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4 1.1 christos <title>Sources for time zone and daylight saving time data</title>
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10 1.1 christos <body>
11 1.1 christos <h1>Sources for 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 1.6 christos <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
30 1.6 christos </ul>
31 1.6 christos </li>
32 1.6 christos <li>Uses of the <code><abbr>tz</abbr></code> database
33 1.6 christos <ul>
34 1.6 christos <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
35 1.6 christos <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
36 1.6 christos <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
37 1.6 christos <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
38 1.6 christos <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
39 1.6 christos </ul>
40 1.6 christos </li>
41 1.6 christos <li>Related data
42 1.6 christos <ul>
43 1.6 christos <li><a href="#other-dbs">Other time zone databases</a></li>
44 1.6 christos <li><a href="#maps">Maps</a></li>
45 1.6 christos <li><a href="#boundaries">Time zone boundaries</a></li>
46 1.6 christos </ul>
47 1.6 christos </li>
48 1.6 christos <li>Timekeeping concepts
49 1.6 christos <ul>
50 1.6 christos <li><a href="#civil">Civil time concepts and history</a></li>
51 1.6 christos <li><a href="#national">National histories of legal time</a></li>
52 1.7 christos <li><a href="#costs">Costs and benefits of time shifts</a></li>
53 1.6 christos <li><a href="#precision">Precision timekeeping</a></li>
54 1.6 christos <li><a href="#notation">Time notation</a></li>
55 1.6 christos <li><a href="#see-also">See also</a></li>
56 1.6 christos </ul>
57 1.6 christos </li>
58 1.6 christos </ul>
59 1.6 christos </nav>
60 1.6 christos
61 1.6 christos <section>
62 1.1 christos <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
63 1.1 christos <p>
64 1.1 christos The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
65 1.1 christos time zone database contains code and data
66 1.1 christos that represent the history of local time
67 1.1 christos for many representative locations around the globe.
68 1.1 christos It is updated periodically to reflect changes made by political bodies
69 1.1 christos to time zone boundaries and daylight saving rules.
70 1.1 christos This database (known as <code><abbr>tz</abbr></code>,
71 1.1 christos <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
72 1.1 christos is used by several implementations,
73 1.1 christos including
74 1.1 christos <a href="https://www.gnu.org/software/libc/">the
75 1.1 christos <abbr title="GNU's Not Unix">GNU</abbr>
76 1.1 christos C Library</a> (used in
77 1.1 christos <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
78 1.1 christos <a href="https://www.android.com">Android</a>,
79 1.1 christos <a href="https://www.freebsd.org">Free<abbr
80 1.1 christos title="Berkeley Software Distribution">BSD</abbr></a>,
81 1.1 christos <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
82 1.1 christos <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
83 1.1 christos <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
84 1.1 christos <a href="https://cygwin.com">Cygwin</a>,
85 1.7 christos <a href="https://mariadb.org">MariaDB</a>,
86 1.1 christos <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
87 1.1 christos <a href="https://www.mysql.com">MySQL</a>,
88 1.1 christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
89 1.1 christos title="Web Operating System">webOS</abbr></a>,
90 1.7 christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
91 1.1 christos title="Advanced Interactive eXecutive">AIX</abbr></a>,
92 1.1 christos <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</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 1.1 christos title="Portable Operating System Interface">POSIX</abbr></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 1.1 christos Since version 2016h, each release has contained a text file named
171 1.1 christos "<samp>version</samp>" whose first (and currently only) line is the version.
172 1.7 christos Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
173 1.7 christos and are also available in an
174 1.1 christos <a href="ftp://ftp.iana.org/tz/releases/"><abbr
175 1.1 christos title="File Transfer Protocol">FTP</abbr> directory</a> via a
176 1.1 christos less-secure protocol.</p>
177 1.1 christos <p>Alternatively, a development repository of code and data can be
178 1.1 christos retrieved from <a href="https://github.com">GitHub</a> via the shell
179 1.1 christos command:</p>
180 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>
181 1.1 christos </code></pre>
182 1.1 christos <p>
183 1.1 christos Since version 2012e, each release has been tagged in development repositories.
184 1.1 christos Untagged commits are less well tested and probably contain
185 1.1 christos more errors.</p>
186 1.1 christos <p>
187 1.1 christos After obtaining the code and data files, see the
188 1.1 christos <code>README</code> file for what to do next.
189 1.1 christos The code lets you compile the <code><abbr>tz</abbr></code> source files into
190 1.2 christos machine-readable binary files, one for each location. The binary files
191 1.5 christos are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
192 1.5 christos specified by <a href="https://tools.ietf.org/html/8536">Internet
193 1.5 christos <abbr>RFC</abbr> 8536</a>.
194 1.2 christos The code also lets
195 1.2 christos you read a <abbr>TZif</abbr> file and interpret timestamps for that
196 1.1 christos location.</p>
197 1.6 christos </section>
198 1.6 christos
199 1.6 christos <section>
200 1.1 christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
201 1.1 christos <p>
202 1.1 christos The <code><abbr>tz</abbr></code> code and data
203 1.1 christos are by no means authoritative. If you find errors, please
204 1.1 christos send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
205 1.1 christos the time zone mailing list. You can also <a
206 1.1 christos href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
207 1.1 christos and browse the <a
208 1.1 christos href="https://mm.icann.org/pipermail/tz/">archive of old
209 1.5 christos messages</a>.
210 1.5 christos <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
211 1.5 christos discussions</a> and corresponding data changes can be
212 1.5 christos generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
213 1.5 christos </p>
214 1.1 christos <p>
215 1.1 christos If your government plans to change its time zone boundaries or
216 1.1 christos daylight saving rules, inform <code>tz (a] iana.org</code> well in
217 1.1 christos advance, as this will coordinate updates to many cell phones,
218 1.8 christos computers, and other devices around the world.
