tz-link.html revision 1.17 1 1.1 christos <!DOCTYPE html>
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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 1.17 christos <a href="https://www.chromium.org/chromium-os/">ChromiumOS</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 1.17 christos <a href="https://musl.libc.org">musl libc</a>,
89 1.1 christos <a href="https://www.mysql.com">MySQL</a>,
90 1.1 christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
91 1.1 christos title="Web Operating System">webOS</abbr></a>,
92 1.7 christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
93 1.1 christos title="Advanced Interactive eXecutive">AIX</abbr></a>,
94 1.6 christos <a href="https://www.apple.com/ios"><abbr
95 1.1 christos title="iPhone OS">iOS</abbr></a>,
96 1.6 christos <a href="https://www.apple.com/macos">macOS</a>,
97 1.1 christos <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
98 1.6 christos <a href="https://www.vmssoftware.com">Open<abbr
99 1.1 christos title="Virtual Memory System">VMS</abbr></a>,
100 1.6 christos <a href="https://www.oracle.com/database/">Oracle Database</a>, and
101 1.1 christos <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
102 1.1 christos <p>
103 1.2 christos Each main entry in the database represents a <dfn>timezone</dfn>
104 1.2 christos for a set of civil-time clocks that have all agreed since 1970.
105 1.2 christos Timezones are typically identified by continent or ocean and then by the
106 1.2 christos name of the largest city within the region containing the clocks.
107 1.1 christos For example, <code>America/New_York</code>
108 1.1 christos represents most of the <abbr title="United States">US</abbr> eastern time zone;
109 1.1 christos <code>America/Phoenix</code> represents most of Arizona, which
110 1.2 christos uses mountain time without daylight saving time (<abbr>DST</abbr>);
111 1.1 christos <code>America/Detroit</code> represents most of Michigan, which uses
112 1.1 christos eastern time but with different <abbr>DST</abbr> rules in 1975;
113 1.1 christos and other entries represent smaller regions like Starke County,
114 1.1 christos Indiana, which switched from central to eastern time in 1991
115 1.1 christos and switched back in 2006.
116 1.17 christos To use the database on a <a
117 1.1 christos href="https://en.wikipedia.org/wiki/POSIX"><abbr
118 1.17 christos title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
119 1.1 christos implementation set the <code><abbr>TZ</abbr></code>
120 1.1 christos environment variable to the location's full name,
121 1.1 christos e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
122 1.1 christos <p>
123 1.2 christos Associated with each timezone is a history of offsets from
124 1.1 christos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
125 1.1 christos Time</a> (<abbr>UT</abbr>), which is <a
126 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
127 1.1 christos Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
128 1.2 christos for timestamps after 1960 this is more precisely <a
129 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
130 1.1 christos Universal Time</a> (<abbr>UTC</abbr>).
131 1.1 christos The database also records when daylight saving time was in use,
132 1.1 christos along with some time zone abbreviations such as <abbr>EST</abbr>
133 1.1 christos for Eastern Standard Time in the <abbr>US</abbr>.</p>
134 1.6 christos </section>
135 1.6 christos
136 1.6 christos <section>
137 1.1 christos <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
138 1.1 christos <p>
139 1.1 christos The following <a
140 1.1 christos href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
141 1.1 christos the latest release's two
142 1.1 christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
143 1.1 christos to a <abbr>GNU</abbr>/Linux or similar host.</p>
144 1.2 christos <pre><code>mkdir tzdb
145 1.1 christos cd tzdb
146 1.1 christos <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
147 1.1 christos wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
148 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 -
149 1.1 christos gzip -dc tzdata-latest.tar.gz | tar -xf -
150 1.1 christos </code></pre>
151 1.1 christos <p>Alternatively, the following shell commands download the same
152 1.1 christos release in a single-tarball format containing extra data
153 1.1 christos useful for regression testing:</p>
154 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>
155 1.1 christos <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
156 1.1 christos </code></pre>
157 1.1 christos <p>These commands use convenience links to the latest release
158 1.1 christos of the <code><abbr>tz</abbr></code> database hosted by the
159 1.1 christos <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
160 1.1 christos of the <a href="https://www.iana.org">Internet Assigned Numbers
161 1.1 christos Authority (IANA)</a>.
162 1.1 christos Older releases are in files named
163 1.1 christos <code>tzcode<var>V</var>.tar.gz</code>,
164 1.1 christos <code>tzdata<var>V</var>.tar.gz</code>, and
165 1.1 christos <code>tzdb-<var>V</var>.tar.lz</code>,
166 1.1 christos where <code><var>V</var></code> is the version.
167 1.1 christos Since 1996, each version has been a four-digit year followed by
168 1.1 christos lower-case letter (<samp>a</samp> through <samp>z</samp>,
169 1.1 christos then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
170 1.1 christos through <samp>zzz</samp>, and so on).
171 1.13 christos Since version 2022a, each release has been distributed in
172 1.10 christos <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
173 1.10 christos ustar interchange format</a>, compressed as described above;
174 1.14 christos older releases use a nearly compatible format.
175 1.1 christos Since version 2016h, each release has contained a text file named
176 1.16 christos "<code>version</code>" whose first (and currently only) line is the version.
177 1.7 christos Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
178 1.7 christos and are also available in an
179 1.1 christos <a href="ftp://ftp.iana.org/tz/releases/"><abbr
180 1.1 christos title="File Transfer Protocol">FTP</abbr> directory</a> via a
181 1.14 christos less secure protocol.</p>
182 1.1 christos <p>Alternatively, a development repository of code and data can be
183 1.1 christos retrieved from <a href="https://github.com">GitHub</a> via the shell
184 1.1 christos command:</p>
185 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>
186 1.1 christos </code></pre>
187 1.1 christos <p>
188 1.1 christos Since version 2012e, each release has been tagged in development repositories.
189 1.1 christos Untagged commits are less well tested and probably contain
190 1.1 christos more errors.</p>
191 1.1 christos <p>
192 1.1 christos After obtaining the code and data files, see the
193 1.1 christos <code>README</code> file for what to do next.
