christos <code><abbr>tz</abbr></code> code and data</a></li>
1199 1.1 christos <li><a href="tz-art.html">Time and the Arts</a></li>
1200 1.1 christos </ul>
1201 1.6 christos </section>
1202 1.6 christos
1203 1.6 christos <footer>
1204 1.1 christos <hr>
1205 1.1 christos This web page is in the public domain, so clarified as of
1206 1.1 christos 2009-05-17 by Arthur David Olson.
1207 1.1 christos <br>
1208 1.1 christos Please send corrections to this web page to the
1209 1.1 christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1210 1.6 christos </footer>
1211 1.1 christos </body>
1212 1.1 christos </html>
1213