219 1.8 christos The change should be officially announced at least a year before it affects
220 1.8 christos how clocks operate; otherwise, there is a good chance that some
221 1.8 christos clocks will operate incorrectly after the change, due
222 1.1 christos to delays in propagating updates to software and data. The shorter
223 1.1 christos the notice, the more likely clock problems will arise; see "<a
224 1.1 christos href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
225 1.1 christos the Timing of Time Zone Changes</a>" for examples.
226 1.8 christos The <code><abbr>tz</abbr></code> data can represent planned changes
227 1.8 christos far into the future, and a long-planned change can easily be reverted
228 1.8 christos or otherwise altered with a year's notice before the change would have
229 1.8 christos affected clocks.
230 1.1 christos </p>
231 1.1 christos <p>
232 1.1 christos Changes to the <code><abbr>tz</abbr></code> code and data are often
233 1.1 christos propagated to clients via operating system updates, so
234 1.1 christos client <code><abbr>tz</abbr></code> data can often be corrected by
235 1.1 christos applying these updates. With GNU/Linux and similar systems, if your
236 1.1 christos maintenance provider has not yet adopted the
237 1.1 christos latest <code><abbr>tz</abbr></code> data, you can often short-circuit
238 1.1 christos the process by tailoring the generic instructions in
239 1.1 christos the <code><abbr>tz</abbr> README</code> file and installing the latest
240 1.1 christos data yourself. System-specific instructions for installing the
241 1.1 christos latest <code><abbr>tz</abbr></code> data have also been published
242 1.1 christos for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
243 1.1 christos <a
244 1.1 christos href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
245 1.1 christos <a
246 1.1 christos href="http://userguide.icu-project.org/datetime/timezone"><abbr
247 1.1 christos title="International Components for Unicode">ICU</abbr></a>,
248 1.9 christos <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr>
249 1.9 christos JDK</a>,
250 1.9 christos <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
251 1.1 christos href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
252 1.9 christos <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
253 1.9 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html#usage">OpenJDK/Oracle JDK</a>.
254 1.1 christos </p>
255 1.1 christos <p>Sources for the <code><abbr>tz</abbr></code> database are
256 1.1 christos <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
257 1.1 christos title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
258 1.1 christos <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
259 1.1 christos with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
260 1.1 christos title="linefeed">LF</abbr></a>,
261 1.1 christos which can be modified by common text editors such
262 1.1 christos as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
263 1.1 christos <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
264 1.1 christos <a href="https://www.vim.org">vim</a>.
265 1.1 christos Specialized source-file editing can be done via the
266 1.1 christos <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
267 1.1 christos zoneinfo</a> package for <a
268 1.1 christos href="https://www.sublimetext.com">Sublime Text</a> and the <a
269 1.1 christos href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
270 1.1 christos zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
271 1.1 christos Studio Code</a>.
272 1.1 christos </p>
273 1.1 christos <p>
274 1.1 christos For further information about updates, please see
275 1.1 christos <a href="https://tools.ietf.org/html/rfc6557">Procedures for
276 1.1 christos Maintaining the Time Zone Database</a> (Internet <abbr
277 1.1 christos title="Request For Comments">RFC</abbr> 6557). More detail can be
278 1.7 christos found in <a href="theory.html">Theory and pragmatics of the
279 1.7 christos <code><abbr>tz</abbr></code> code and data</a>.
280 1.2 christos <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
281 1.2 christos displays changes between recent <code><abbr>tzdb</abbr></code> versions.
282 1.1 christos </p>
283 1.6 christos </section>
284 1.6 christos
285 1.6 christos <section>
286 1.1 christos <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
287 1.1 christos <ul>
288 1.1 christos <li>The article
289 1.1 christos <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
290 1.1 christos an encyclopedic summary.</li>
291 1.1 christos <li><a href="tz-how-to.html">How to Read the
292 1.1 christos tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
293 1.1 christos database format.</li>
294 1.1 christos <li><a
295 1.1 christos href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
296 1.1 christos literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
297 1.1 christos database's style.</li>
298 1.1 christos </ul>
299 1.6 christos </section>
300 1.6 christos
301 1.6 christos <section>
302 1.1 christos <h2 id="web">Web sites using recent versions of the
303 1.1 christos <code><abbr>tz</abbr></code> database</h2>
304 1.1 christos <p>
305 1.1 christos These are listed roughly in ascending order of complexity and fanciness.
306 1.1 christos </p>
307 1.1 christos <ul>
308 1.1 christos <li><a href="https://time.is">Time.is</a> shows locations'
309 1.1 christos time and zones.</li>
310 1.1 christos <li><a href="https://www.timejones.com">TimeJones.com</a>,
311 1.1 christos <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
312 1.7 christos <a href="https://www.worldclock.com">The World Clock</a>
313 1.1 christos are time zone converters.</li>
314 1.8 christos <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
315 1.8 christos publishes <code><abbr>tzdb</abbr></code>-derived data in
316 1.8 christos <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
317 1.8 christos title="comma-separated values">CSV</abbr></a> and
318 1.8 christos in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
319 1.8 christos title="Structured Query Language">SQL</abbr></a> form.</li>
320 1.1 christos <li><a
321 1.6 christos href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
322 1.1 christos lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
323 1.1 christos <li><a
324 1.1 christos href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
325 1.1 christos Time in 1000 Places</a> uses descriptions of the values.</li>
326 1.6 christos <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
327 1.1 christos uses a pulldown menu.</li>
328 1.7 christos <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
329 1.1 christos timezone information for all countries</a> displays tables of DST rules.