194 1.1 christos The code lets you compile the <code><abbr>tz</abbr></code> source files into
195 1.2 christos machine-readable binary files, one for each location. The binary files
196 1.17 christos are in a special format specified by
197 1.17 christos <a href="https://datatracker.ietf.org/doc/html/8536">The
198 1.17 christos Time Zone Information Format (<abbr>TZif</abbr>)</a>
199 1.17 christos (Internet <abbr title="Request For Comments">RFC</abbr> 8536).
200 1.2 christos The code also lets
201 1.2 christos you read a <abbr>TZif</abbr> file and interpret timestamps for that
202 1.1 christos location.</p>
203 1.6 christos </section>
204 1.6 christos
205 1.6 christos <section>
206 1.1 christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
207 1.1 christos <p>
208 1.1 christos The <code><abbr>tz</abbr></code> code and data
209 1.1 christos are by no means authoritative. If you find errors, please
210 1.17 christos email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
211 1.17 christos the time zone mailing list. See
212 1.17 christos <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
213 1.17 christos list's main page</a> to subscribe or to browse its archive of old messages.
214 1.17 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 1.17 christos for <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><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 1.17 christos <a href="https://gedit-technology.github.io/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 1.17 christos Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
265 1.17 christos 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 1.17 christos uses <a href="https://www.json.org/json-en.html"><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 1.17 christos <li><a href="https://github.com/libical/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 1.17 christos runtime library with a <a
444 1.17 christos href="https://en.cppreference.com/w/cpp/chrono"><code>std::chrono</code> API</a>
445 1.17 christos that is a standard part of C++.
446 1.1 christos It is freely available under the
447 1.1 christos <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
448 1.13 christos <li><a id="ICU" href="https://icu.unicode.org">International Components for
449 1.1 christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
450 1.14 christos href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
451 1.1 christos libraries for internationalization that
452 1.1 christos has a compiler from <code><abbr>tz</abbr></code> source
453 1.1 christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
454 1.1 christos (mentioned <a href="#CLDR">below</a>)
455 1.1 christos into an <abbr>ICU</abbr>-specific format.
456 1.1 christos <abbr>ICU</abbr> is freely available under a
457 1.1 christos <abbr>BSD</abbr>-style license.</li>
458 1.1 christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
459 1.1 christos the <a href="https://elixir-lang.org">Elixir</a> language downloads
460 1.7 christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
461 1.1 christos title="Application Program Interface">API</abbr>s for use. It is
462 1.1 christos freely available under the <abbr>MIT</abbr> license.</li>
463 1.1 christos <li>Java-based compilers and libraries include:
464 1.1 christos <ul>
465 1.1 christos <li>The <a
466 1.9 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
467 1.1 christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
468 1.17 christos <a href="https://openjdk.org">OpenJDK</a> and
469 1.17 christos <a href="https://jdk.java.net">Oracle JDK</a>.
470 1.9 christos Although its source code is proprietary, its executable is available under the
471 1.9 christos <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
472 1.9 christos Timezone Updater License Agreement</a>.</li>
473 1.1 christos <li>The <a
474 1.4 christos href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
475 1.6 christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
476 1.4 christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
477 1.1 christos which is freely available under a <abbr>BSD</abbr>-style license.</li>
478 1.4 christos <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
479 1.1 christos and time <abbr>API</abbr></a> contains a class
480 1.1 christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
481 1.1 christos <code><abbr>tz</abbr></code> source into a binary format. It inspired
482 1.1 christos Java 8 <code>java.time</code>, which its users should migrate to once
483 1.1 christos they can assume Java 8 or later. It is available under the <a
484 1.1 christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
485 1.9 christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
486 1.7 christos href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
487 1.9 christos are alternatives to TZUpdater. IANA Updater's license is unclear;
488 1.9 christos ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
489 1.4 christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
490 1.4 christos time library for Android</a> and
491 1.17 christos <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,
492 1.4 christos time and interval library for Java</a> compile
493 1.4 christos <code><abbr>tz</abbr></code> source into a binary format.
494 1.4 christos Time4A is available under the Apache License and Time4J is
495 1.1 christos available under the <a
496 1.1 christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
497 1.1 christos General Public License (<abbr title="Lesser General Public
498 1.1 christos License">LGPL</abbr>)</a>.</li>
499 1.1 christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
500 1.1 christos Java-based libraries.</li>
501 1.1 christos </ul>
502 1.1 christos <li><a href="https://nodatime.org">Noda Time – Date and
503 1.1 christos time <abbr>API</abbr> for .NET</a>
504 1.4 christos is like Joda-Time and Time4J, but for the .NET framework instead of Java.
505 1.4 christos It is freely available under the Apache License.</li>
506 1.8 christos <li>Many modern
507 1.8 christos <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
508 1.8 christos runtimes support <code><abbr>tz</abbr></code> natively via the
509 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>.
510 1.10 christos This can be used as-is or with most of the following libraries,
511 1.16 christos many of which also support runtimes lacking the <code>timeZone</code> option.
512 1.1 christos <ul>
513 1.8 christos <li>The <a
514 1.16 christos href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
515 1.8 christos timezone polyfill</a>
516 1.8 christos is freely available under a <abbr>BSD</abbr>-style license.</li>
517 1.17 christos <li>The <a href="https://date-fns.org">date-fns</a>
518 1.10 christos library manipulates timezone-aware timestamps in browsers and
519 1.10 christos in <a href="https://nodejs.org/en/">Node.js</a>.
520 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
521 1.10 christos <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
522 1.10 christos minimalist replacement for the date and time API of
523 1.10 christos the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
524 1.10 christos manipulation library.
525 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
526 1.10 christos <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
527 1.16 christos timezone support for the <code>Intl</code> API.
528 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
529 1.1 christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
530 1.10 christos Moment.js plugin.
531 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
532 1.8 christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
533 1.8 christos JavaScript library that supports date arithmetic that is time zone
534 1.8 christos aware. It is freely available under the <abbr>MIT</abbr> license.</li>
535 1.10 christos <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
536 1.10 christos supports live <code><abbr>tzdb</abbr></code> updates,
537 1.10 christos astronomical and atomic time, a command-line interface,
538 1.10 christos and full <a
539 1.10 christos href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
540 1.10 christos Its companion <a
541 1.10 christos href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
542 1.10 christos can generate <abbr>TZif</abbr> and other files, and a companion website
543 1.10 christos <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
544 1.10 christos convert timestamps, view transition histories, and download code and data.
545 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
546 1.1 christos </ul>
547 1.10 christos The proposed <a
548 1.16 christos href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
549 1.10 christos objects</a> let programs access an abstract view of
550 1.10 christos <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
551 1.10 christos href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
552 1.16 christos problematic <code>Date</code> objects</a> when working with dates and times.
553 1.17 christos <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
554 1.1 christos compiler from <code><abbr>tz</abbr></code> source into
555 1.17 christos <a href="https://julialang.org">Julia</a>. It is freely available
556 1.1 christos under the <abbr>MIT</abbr> license.</li>
557 1.6 christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
558 1.6 christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
559 1.6 christos title="Free Pascal Compiler">FPC</abbr></a>
560 1.2 christos compiles from <code><abbr>tz</abbr></code> source into
561 1.2 christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
562 1.2 christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
563 1.2 christos and <a
564 1.2 christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
565 1.2 christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
566 1.17 christos <li><a href="https://pythonhosted.org/pytz/">pytz – World Timezone
567 1.1 christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
568 1.1 christos <a href="https://www.python.org">Python</a>.
569 1.9 christos It is freely available under a <abbr>BSD</abbr>-style license.
570 1.11 christos In code that can assume Python 3.6 or later it is largely superseded; see <a
571 1.11 christos href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
572 1.11 christos The Fastest Footgun in the West</a>.</li>
573 1.1 christos <li><a href="https://tzinfo.github.io">TZInfo –
574 1.1 christos Ruby Timezone Library</a>
575 1.1 christos compiles <code><abbr>tz</abbr></code> source into
576 1.1 christos <a href="https://www.ruby-lang.org/en/">Ruby</a>.
577 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
578 1.13 christos <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
579 1.1 christos Library</a> is
580 1.1 christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
581 1.1 christos library that compiles <code><abbr>tz</abbr></code> source into a time
582 1.1 christos zone repository whose format
583 1.1 christos is either proprietary or an <abbr>XML</abbr>-encoded
584 1.1 christos representation.</li>
585 1.1 christos <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
586 1.1 christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
587 1.1 christos source into text files, along with a runtime that can read those
588 1.1 christos files. Tcl is freely available under a <abbr>BSD</abbr>-style
589 1.1 christos license.</li>
590 1.1 christos </ul>
591 1.6 christos </section>
592 1.6 christos
593 1.6 christos <section>
594 1.2 christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
595 1.1 christos <ul>
596 1.1 christos <li>The <a
597 1.1 christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
598 1.1 christos Library</a>
599 1.1 christos has an independent, thread-safe implementation of
600 1.2 christos a <abbr>TZif</abbr> file reader.
601 1.1 christos This library is freely available under the LGPL
602 1.1 christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
603 1.1 christos <li><a href="https://www.gnome.org">GNOME</a>'s
604 1.1 christos <a href="https://developer.gnome.org/glib/">GLib</a> has
605 1.2 christos a <abbr>TZif</abbr> file reader written in C that
606 1.1 christos creates a <code>GTimeZone</code> object representing sets
607 1.1 christos of <abbr>UT</abbr> offsets.
608 1.1 christos It is freely available under the <abbr>LGPL</abbr>.</li>
609 1.1 christos <li>The
610 1.1 christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
611 1.1 christos <code>baltzo::TimeZoneUtil</code> component contains a C++
612 1.2 christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under
613 1.1 christos the Apache License.</li>
614 1.1 christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
615 1.1 christos library that translates between <abbr>UT</abbr> and civil time and
616 1.2 christos can read <abbr>TZif</abbr> files. It is freely available under the Apache
617 1.1 christos License.</li>
618 1.16 christos <li>The
619 1.16 christos <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
620 1.16 christos package</a> contains Python code
621 1.16 christos to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
622 1.17 christos <code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
623 1.16 christos For example, it maps <code>"Africa/Cairo"</code>
624 1.16 christos to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
625 1.16 christos an approximation valid for Cairo timestamps from 2023 on.
626 1.17 christos This can help porting to platforms that support only proleptic TZ.
627 1.16 christos The package is freely available under the MIT license.</li>
628 1.1 christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
629 1.2 christos library that reads <abbr>TZif</abbr> files and converts
630 1.2 christos timestamps from one time zone or format to another.
631 1.1 christos It is used by <a href="https://secure.php.net"><abbr
632 1.1 christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
633 1.1 christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
634 1.1 christos and <a href="https://www.mongodb.com">MongoDB</a>.
635 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
636 1.1 christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
637 1.2 christos <abbr>TZif</abbr> file reader.</li>
638 1.4 christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
639 1.1 christos DateTime::TimeZone::Tzfile</a>
640 1.2 christos is a <abbr>TZif</abbr> file reader written in Perl.
641 1.1 christos It is freely available under the same terms as Perl
642 1.1 christos (dual <abbr>GPL</abbr> and Artistic license).</li>
643 1.9 christos <li>Python has a <a id="python-zoneinfo"
644 1.11 christos href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
645 1.8 christos class</a> that reads <abbr>TZif</abbr> data and creates objects
646 1.8 christos that represent <code><abbr>tzdb</abbr></code> timezones.
647 1.8 christos Python is freely available under the
648 1.11 christos <a href="https://docs.python.org/3/license.html">Python Software Foundation
649 1.8 christos License</a>.