330 1.1 christos <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
331 1.1 christos Worldwide</a> lets you sort zone names and convert times.</li>
332 1.1 christos <li><a href="https://24timezones.com">24TimeZones</a> has a world
333 1.1 christos time map and a time converter.</li>
334 1.1 christos <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
335 1.1 christos calculates the current time difference between locations.</li>
336 1.1 christos <li><a href="http://www.wx-now.com">Weather Now</a> and
337 1.1 christos <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
338 1.1 christos </ul>
339 1.6 christos </section>
340 1.6 christos
341 1.6 christos <section>
342 1.1 christos <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
343 1.1 christos <ul>
344 1.1 christos <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
345 1.1 christos <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
346 1.1 christos Distribution Service (tzdist) working group</a> defined <a
347 1.1 christos href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
348 1.1 christos (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
349 1.1 christos along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
350 1.1 christos (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
351 1.2 christos transferring time zone data by reference.
352 1.8 christos <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
353 1.8 christos implementations</a> are available.
354 1.2 christos The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
355 1.9 christos mailing list</a> discusses possible extensions.</li>
356 1.1 christos <li>The <a href="https://tools.ietf.org/html/rfc5545">
357 1.1 christos Internet Calendaring and Scheduling Core Object Specification
358 1.1 christos (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
359 1.1 christos covers time zone
360 1.1 christos data; see its VTIMEZONE calendar component.
361 1.1 christos The iCalendar format requires specialized parsers and generators; a
362 1.1 christos variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
363 1.1 christos (Internet <abbr>RFC</abbr> 6321) uses
364 1.1 christos <a href="https://www.w3.org/XML/"><abbr
365 1.1 christos title="Extensible Markup Language">XML</abbr></a> format, and a variant
366 1.1 christos <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
367 1.1 christos (Internet <abbr>RFC</abbr> 7265)
368 1.1 christos uses <a href="https://www.json.org"><abbr
369 1.1 christos title="JavaScript Object Notation">JSON</abbr></a> format.</li>
370 1.1 christos </ul>
371 1.6 christos </section>
372 1.6 christos
373 1.6 christos <section>
374 1.1 christos <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
375 1.7 christos <p>Although some of these do not fully support
376 1.7 christos <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
377 1.7 christos distributions you can generally work around compatibility problems by
378 1.7 christos running the command <code>make rearguard_tarballs</code> and compiling
379 1.7 christos from the resulting tarballs instead.</p>
380 1.1 christos <ul>
381 1.1 christos <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
382 1.1 christos href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
383 1.1 christos program that compiles
384 1.1 christos <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
385 1.1 christos Vzic is freely
386 1.1 christos available under the <a
387 1.1 christos href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
388 1.1 christos General Public License (<abbr
389 1.1 christos title="General Public License">GPL</abbr>)</a>.</li>
390 1.1 christos <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
391 1.1 christos database conversion utility</a> is like Vzic, except for the <a
392 1.6 christos href="https://dotnet.microsoft.com">.NET framework</a>
393 1.1 christos and with a <abbr>BSD</abbr>-style license.</li>
394 1.1 christos <li><a
395 1.4 christos href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
396 1.1 christos contains a script <code>parse_olson</code> that compiles
397 1.1 christos <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
398 1.1 christos modules. It is part of the Perl <a
399 1.4 christos href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
400 1.4 christos which is freely
401 1.1 christos available under both the <abbr>GPL</abbr> and the Perl Artistic
402 1.1 christos License. DateTime::TimeZone also contains a script
403 1.1 christos <code>tests_from_zdump</code> that generates test cases for each clock
404 1.1 christos transition in the <code><abbr>tz</abbr></code> database.</li>
405 1.1 christos <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
406 1.1 christos Database Parser</a> is a
407 1.1 christos <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
408 1.9 christos runtime library with <a
409 1.9 christos href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
410 1.9 christos adopted by
411 1.6 christos <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
412 1.9 christos the current iteration of the C++ standard.
413 1.1 christos It is freely available under the
414 1.1 christos <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
415 1.1 christos <li><a id="ICU" href="http://site.icu-project.org">International Components for
416 1.1 christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
417 1.1 christos href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
418 1.1 christos libraries for internationalization that
419 1.1 christos has a compiler from <code><abbr>tz</abbr></code> source
420 1.1 christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
421 1.1 christos (mentioned <a href="#CLDR">below</a>)
422 1.1 christos into an <abbr>ICU</abbr>-specific format.
423 1.1 christos <abbr>ICU</abbr> is freely available under a
424 1.1 christos <abbr>BSD</abbr>-style license.</li>
425 1.1 christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
426 1.1 christos the <a href="https://elixir-lang.org">Elixir</a> language downloads
427 1.7 christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
428 1.1 christos title="Application Program Interface">API</abbr>s for use. It is
429 1.1 christos freely available under the <abbr>MIT</abbr> license.</li>
430 1.1 christos <li>Java-based compilers and libraries include:
431 1.1 christos <ul>
432 1.1 christos <li>The <a
433 1.9 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
434 1.1 christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
435 1.9 christos <a href="https://openjdk.java.net/">OpenJDK</a> and
436 1.9 christos <a href="https://jdk.java.net/">Oracle JDK</a>.
437 1.9 christos Although its source code is proprietary, its executable is available under the
438 1.9 christos <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
439 1.9 christos Timezone Updater License Agreement</a>.</li>
440 1.1 christos <li>The <a
441 1.4 christos href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
442 1.6 christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
443 1.4 christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
444 1.1 christos which is freely available under a <abbr>BSD</abbr>-style license.</li>
445 1.4 christos <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
446 1.1 christos and time <abbr>API</abbr></a> contains a class
447 1.1 christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
448 1.1 christos <code><abbr>tz</abbr></code> source into a binary format. It inspired
449 1.1 christos Java 8 <code>java.time</code>, which its users should migrate to once
450 1.1 christos they can assume Java 8 or later. It is available under the <a
451 1.1 christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
452 1.9 christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
453 1.7 christos href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
454 1.9 christos are alternatives to TZUpdater. IANA Updater's license is unclear;
455 1.9 christos ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
456 1.4 christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
457 1.4 christos time library for Android</a> and
458 1.4 christos <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
459 1.4 christos time and interval library for Java</a> compile
460 1.4 christos <code><abbr>tz</abbr></code> source into a binary format.
461 1.4 christos Time4A is available under the Apache License and Time4J is
462 1.1 christos available under the <a
463 1.1 christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
464 1.1 christos General Public License (<abbr title="Lesser General Public
465 1.1 christos License">LGPL</abbr>)</a>.</li>
466 1.1 christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
467 1.1 christos Java-based libraries.</li>
468 1.1 christos </ul>
469 1.1 christos <li><a href="https://nodatime.org">Noda Time – Date and
470 1.1 christos time <abbr>API</abbr> for .NET</a>
471 1.4 christos is like Joda-Time and Time4J, but for the .NET framework instead of Java.
472 1.4 christos It is freely available under the Apache License.</li>
473 1.8 christos <li>Many modern
474 1.8 christos <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
475 1.8 christos runtimes support <code><abbr>tz</abbr></code> natively via the
476 1.8 christos <samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>.
477 1.8 christos On other runtimes, you can use one of the following.
478 1.1 christos <ul>
479 1.8 christos <li>The <a
480 1.8 christos href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
481 1.8 christos timezone polyfill</a>
482 1.8 christos is freely available under a <abbr>BSD</abbr>-style license.</li>
483 1.1 christos <li><a
484 1.1 christos href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
485 1.1 christos compiles time zone data into a compact form designed for
486 1.1 christos JavaScript. It is freely available under a combination of
487 1.1 christos the <abbr>MIT</abbr> license and the Apache License.</li>
488 1.1 christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
489 1.1 christos plugin for the <a href="https://momentjs.com">Moment.js</a> date
490 1.1 christos manipulation library. It is freely available under the <abbr>MIT</abbr>
491 1.1 christos license.</li>
492 1.8 christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
493 1.8 christos JavaScript library that supports date arithmetic that is time zone
494 1.8 christos aware. It is freely available under the <abbr>MIT</abbr> license.</li>
495 1.1 christos <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
496 1.1 christos <abbr>API</abbr> is upward compatible with standard JavaScript
497 1.1 christos Dates. It is freely available under the Apache License.</li>
498 1.1 christos </ul>
499 1.1 christos <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
500 1.1 christos compiler from <code><abbr>tz</abbr></code> source into
501 1.1 christos <a href="https://julialang.org/">Julia</a>. It is freely available
502 1.1 christos under the <abbr>MIT</abbr> license.</li>
503 1.6 christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
504 1.6 christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
505 1.6 christos title="Free Pascal Compiler">FPC</abbr></a>
506 1.2 christos compiles from <code><abbr>tz</abbr></code> source into
507 1.2 christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
508 1.2 christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
509 1.2 christos and <a
510 1.2 christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
511 1.2 christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
512 1.1 christos <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
513 1.1 christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
514 1.1 christos <a href="https://www.python.org">Python</a>.
515 1.9 christos It is freely available under a <abbr>BSD</abbr>-style license.
516 1.9 christos In code that can assume Python 3.9 or later it is superseded by
517 1.9 christos <a href="#python-zoneinfo"><code>zoneinfo.ZoneInfo</code></a>
518 1.9 christos and the <a href="#pypi-tzdata"><code>tzdata</code> Python package</a>.</li>
519 1.1 christos <li><a href="https://tzinfo.github.io">TZInfo –
520 1.1 christos Ruby Timezone Library</a>
521 1.1 christos compiles <code><abbr>tz</abbr></code> source into
522 1.1 christos <a href="https://www.ruby-lang.org/en/">Ruby</a>.
523 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
524 1.1 christos <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
525 1.1 christos Library</a> is
526 1.1 christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
527 1.1 christos library that compiles <code><abbr>tz</abbr></code> source into a time
528 1.1 christos zone repository whose format
529 1.1 christos is either proprietary or an <abbr>XML</abbr>-encoded
530 1.1 christos representation.</li>
531 1.1 christos <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
532 1.1 christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
533 1.1 christos source into text files, along with a runtime that can read those
534 1.1 christos files. Tcl is freely available under a <abbr>BSD</abbr>-style
535 1.1 christos license.</li>
536 1.1 christos </ul>
537 1.6 christos </section>
538 1.6 christos
539 1.6 christos <section>
540 1.2 christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
541 1.1 christos <ul>
542 1.1 christos <li>The <a
543 1.1 christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
544 1.1 christos Library</a>
545 1.1 christos has an independent, thread-safe implementation of
546 1.2 christos a <abbr>TZif</abbr> file reader.
547 1.1 christos This library is freely available under the LGPL
548 1.1 christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
549 1.1 christos <li><a href="https://www.gnome.org">GNOME</a>'s
550 1.1 christos <a href="https://developer.gnome.org/glib/">GLib</a> has
551 1.2 christos a <abbr>TZif</abbr> file reader written in C that
552 1.1 christos creates a <code>GTimeZone</code> object representing sets
553 1.1 christos of <abbr>UT</abbr> offsets.
554 1.1 christos It is freely available under the <abbr>LGPL</abbr>.</li>
555 1.1 christos <li>The
556 1.1 christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
557 1.1 christos <code>baltzo::TimeZoneUtil</code> component contains a C++
558 1.2 christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under
559 1.1 christos the Apache License.</li>
560 1.1 christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
561 1.1 christos library that translates between <abbr>UT</abbr> and civil time and
562 1.2 christos can read <abbr>TZif</abbr> files. It is freely available under the Apache
563 1.1 christos License.</li>
564 1.1 christos <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
565 1.2 christos is a <abbr>TZif</abbr> file reader written in Java.