650 1.17 christos A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
651 1.8 christos <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
652 1.8 christos supplies TZif data if the underlying system data cannot be found;
653 1.8 christos it is freely available under the Apache License.</li>
654 1.1 christos <li>The
655 1.1 christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
656 1.1 christos library contains a Python tool that
657 1.2 christos converts <abbr>TZif</abbr> data into
658 1.1 christos <abbr>JSON</abbr>-format data suitable for use
659 1.1 christos in its JavaScript library for time zone conversion. Dates before 1970
660 1.1 christos are not supported.</li>
661 1.1 christos <li>The <a
662 1.1 christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
663 1.1 christos package contains <a href="https://www.haskell.org">Haskell</a> code that
664 1.2 christos parses and uses <abbr>TZif</abbr> data. It is freely
665 1.1 christos available under a <abbr>BSD</abbr>-style license.</li>
666 1.1 christos </ul>
667 1.6 christos </section>
668 1.6 christos
669 1.6 christos <section>
670 1.1 christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
671 1.1 christos <ul>
672 1.1 christos <li><a href="https://foxclocks.org">FoxClocks</a>
673 1.1 christos is an extension for <a href="https://www.google.com/chrome/">Google
674 1.16 christos Chrome</a>, <a
675 1.1 christos href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
676 1.1 christos href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
677 1.1 christos It displays multiple clocks in the application window, and has a mapping
678 1.1 christos interface to <a href="https://www.google.com/earth/">Google Earth</a>.
679 1.1 christos It is freely available under the <abbr>GPL</abbr>.</li>
680 1.1 christos <li><a href="https://golang.org">Go programming language</a>
681 1.1 christos implementations contain a copy of a 32-bit subset of a recent
682 1.1 christos <code><abbr>tz</abbr></code> database in a
683 1.1 christos Go-specific format.</li>
684 1.1 christos <li>Microsoft Windows 8.1
685 1.1 christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
686 1.3 christos data (mentioned <a href="#CLDR">below</a>) used by the
687 1.3 christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
688 1.3 christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
689 1.3 christos <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
690 1.3 christos <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
691 1.1 christos <a id="System.TimeZoneInfo"
692 1.1 christos href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
693 1.1 christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
694 1.1 christos the older, proprietary method of Microsoft Windows 2000 and later,
695 1.1 christos which stores time zone data in the
696 1.1 christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
697 1.1 christos <a
698 1.1 christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
699 1.1 christos Tzid table</a> or <a
700 1.6 christos href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
701 1.1 christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
702 1.1 christos to <code><abbr>tz</abbr></code> names.
703 1.3 christos These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
704 1.3 christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
705 1.1 christos <li><a
706 1.1 christos href="https://www.oracle.com/java/index.html">Oracle
707 1.1 christos Java</a> contains a copy of a subset of a recent
708 1.1 christos <code><abbr>tz</abbr></code> database in a
709 1.1 christos Java-specific format.</li>
710 1.1 christos </ul>
711 1.6 christos </section>
712 1.6 christos
713 1.6 christos <section>
714 1.1 christos <h2 id="other-dbs">Other time zone databases</h2>
715 1.1 christos <ul>
716 1.1 christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
717 1.4 christos is Astrodienst's Web version of Shanks and Pottenger's out-of-print
718 1.4 christos time zone history atlases
719 1.4 christos <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
720 1.9 christos <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
721 1.4 christos Although these extensive atlases
722 1.4 christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
723 1.4 christos sources for much of the older <code><abbr>tz</abbr></code> data</a>,
724 1.4 christos they are unreliable as Shanks appears to have
725 1.1 christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
726 1.1 christos sources and do not indicate which entries are guesswork.</li>
727 1.1 christos <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
728 1.1 christos its own <code>tztab</code>(4) format.</li>
729 1.1 christos <li>Microsoft Windows has proprietary data mentioned
730 1.1 christos <a href="#System.TimeZoneInfo">above</a>.</li>
731 1.1 christos <li><a href="https://www.worldtimeserver.com">World Time Server</a>
732 1.1 christos is another time zone database.</li>
733 1.1 christos <li>The <a
734 1.1 christos href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
735 1.1 christos Schedules Information Manual</a> of the
736 1.1 christos International Air Transport Association
737 1.1 christos gives current time zone rules for airports served by commercial aviation.</li>
738 1.1 christos </ul>
739 1.6 christos </section>
740 1.6 christos
741 1.6 christos <section>
742 1.1 christos <h2 id="maps">Maps</h2>
743 1.1 christos <ul>
744 1.9 christos <li>The <a
745 1.9 christos href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
746 1.9 christos and Regional Maps section</a> of <em>The World Factbook</em>, published by the
747 1.9 christos <a href="https://www.cia.gov">US Central Intelligence
748 1.9 christos Agency (<abbr
749 1.9 christos title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
750 1.9 christos zone map; the
751 1.1 christos <a
752 1.6 christos href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
753 1.1 christos Library Map Collection</a>
754 1.1 christos of the University of Texas at Austin has copies of
755 1.1 christos recent editions.
756 1.1 christos The pictorial quality is good,
757 1.1 christos but the maps do not indicate daylight saving time,
758 1.1 christos and parts of the data are a few years out of date.</li>
759 1.6 christos <li><a href="https://www.worldtimezone.com">World Time Zone Map
760 1.6 christos with current time</a>
761 1.1 christos has several fancy time zone maps; it covers Russia particularly well.
762 1.1 christos The maps' pictorial quality is not quite as good as the
763 1.1 christos <abbr>CIA</abbr>'s
764 1.1 christos but the maps are more up to date.</li>
765 1.1 christos <li><a
766 1.1 christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
767 1.1 christos much is time wrong around the world?</a> maps the difference between
768 1.1 christos mean solar and standard time, highlighting areas such as western China
769 1.1 christos where the two differ greatly. It's a bit out of date, unfortunately.</li>
770 1.1 christos </ul>
771 1.6 christos </section>
772 1.6 christos
773 1.6 christos <section>
774 1.1 christos <h2 id="boundaries">Time zone boundaries</h2>
775 1.2 christos <p>Geographical boundaries between timezones are available
776 1.9 christos from several <a
777 1.9 christos href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
778 1.9 christos geolocation</a>
779 1.1 christos services and other sources.</p>
780 1.1 christos <ul>
781 1.1 christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
782 1.1 christos Boundary Builder</a> extracts
783 1.1 christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
784 1.2 christos boundaries of <code><abbr>tzdb</abbr></code> timezones.