566 1.1 christos It is freely available under the <abbr>LGPL</abbr>.</li>
567 1.1 christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
568 1.2 christos library that reads <abbr>TZif</abbr> files and converts
569 1.2 christos timestamps from one time zone or format to another.
570 1.1 christos It is used by <a href="https://secure.php.net"><abbr
571 1.1 christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
572 1.1 christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
573 1.1 christos and <a href="https://www.mongodb.com">MongoDB</a>.
574 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
575 1.1 christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
576 1.2 christos <abbr>TZif</abbr> file reader.</li>
577 1.4 christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
578 1.1 christos DateTime::TimeZone::Tzfile</a>
579 1.2 christos is a <abbr>TZif</abbr> file reader written in Perl.
580 1.1 christos It is freely available under the same terms as Perl
581 1.1 christos (dual <abbr>GPL</abbr> and Artistic license).</li>
582 1.9 christos <li>Python has a <a id="python-zoneinfo"
583 1.8 christos href="https://docs.python.org/3.9/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
584 1.8 christos class</a> that reads <abbr>TZif</abbr> data and creates objects
585 1.8 christos that represent <code><abbr>tzdb</abbr></code> timezones.
586 1.8 christos Python is freely available under the
587 1.8 christos <a href="https://docs.python.org/3.9/license.html">Python Software Foundation
588 1.8 christos License</a>.
589 1.9 christos A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
590 1.8 christos <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
591 1.8 christos supplies TZif data if the underlying system data cannot be found;
592 1.8 christos it is freely available under the Apache License.</li>
593 1.1 christos <li>The
594 1.1 christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
595 1.1 christos library contains a Python tool that
596 1.2 christos converts <abbr>TZif</abbr> data into
597 1.1 christos <abbr>JSON</abbr>-format data suitable for use
598 1.1 christos in its JavaScript library for time zone conversion. Dates before 1970
599 1.1 christos are not supported.</li>
600 1.1 christos <li>The <a
601 1.1 christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
602 1.1 christos package contains <a href="https://www.haskell.org">Haskell</a> code that
603 1.2 christos parses and uses <abbr>TZif</abbr> data. It is freely
604 1.1 christos available under a <abbr>BSD</abbr>-style license.</li>
605 1.1 christos </ul>
606 1.6 christos </section>
607 1.6 christos
608 1.6 christos <section>
609 1.1 christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
610 1.1 christos <ul>
611 1.1 christos <li><a href="https://foxclocks.org">FoxClocks</a>
612 1.1 christos is an extension for <a href="https://www.google.com/chrome/">Google
613 1.1 christos Chrome</a> and for <a
614 1.1 christos href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
615 1.1 christos Toolkit</a> applications like <a
616 1.1 christos href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
617 1.1 christos href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
618 1.1 christos It displays multiple clocks in the application window, and has a mapping
619 1.1 christos interface to <a href="https://www.google.com/earth/">Google Earth</a>.
620 1.1 christos It is freely available under the <abbr>GPL</abbr>.</li>
621 1.1 christos <li><a href="https://golang.org">Go programming language</a>
622 1.1 christos implementations contain a copy of a 32-bit subset of a recent
623 1.1 christos <code><abbr>tz</abbr></code> database in a
624 1.1 christos Go-specific format.</li>
625 1.1 christos <li>Microsoft Windows 8.1
626 1.1 christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
627 1.3 christos data (mentioned <a href="#CLDR">below</a>) used by the
628 1.3 christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
629 1.3 christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
630 1.3 christos <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
631 1.3 christos <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
632 1.1 christos <a id="System.TimeZoneInfo"
633 1.1 christos href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
634 1.1 christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
635 1.1 christos the older, proprietary method of Microsoft Windows 2000 and later,
636 1.1 christos which stores time zone data in the
637 1.1 christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
638 1.1 christos <a
639 1.1 christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
640 1.1 christos Tzid table</a> or <a
641 1.6 christos href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
642 1.1 christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
643 1.1 christos to <code><abbr>tz</abbr></code> names.
644 1.3 christos These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
645 1.3 christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
646 1.1 christos <li><a
647 1.1 christos href="https://www.oracle.com/java/index.html">Oracle
648 1.1 christos Java</a> contains a copy of a subset of a recent
649 1.1 christos <code><abbr>tz</abbr></code> database in a
650 1.1 christos Java-specific format.</li>
651 1.2 christos <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
652 1.1 christos Master</a> is a Microsoft Windows clock program that can automatically
653 1.4 christos download, compile and use <code><abbr>tz</abbr></code> releases.
654 1.4 christos The Basic version is free.</li>
655 1.1 christos <li><a
656 1.1 christos href="http://veladg.com/velaterra.html">VelaTerra</a> is
657 1.1 christos a macOS program. Its developers
658 1.1 christos <a href="http://veladg.com/tzoffer.html">offer free
659 1.1 christos licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
660 1.1 christos </ul>
661 1.6 christos </section>
662 1.6 christos
663 1.6 christos <section>
664 1.1 christos <h2 id="other-dbs">Other time zone databases</h2>
665 1.1 christos <ul>
666 1.1 christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
667 1.4 christos is Astrodienst's Web version of Shanks and Pottenger's out-of-print
668 1.4 christos time zone history atlases
669 1.4 christos <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
670 1.9 christos <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
671 1.4 christos Although these extensive atlases
672 1.4 christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
673 1.4 christos sources for much of the older <code><abbr>tz</abbr></code> data</a>,
674 1.4 christos they are unreliable as Shanks appears to have
675 1.1 christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
676 1.1 christos sources and do not indicate which entries are guesswork.</li>
677 1.1 christos <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
678 1.1 christos its own <code>tztab</code>(4) format.</li>
679 1.1 christos <li>Microsoft Windows has proprietary data mentioned
680 1.1 christos <a href="#System.TimeZoneInfo">above</a>.</li>
681 1.1 christos <li><a href="https://www.worldtimeserver.com">World Time Server</a>
682 1.1 christos is another time zone database.</li>
683 1.1 christos <li>The <a
684 1.1 christos href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
685 1.1 christos Schedules Information Manual</a> of the
686 1.1 christos International Air Transport Association
687 1.1 christos gives current time zone rules for airports served by commercial aviation.</li>
688 1.1 christos </ul>
689 1.6 christos </section>
690 1.6 christos
691 1.6 christos <section>
692 1.1 christos <h2 id="maps">Maps</h2>
693 1.1 christos <ul>
694 1.9 christos <li>The <a
695 1.9 christos href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
696 1.9 christos and Regional Maps section</a> of <em>The World Factbook</em>, published by the
697 1.9 christos <a href="https://www.cia.gov">US Central Intelligence
698 1.9 christos Agency (<abbr
699 1.9 christos title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
700 1.9 christos zone map; the
701 1.1 christos <a
702 1.6 christos href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
703 1.1 christos Library Map Collection</a>
704 1.1 christos of the University of Texas at Austin has copies of
705 1.1 christos recent editions.
706 1.1 christos The pictorial quality is good,
707 1.1 christos but the maps do not indicate daylight saving time,
708 1.1 christos and parts of the data are a few years out of date.</li>
709 1.6 christos <li><a href="https://www.worldtimezone.com">World Time Zone Map
710 1.6 christos with current time</a>
711 1.1 christos has several fancy time zone maps; it covers Russia particularly well.
712 1.1 christos The maps' pictorial quality is not quite as good as the
713 1.1 christos <abbr>CIA</abbr>'s
714 1.1 christos but the maps are more up to date.</li>
715 1.1 christos <li><a
716 1.1 christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
717 1.1 christos much is time wrong around the world?</a> maps the difference between
718 1.1 christos mean solar and standard time, highlighting areas such as western China
719 1.1 christos where the two differ greatly. It's a bit out of date, unfortunately.</li>
720 1.1 christos </ul>
721 1.6 christos </section>
722 1.6 christos
723 1.6 christos <section>
724 1.1 christos <h2 id="boundaries">Time zone boundaries</h2>
725 1.2 christos <p>Geographical boundaries between timezones are available
726 1.9 christos from several <a
727 1.9 christos href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
728 1.9 christos geolocation</a>
729 1.1 christos services and other sources.</p>
730 1.1 christos <ul>
731 1.1 christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
732 1.1 christos Boundary Builder</a> extracts
733 1.1 christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
734 1.2 christos boundaries of <code><abbr>tzdb</abbr></code> timezones.
735 1.1 christos Its code is freely available under the <abbr>MIT</abbr> license, and
736 1.1 christos its data entries are freely available under the
737 1.1 christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
738 1.1 christos Open Database License</a>. The maps' borders appear to be quite accurate.</li>
739 1.1 christos <li>Programmatic interfaces that map geographical coordinates via tz_world to
740 1.2 christos <code><abbr>tzdb</abbr></code> timezones include:
741 1.1 christos <ul>
742 1.1 christos <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
743 1.1 christos written in <a
744 1.1 christos href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
745 1.1 christos and is freely available under the <abbr>MIT</abbr> license.</li>
746 1.1 christos <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
747 1.1 christos is written in Go and is freely available under the Apache License.</li>
748 1.1 christos <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
749 1.1 christos in both Java and
750 1.1 christos <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
751 1.1 christos form, is freely available under the MIT license.</li>
752 1.1 christos <li>For <a href="https://nodejs.org/en/">Node.js</a>,
753 1.1 christos the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
754 1.1 christos is freely available under the MIT license, and
755 1.1 christos the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
756 1.1 christos is in the public domain.</li>
757 1.1 christos <li>The <a
758 1.1 christos href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
759 1.1 christos library for Python is freely available under the MIT license.