785 1.1 christos Its code is freely available under the <abbr>MIT</abbr> license, and
786 1.1 christos its data entries are freely available under the
787 1.1 christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
788 1.16 christos Open Database License</a>. The borders appear to be quite accurate.
789 1.16 christos Its main web page lists more than twenty libraries
790 1.16 christos for looking up a timezone name from a GPS coordinate.</li>
791 1.1 christos <li>Free access via a network API, if you register a key, is provided by
792 1.4 christos the <a
793 1.4 christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
794 1.4 christos Timezone web service</a>, the <a
795 1.4 christos href="https://developers.google.com/maps/documentation/timezone/intro">Google
796 1.4 christos Maps Time Zone API</a>, and
797 1.8 christos the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
798 1.1 christos Commercial network API access is provided
799 1.1 christos by <a href="https://askgeo.com">AskGeo</a>
800 1.1 christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
801 1.1 christos </li>
802 1.1 christos <li>"<a
803 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
804 1.1 christos to get a time zone from a location using latitude and longitude
805 1.1 christos coordinates?</a>" discusses other geolocation possibilities.</li>
806 1.1 christos <li><a href="http://statoids.com/statoids.html">Administrative
807 1.1 christos Divisions of Countries ("Statoids")</a> lists
808 1.1 christos political subdivision data related to time zones.</li>
809 1.13 christos <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
810 1.1 christos – GIS and Database Tools</a> includes a Manifold-format map of
811 1.16 christos world time zone boundaries circa 2007, distributed under the
812 1.1 christos <abbr>GPL</abbr>.</li>
813 1.1 christos <li>A ship within the <a
814 1.1 christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
815 1.1 christos waters</a> of any nation uses that nation's time. In international
816 1.1 christos waters, time zone boundaries are meridians 15° apart, except that
817 1.1 christos <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
818 1.1 christos wide and are separated by
819 1.1 christos the 180° meridian (not by the International Date Line, which is
820 1.1 christos for land and territorial waters only). A captain can change ship's
821 1.1 christos clocks any time after entering a new time zone; midnight changes are
822 1.1 christos common.</li>
823 1.1 christos </ul>
824 1.6 christos </section>
825 1.6 christos
826 1.6 christos <section>
827 1.1 christos <h2 id="civil">Civil time concepts and history</h2>
828 1.1 christos <ul>
829 1.1 christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
830 1.1 christos Walk through Time</a>
831 1.1 christos surveys the evolution of timekeeping.</li>
832 1.4 christos <li>The history of daylight saving time is surveyed in <a
833 1.4 christos href="http://www.webexhibits.org/daylightsaving/">About Daylight
834 1.4 christos Saving Time – History, rationale, laws & dates</a> and summarized in
835 1.4 christos <a href="http://seizethedaylight.com/dst/">A Brief
836 1.4 christos History of Daylight Saving Time</a>.</li>
837 1.4 christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
838 1.4 christos Lords</a> discusses how authoritarians manipulate civil time.</li>
839 1.1 christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
840 1.1 christos contains guidelines and best practices for software applications that
841 1.1 christos deal with civil time.</li>
842 1.16 christos <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
843 1.1 christos the International Date Line</a> tells the story of the most important
844 1.1 christos time zone boundary.</li>
845 1.1 christos <li><a href="http://statoids.com/tconcept.html">Basic Time
846 1.1 christos Zone Concepts</a> discusses terminological issues behind time zones.</li>
847 1.1 christos </ul>
848 1.6 christos </section>
849 1.6 christos
850 1.6 christos <section>
851 1.1 christos <h2 id="national">National histories of legal time</h2>
852 1.1 christos <dl>
853 1.1 christos <dt>Australia</dt>
854 1.6 christos <dd>The Parliamentary Library commissioned a <a
855 1.16 christos href="https://parlinfo.aph.gov.au/parlInfo/download/library/prspub/359V6/upload_binary/359v60.pdf">research
856 1.1 christos paper on daylight saving time in Australia</a>.
857 1.1 christos The Bureau of Meteorology publishes a list of <a
858 1.1 christos href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
859 1.1 christos Dates of Daylight Savings Time within Australia</a>.</dd>
860 1.1 christos <dt>Belgium</dt>
861 1.7 christos <dd>The Royal Observatory of Belgium maintains a table of time in
862 1.7 christos Belgium (in
863 1.16 christos <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
864 1.7 christos hreflang="nl">Dutch</a> and <a
865 1.16 christos href="https://robinfo.oma.be/fr/astro-info/heure/"
866 1.7 christos hreflang="fr">French</a>).</dd>
867 1.1 christos <dt>Brazil</dt>
868 1.1 christos <dd>The Time Service Department of the National Observatory
869 1.1 christos records <a href="http://pcdsh01.on.br/DecHV.html"
870 1.1 christos hreflang="pt-BR">Brazil's daylight saving time decrees (in
871 1.1 christos Portuguese)</a>.</dd>
872 1.1 christos <dt>Canada</dt>
873 1.1 christos <dd>National Research Council Canada publishes current
874 1.1 christos and some older information about <a
875 1.6 christos href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
876 1.6 christos zones and daylight saving time</a>.</dd>
877 1.1 christos <dt>Chile</dt>
878 1.1 christos <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
879 1.11 christos <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
880 1.1 christos Chile's official time (in Spanish)</a>.</dd>
881 1.3 christos <dt>China</dt>
882 1.3 christos <dd>The Hong Kong Observatory maintains a
883 1.7 christos <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
884 1.3 christos summer time in Hong Kong</a>,
885 1.3 christos and Macau's Meteorological and Geophysical Bureau maintains a <a
886 1.7 christos href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
887 1.3 christos history for Macau</a>.
888 1.3 christos Unfortunately the latter is incomplete and has errors.</dd>
889 1.1 christos <dt>Czech Republic</dt>
890 1.1 christos <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
891 1.1 christos hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
892 1.10 christos summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
893 1.1 christos <dt>Germany</dt>
894 1.1 christos <dd>The National Institute for Science and Technology maintains the <a
895 1.1 christos href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
896 1.1 christos of Legal Time in Germany</a>.</dd>
897 1.1 christos <dt>Israel</dt>
898 1.17 christos <dd><a
899 1.17 christos href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
900 1.17 christos lists official time-change announcements and laws since 1940,
901 1.17 christos almost all in Hebrew.</dd>
902 1.2 christos <dt>Malaysia</dt>
903 1.2 christos <dd>See Singapore <a href="#Singapore">below</a>.</dd>
904 1.1 christos <dt>Mexico</dt>
905 1.1 christos <dd>The Investigation and Analysis Service of the Mexican Library of
906 1.1 christos Congress has published a <a
907 1.13 christos href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
908 1.1 christos hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
909 1.1 christos <dt>Netherlands</dt>
910 1.16 christos <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
911 1.1 christos hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
912 1.1 christos covers the history of local time in the Netherlands from ancient times.</dd>
913 1.1 christos <dt>New Zealand</dt>
914 1.1 christos <dd>The Department of Internal Affairs maintains a brief <a
915 1.1 christos href="https://www.dia.govt.nz/Daylight-Saving-History">History of
916 1.6 christos Daylight Saving</a>.</dd>
917 1.14 christos <dt>Palestine</dt>
918 1.14 christos <dd>The Ministry of Telecom and IT publishes a <a
919 1.16 christos href="https://mtit.pna.ps/home/TimeZone"
920 1.14 christos hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
921 1.10 christos <dt>Portugal</dt>
922 1.10 christos <dd>The Lisbon Astronomical Observatory publishes a
923 1.10 christos <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
924 1.10 christos legal time (in Portuguese)</a>.</dd>
925 1.1 christos <dt>Singapore</dt>
926 1.1 christos <dd><a id="Singapore"
927 1.7 christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
928 1.1 christos is Singapore in the "Wrong" Time Zone?</a> details the
929 1.1 christos history of legal time in Singapore and Malaysia.</dd>
930 1.1 christos <dt>United Kingdom</dt>
931 1.1 christos <dd><a
932 1.1 christos href="https://www.polyomino.org.uk/british-time/">History of
933 1.1 christos legal time in Britain</a> discusses in detail the country
934 1.6 christos with perhaps the best-documented history of clock adjustments.</dd>
935 1.1 christos <dt>United States</dt>
936 1.1 christos <dd>The Department of Transportation's <a
937 1.1 christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
938 1.1 christos Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
939 1.1 christos <dt>Uruguay</dt>
940 1.1 christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
941 1.1 christos Navy (SOHMA) publishes an annual <a
942 1.13 christos href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
943 1.1 christos (in Spanish)</a>.</dd>
944 1.1 christos </dl>
945 1.6 christos </section>
946 1.6 christos
947 1.6 christos <section>
948 1.7 christos <h2 id="costs">Costs and benefits of time shifts</h2>
949 1.7 christos <p>Various sources argue for and against daylight saving time and time
950 1.7 christos zone shifts, and many scientific studies have been conducted. This
951 1.8 christos section summarizes reviews and position statements based on
952 1.8 christos scientific literature in the area.</p>
953 1.7 christos <ul>
954 1.15 christos <li>In 2022 the American Medical Association issued a
955 1.15 christos <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
956 1.15 christos supporting permanent standard time</a> on health grounds.</li>
957 1.7 christos <li>Carey RN, Sarma KM.
958 1.7 christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
959 1.7 christos daylight saving time on road traffic collision risk: a systematic
960 1.7 christos review</a>.
961 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>.
962 1.7 christos This reviews research literature and concludes that the evidence
963 1.7 christos neither supports nor refutes road safety benefits from
964 1.7 christos shifts in time zones.</li>
965 1.7 christos <li>Havranek T, Herman D, Irsova D.
966 1.13 christos <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
967 1.16 christos daylight saving save electricity? A meta-analysis</a>.
968 1.13 christos <em>Energy J.</em> 2018;39(2):35–61.
969 1.7 christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
970 1.7 christos This analyzes research literature and concludes, "Electricity savings
971 1.7 christos are larger for countries farther away from the equator, while
972 1.10 christos subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
973 1.16 christos <li>Malow BA. <a
974 1.16 christos href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
975 1.16 christos to abolish the clock change and adopt permanent
976 1.16 christos standard time in the United States:
977 1.16 christos a Sleep Research Society position statement</a>.
978 1.15 christos <em>Sleep.</em> 2022;45(12):zsac236.
979 1.15 christos doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
980 1.15 christos After reviewing the scientific literature, the Sleep Research Society
981 1.15 christos advocates permanent standard time due to its health benefits.
982 1.15 christos <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
983 1.15 christos <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
984 1.15 christos is the optimal choice for health and safety:
985 1.8 christos an American Academy of Sleep Medicine position statement</a>.
986 1.16 christos <em>J Clin Sleep Med.</em> 2024;20(1):121–125.
987 1.15 christos doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
988 1.15 christos The AASM argues for permanent standard time due to health and safety risks
989 1.15 christos and economic costs of both <abbr>DST</abbr> transitions and
990 1.15 christos permanent <abbr>DST</abbr>.</li>
991 1.13 christos <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
992 1.16 christos <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
993 1.13 christos should we abolish Daylight Saving Time?</a>
994 1.15 christos <em>J Biol Rhythms.</em> 2019;34(3):227–230.