760 1.1 christos <li>The <a
761 1.1 christos href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
762 1.1 christos library for Ruby is freely available under the MIT license.</li>
763 1.1 christos </ul></li>
764 1.1 christos <li>Free access via a network API, if you register a key, is provided by
765 1.4 christos the <a
766 1.4 christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
767 1.4 christos Timezone web service</a>, the <a
768 1.4 christos href="https://developers.google.com/maps/documentation/timezone/intro">Google
769 1.4 christos Maps Time Zone API</a>, and
770 1.8 christos the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
771 1.1 christos Commercial network API access is provided
772 1.1 christos by <a href="https://askgeo.com">AskGeo</a>
773 1.1 christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
774 1.1 christos </li>
775 1.1 christos <li>"<a
776 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
777 1.1 christos to get a time zone from a location using latitude and longitude
778 1.1 christos coordinates?</a>" discusses other geolocation possibilities.</li>
779 1.1 christos <li><a href="http://statoids.com/statoids.html">Administrative
780 1.1 christos Divisions of Countries ("Statoids")</a> lists
781 1.1 christos political subdivision data related to time zones.</li>
782 1.7 christos <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
783 1.1 christos zone boundaries for multizone countries</a> summarizes legal
784 1.1 christos boundaries between time zones within countries.</li>
785 1.1 christos <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
786 1.1 christos – GIS and Database Tools</a> includes a Manifold-format map of
787 1.1 christos world time zone boundaries distributed under the
788 1.1 christos <abbr>GPL</abbr>.</li>
789 1.1 christos <li>A ship within the <a
790 1.1 christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
791 1.1 christos waters</a> of any nation uses that nation's time. In international
792 1.1 christos waters, time zone boundaries are meridians 15° apart, except that
793 1.1 christos <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
794 1.1 christos wide and are separated by
795 1.1 christos the 180° meridian (not by the International Date Line, which is
796 1.1 christos for land and territorial waters only). A captain can change ship's
797 1.1 christos clocks any time after entering a new time zone; midnight changes are
798 1.1 christos common.</li>
799 1.1 christos </ul>
800 1.6 christos </section>
801 1.6 christos
802 1.6 christos <section>
803 1.1 christos <h2 id="civil">Civil time concepts and history</h2>
804 1.1 christos <ul>
805 1.1 christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
806 1.1 christos Walk through Time</a>
807 1.1 christos surveys the evolution of timekeeping.</li>
808 1.4 christos <li>The history of daylight saving time is surveyed in <a
809 1.4 christos href="http://www.webexhibits.org/daylightsaving/">About Daylight
810 1.4 christos Saving Time – History, rationale, laws & dates</a> and summarized in
811 1.4 christos <a href="http://seizethedaylight.com/dst/">A Brief
812 1.4 christos History of Daylight Saving Time</a>.</li>
813 1.4 christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
814 1.4 christos Lords</a> discusses how authoritarians manipulate civil time.</li>
815 1.1 christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
816 1.1 christos contains guidelines and best practices for software applications that
817 1.1 christos deal with civil time.</li>
818 1.1 christos <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
819 1.1 christos the International Date Line</a> tells the story of the most important
820 1.1 christos time zone boundary.</li>
821 1.1 christos <li><a href="http://statoids.com/tconcept.html">Basic Time
822 1.1 christos Zone Concepts</a> discusses terminological issues behind time zones.</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="national">National histories of legal time</h2>
828 1.1 christos <dl>
829 1.1 christos <dt>Australia</dt>
830 1.6 christos <dd>The Parliamentary Library commissioned a <a
831 1.1 christos href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
832 1.1 christos paper on daylight saving time in Australia</a>.
833 1.1 christos The Bureau of Meteorology publishes a list of <a
834 1.1 christos href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
835 1.1 christos Dates of Daylight Savings Time within Australia</a>.</dd>
836 1.1 christos <dt>Belgium</dt>
837 1.7 christos <dd>The Royal Observatory of Belgium maintains a table of time in
838 1.7 christos Belgium (in
839 1.7 christos <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
840 1.7 christos hreflang="nl">Dutch</a> and <a
841 1.7 christos href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
842 1.7 christos hreflang="fr">French</a>).</dd>
843 1.1 christos <dt>Brazil</dt>
844 1.1 christos <dd>The Time Service Department of the National Observatory
845 1.1 christos records <a href="http://pcdsh01.on.br/DecHV.html"
846 1.1 christos hreflang="pt-BR">Brazil's daylight saving time decrees (in
847 1.1 christos Portuguese)</a>.</dd>
848 1.1 christos <dt>Canada</dt>
849 1.1 christos <dd>National Research Council Canada publishes current
850 1.1 christos and some older information about <a
851 1.6 christos href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
852 1.6 christos zones and daylight saving time</a>.</dd>
853 1.1 christos <dt>Chile</dt>
854 1.1 christos <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
855 1.4 christos <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
856 1.1 christos Chile's official time (in Spanish)</a>.</dd>
857 1.3 christos <dt>China</dt>
858 1.3 christos <dd>The Hong Kong Observatory maintains a
859 1.7 christos <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
860 1.3 christos summer time in Hong Kong</a>,
861 1.3 christos and Macau's Meteorological and Geophysical Bureau maintains a <a
862 1.7 christos href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
863 1.3 christos history for Macau</a>.
864 1.3 christos Unfortunately the latter is incomplete and has errors.</dd>
865 1.1 christos <dt>Czech Republic</dt>
866 1.1 christos <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
867 1.1 christos hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
868 1.1 christos summarizes and cites historical DST regulations.</dd>
869 1.1 christos <dt>Germany</dt>
870 1.1 christos <dd>The National Institute for Science and Technology maintains the <a
871 1.1 christos href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
872 1.1 christos of Legal Time in Germany</a>.</dd>
873 1.1 christos <dt>Israel</dt>
874 1.1 christos <dd>The Interior Ministry periodically issues <a
875 1.1 christos href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
876 1.1 christos hreflang="he">announcements (in Hebrew)</a>.</dd>
877 1.2 christos <dt>Malaysia</dt>
878 1.2 christos <dd>See Singapore <a href="#Singapore">below</a>.</dd>
879 1.1 christos <dt>Mexico</dt>
880 1.1 christos <dd>The Investigation and Analysis Service of the Mexican Library of
881 1.1 christos Congress has published a <a
882 1.1 christos href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
883 1.1 christos hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
884 1.1 christos <dt>Netherlands</dt>
885 1.1 christos <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
886 1.1 christos hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
887 1.1 christos covers the history of local time in the Netherlands from ancient times.</dd>
888 1.1 christos <dt>New Zealand</dt>
889 1.1 christos <dd>The Department of Internal Affairs maintains a brief <a
890 1.1 christos href="https://www.dia.govt.nz/Daylight-Saving-History">History of
891 1.6 christos Daylight Saving</a>.</dd>
892 1.1 christos <dt>Singapore</dt>
893 1.1 christos <dd><a id="Singapore"
894 1.7 christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
895 1.1 christos is Singapore in the "Wrong" Time Zone?</a> details the
896 1.1 christos history of legal time in Singapore and Malaysia.</dd>
897 1.1 christos <dt>United Kingdom</dt>
898 1.1 christos <dd><a
899 1.1 christos href="https://www.polyomino.org.uk/british-time/">History of
900 1.1 christos legal time in Britain</a> discusses in detail the country
901 1.6 christos with perhaps the best-documented history of clock adjustments.</dd>
902 1.1 christos <dt>United States</dt>
903 1.1 christos <dd>The Department of Transportation's <a
904 1.1 christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
905 1.1 christos Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
906 1.1 christos <dt>Uruguay</dt>
907 1.1 christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
908 1.1 christos Navy (SOHMA) publishes an annual <a
909 1.7 christos href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
910 1.1 christos (in Spanish)</a>.</dd>
911 1.1 christos </dl>
912 1.6 christos </section>
913 1.6 christos
914 1.6 christos <section>
915 1.7 christos <h2 id="costs">Costs and benefits of time shifts</h2>
916 1.7 christos <p>Various sources argue for and against daylight saving time and time
917 1.7 christos zone shifts, and many scientific studies have been conducted. This
918 1.8 christos section summarizes reviews and position statements based on
919 1.8 christos scientific literature in the area.</p>
920 1.7 christos <ul>
921 1.7 christos <li>Carey RN, Sarma KM.