995 1.13 christos doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
996 1.15 christos The Society for Research on Biological Rhythms
997 1.13 christos opposes DST changes and permanent DST, and advocates that governments adopt
998 1.13 christos "permanent Standard Time for the health and safety of their citizens".</li>
999 1.7 christos </ul>
1000 1.7 christos </section>
1001 1.7 christos
1002 1.7 christos <section>
1003 1.1 christos <h2 id="precision">Precision timekeeping</h2>
1004 1.1 christos <ul>
1005 1.1 christos <li><a
1006 1.1 christos href="http://leapsecond.com/hpan/an1289.pdf">The
1007 1.1 christos Science of Timekeeping</a> is a thorough introduction
1008 1.1 christos to the theory and practice of precision timekeeping.</li>
1009 1.1 christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
1010 1.14 christos Time 2016</a> contains several freely readable papers.</li>
1011 1.13 christos <li><a href="https://www.ntp.org"><abbr
1012 1.1 christos title="Network Time Protocol">NTP</abbr>: The Network
1013 1.1 christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
1014 1.1 christos discusses how to synchronize clocks of
1015 1.1 christos Internet hosts.</li>
1016 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>
1017 1.2 christos family of software algorithms can achieve accuracy to a few tens of
1018 1.2 christos nanoseconds in scalable server farms without special hardware.</li>
1019 1.1 christos <li>The <a
1020 1.1 christos href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
1021 1.1 christos Time Protocol</a> (<abbr
1022 1.1 christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
1023 1.2 christos can achieve submicrosecond clock accuracy on a local area network
1024 1.2 christos with special-purpose hardware.</li>
1025 1.1 christos <li><a
1026 1.13 christos href="https://datatracker.ietf.org/doc/html/rfc4833">Timezone
1027 1.1 christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1028 1.1 christos (Internet <abbr>RFC</abbr> 4833)
1029 1.1 christos specifies a <a
1030 1.1 christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1031 1.1 christos option for a server to configure
1032 1.1 christos a client's time zone and daylight saving settings automatically.</li>
1033 1.11 christos <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1034 1.11 christos Scales</a> describes astronomical time scales like
1035 1.1 christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1036 1.1 christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1037 1.1 christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1038 1.1 christos <li>The <a href="https://www.iau.org"><abbr
1039 1.1 christos title="International Astronomical Union">IAU</abbr></a>'s <a
1040 1.13 christos href="https://www.iausofa.org"><abbr
1041 1.1 christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1042 1.1 christos collection contains C and <a
1043 1.1 christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1044 1.1 christos code for converting among time scales like
1045 1.1 christos <abbr title="International Atomic Time">TAI</abbr>,
1046 1.1 christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1047 1.13 christos <abbr>UTC</abbr>. It is freely available under the
1048 1.13 christos <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
1049 1.1 christos <li><a
1050 1.1 christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1051 1.1 christos – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1052 1.1 christos diverse local time
1053 1.1 christos scales used by each landed mission on Mars.</li>
1054 1.1 christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1055 1.1 christos dedicated not only to leap seconds but to precise time and frequency
1056 1.1 christos in general. It covers the state of the art in amateur timekeeping, and
1057 1.1 christos how the art has progressed over the past few decades.</li>
1058 1.7 christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1059 1.7 christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1060 1.7 christos and time-signal emissions</a>, International Telecommunication Union –
1061 1.7 christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1062 1.1 christos <li><a
1063 1.1 christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1064 1.1 christos title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1065 1.1 christos Bulletins</a> contains official publications of the International
1066 1.1 christos Earth Rotation and Reference Systems Service, which decides when leap
1067 1.4 christos seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1068 1.9 christos via an optional "<code>right</code>" configuration where a computer's internal
1069 1.9 christos <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1070 1.9 christos as opposed to the default "<code>posix</code>" configuration
1071 1.9 christos where the internal clock ignores leap seconds.
1072 1.9 christos The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1073 1.9 christos <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1074 1.9 christos timestamps starting with <code>time_t</code> 78 796 800,
1075 1.9 christos which corresponds to the first leap second
1076 1.9 christos 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1077 1.9 christos and to
1078 1.9 christos 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1079 1.9 christos In practice the two configurations also agree for timestamps before
1080 1.9 christos 1972 even though the historical situation is messy, partly because
1081 1.9 christos neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1082 1.14 christos is well-defined for sufficiently old timestamps.</li>
1083 1.17 christos <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
1084 1.17 christos <abbr>NTP</abbr> Leap Second File</a> covers the text file
1085 1.17 christos <code>leap-seconds.list</code>, which lists the currently known leap seconds.
1086 1.17 christos The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
1087 1.17 christos <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
1088 1.17 christos <a href="https://www.ntp.org">classic
1089 1.17 christos <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
1090 1.17 christos and <a href="https://ntpsec.org">NTPsec</a>.
1091 1.17 christos The <code><abbr>tz</abbr></code> database also distributes leap second
1092 1.17 christos information in a differently-formatted <code>leapseconds</code> text file,
1093 1.17 christos as well as in the "<code>right</code>" configuration in binary form; for
1094 1.17 christos example, <code>right/UTC</code> can be used
1095 1.17 christos by <a href="https://chrony-project.org"><code>chrony</code></a>,
1096 1.17 christos another <abbr>NTP</abbr> implementation.</li>
1097 1.1 christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1098 1.1 christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1099 1.1 christos leap second so that they disagree with <abbr>UTC</abbr> by at most a
1100 1.1 christos half second, even though every <abbr>POSIX</abbr> minute has exactly
1101 1.4 christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1102 1.1 christos "<code>posix</code>" configuration, is <a
1103 1.1 christos href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1104 1.17 christos the abovementioned <abbr>NTP</abbr> implementations, <a
1105 1.12 christos href="https://github.com/google/unsmear">supports</a> conversion between
1106 1.12 christos <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1107 1.7 christos cloud service providers. However, according to
1108 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">§3.7.1 of
1109 1.7 christos Network Time Protocol Best Current Practices</a>
1110 1.7 christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1111 1.9 christos applications requiring accurate <abbr>UTC</abbr> or civil time,
1112 1.7 christos and is intended for use only in single, well-controlled environments.</li>
1113 1.1 christos <li>The <a
1114 1.1 christos href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1115 1.1 christos Second Discussion List</a> covers <a
1116 1.1 christos href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1117 1.1 christos and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1118 1.1 christos discussed further in
1119 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1120 1.1 christos leap second: its history and possible future</a>.
1121 1.1 christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1122 1.1 christos might be redefined
1123 1.1 christos without Leap Seconds</a> gives pointers on this
1124 1.14 christos contentious issue.
1125 1.14 christos The General Conference on Weights and Measures
1126 1.16 christos <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
1127 1.17 christos to discontinue the use of leap seconds by 2035, and requested that no
1128 1.17 christos discontinuous adjustments be made to UTC for at least a century.
1129 1.16 christos The World Radiocommunication Conference <a
1130 1.16 christos href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
1131 1.16 christos in 2023</a> to cooperate with this process.
1132 1.17 christos <a href="https://www.preprints.org/manuscript/202406.0043/v1">A proposal
1133 1.17 christos to change the leap-second adjustments to Coordinated Universal Time</a>
1134 1.17 christos (doi:<a href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>)
1135 1.17 christos would replace leap seconds with 13-second leap smears occurring once per
1136 1.17 christos decade until 2100, with leap smears after that gradually increasing in size.
1137 1.17 christos However, there is still no consensus on whether this is the best way
1138 1.17 christos to replace leap seconds.
1139 1.14 christos </li>
1140 1.1 christos </ul>
1141 1.6 christos </section>
1142 1.6 christos
1143 1.6 christos <section>
1144 1.1 christos <h2 id="notation">Time notation</h2>
1145 1.1 christos <ul>
1146 1.13 christos <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1147 1.1 christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1148 1.1 christos zone names, abbreviations, identifiers, and formats. For example, it
1149 1.1 christos contains French translations for "Eastern European Summer Time",
1150 1.1 christos "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1151 1.1 christos "Bucharest". Its
1152 1.1 christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1153 1.1 christos charts</a> show these values for many locales. Data values are available in
1154 1.1 christos both <abbr title="Locale Data Markup Language">LDML</abbr>
1155 1.1 christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1156 1.1 christos <li>
1157 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1158 1.6 christos the international standard date and time notation</a> covers
1159 1.1 christos <a
1160 1.6 christos href="https://www.iso.org/standard/70907.html"><em><abbr
1161 1.6 christos title="International Organization for Standardization">ISO</abbr>
1162 1.6 christos 8601-1:2019 – Date and time – Representations for information
1163 1.6 christos interchange – Part 1: Basic rules</em></a>.</li>
1164 1.1 christos <li>
1165 1.6 christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1166 1.1 christos Schema: Datatypes – dateTime</a> specifies a format inspired by
1167 1.1 christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1168 1.13 christos <li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">§3.3 of
1169 1.1 christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1170 1.1 christos specifies the time notation used in email and <a
1171 1.1 christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1172 1.1 christos headers.</li>
1173 1.1 christos <li>
1174 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time
1175 1.1 christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1176 1.17 christos specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
1177 1.17 christos An extension, <a href="https://datatracker.ietf.org/doc/html/rfc9557">Date
1178 1.17 christos and Time on the Internet: Timestamps with Additional Information</a>
1179 1.17 christos (Internet <abbr>RFC</abbr> 9557) extends this profile
1180 1.17 christos to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
1181 1.17 christos via suffixes like "<code>[Asia/Tokyo]</code>".
1182 1.1 christos <li>
1183 1.7 christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1184 1.1 christos Formats on the Web</a> surveys web- and Internet-oriented date and time
1185 1.1 christos formats.</li>
1186 1.1 christos <li>Alphabetic time zone abbreviations should not be used as unique
1187 1.1 christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1188 1.1 christos practice. For example, in English-speaking North America
1189 1.1 christos "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1190 1.1 christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1191 1.1 christos and French-speaking North Americans prefer
1192 1.1 christos "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1193 1.1 christos "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1194 1.2 christos database contains English abbreviations for many timestamps;
1195 1.1 christos unfortunately some of these abbreviations were merely the database maintainers'
1196 1.1 christos inventions, and these have been removed when possible.</li>
1197 1.1 christos <li>Numeric time zone abbreviations typically count hours east of
1198 1.1 christos <abbr>UT</abbr>, e.g., +09 for Japan and
1199 1.17 christos −10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
1200 1.17 christos <code><abbr>TZ</abbr></code> settings use the opposite convention.
1201 1.1 christos For example, one might use <code><abbr>TZ</abbr>="<abbr
1202 1.1 christos title="Japan Standard Time">JST</abbr>-9"</code> and
1203 1.1 christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1204 1.1 christos for Japan and Hawaii, respectively. If the
1205 1.1 christos <code><abbr>tz</abbr></code> database is available, it is usually better to use
1206 1.1 christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1207 1.1 christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1208 1.2 christos confusion, handle old timestamps better, and insulate you better from
1209 1.1 christos any future changes to the rules. One should never set
1210 1.1 christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1211 1.1 christos <code>"GMT-9"</code>, though, since this would incorrectly imply that
1212 1.1 christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1213 1.1 christos is called "<abbr>GMT</abbr>".</li>
1214 1.1 christos </ul>
1215 1.6 christos </section>
1216 1.6 christos
1217 1.6 christos <section>
1218 1.1 christos <h2 id="see-also">See also</h2>
1219 1.1 christos <ul>
1220 1.7 christos <li><a href="theory.html">Theory and pragmatics of the
1221 1.7 christos <code><abbr>tz</abbr></code> code and data</a></li>
1222 1.1 christos <li><a href="tz-art.html">Time and the Arts</a></li>
1223 1.1 christos </ul>
1224 1.6 christos </section>
1225 1.6 christos
1226 1.6 christos <footer>
1227 1.1 christos <hr>
1228 1.1 christos This web page is in the public domain, so clarified as of
1229 1.1 christos 2009-05-17 by Arthur David Olson.
1230 1.1 christos <br>
1231 1.1 christos Please send corrections to this web page to the
1232 1.1 christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1233 1.6 christos </footer>
1234 1.1 christos </body>
1235 1.1 christos </html>
1236