922 1.7 christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
923 1.7 christos daylight saving time on road traffic collision risk: a systematic
924 1.7 christos review</a>.
925 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>.
926 1.7 christos This reviews research literature and concludes that the evidence
927 1.7 christos neither supports nor refutes road safety benefits from
928 1.7 christos shifts in time zones.</li>
929 1.7 christos <li>Havranek T, Herman D, Irsova D.
930 1.7 christos <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
931 1.7 christos saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
932 1.7 christos 2018;39(2).
933 1.7 christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
934 1.7 christos This analyzes research literature and concludes, "Electricity savings
935 1.7 christos are larger for countries farther away from the equator, while
936 1.7 christos subtropical regions consume more electricity because of DST."</li>
937 1.8 christos <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
938 1.8 christos <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
939 1.8 christos an American Academy of Sleep Medicine position statement</a>.
940 1.8 christos <em>J Clin Sleep Med.</em>
941 1.8 christos 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
942 1.8 christos This argues for permanent standard time due to health risks of both
943 1.8 christos DST transitions and permanent DST.</li>
944 1.7 christos <li>Roenneberg T, Winnebeck EC, Klerman EB.
945 1.7 christos <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
946 1.7 christos saving time and artificial time zones – a battle between
947 1.7 christos biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
948 1.7 christos doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
949 1.7 christos This reviews evidence about the health effects of DST and concludes,
950 1.7 christos "In summary, the scientific literature strongly argues against the
951 1.7 christos switching between DST and Standard Time and even more so against
952 1.7 christos adopting DST permanently."</li>
953 1.7 christos </ul>
954 1.7 christos </section>
955 1.7 christos
956 1.7 christos <section>
957 1.1 christos <h2 id="precision">Precision timekeeping</h2>
958 1.1 christos <ul>
959 1.1 christos <li><a
960 1.1 christos href="http://leapsecond.com/hpan/an1289.pdf">The
961 1.1 christos Science of Timekeeping</a> is a thorough introduction
962 1.1 christos to the theory and practice of precision timekeeping.</li>
963 1.1 christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
964 1.1 christos Time 2016</a> contains several freely-readable papers.</li>
965 1.1 christos <li><a href="http://www.ntp.org"><abbr
966 1.1 christos title="Network Time Protocol">NTP</abbr>: The Network
967 1.1 christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
968 1.1 christos discusses how to synchronize clocks of
969 1.1 christos Internet hosts.</li>
970 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>
971 1.2 christos family of software algorithms can achieve accuracy to a few tens of
972 1.2 christos nanoseconds in scalable server farms without special hardware.</li>
973 1.1 christos <li>The <a
974 1.1 christos href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
975 1.1 christos Time Protocol</a> (<abbr
976 1.1 christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
977 1.2 christos can achieve submicrosecond clock accuracy on a local area network
978 1.2 christos with special-purpose hardware.</li>
979 1.1 christos <li><a
980 1.1 christos href="https://tools.ietf.org/html/rfc4833">Timezone
981 1.1 christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
982 1.1 christos (Internet <abbr>RFC</abbr> 4833)
983 1.1 christos specifies a <a
984 1.1 christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
985 1.1 christos option for a server to configure
986 1.1 christos a client's time zone and daylight saving settings automatically.</li>
987 1.1 christos <li><a
988 1.1 christos href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
989 1.1 christos Times</a> explains more abstruse astronomical time scales like
990 1.1 christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
991 1.1 christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
992 1.1 christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.
993 1.1 christos <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
994 1.1 christos Scales</a> goes into more detail, particularly for historical variants.</li>
995 1.1 christos <li>The <a href="https://www.iau.org"><abbr
996 1.1 christos title="International Astronomical Union">IAU</abbr></a>'s <a
997 1.1 christos href="http://www.iausofa.org"><abbr
998 1.1 christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
999 1.1 christos collection contains C and <a
1000 1.1 christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1001 1.1 christos code for converting among time scales like
1002 1.1 christos <abbr title="International Atomic Time">TAI</abbr>,
1003 1.1 christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1004 1.1 christos <abbr>UTC</abbr>.</li>
1005 1.1 christos <li><a
1006 1.1 christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1007 1.1 christos – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1008 1.1 christos diverse local time
1009 1.1 christos scales used by each landed mission on Mars.</li>
1010 1.1 christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1011 1.1 christos dedicated not only to leap seconds but to precise time and frequency
1012 1.1 christos in general. It covers the state of the art in amateur timekeeping, and
1013 1.1 christos how the art has progressed over the past few decades.</li>
1014 1.7 christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1015 1.7 christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1016 1.7 christos and time-signal emissions</a>, International Telecommunication Union –
1017 1.7 christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1018 1.1 christos <li><a
1019 1.1 christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1020 1.1 christos title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1021 1.1 christos Bulletins</a> contains official publications of the International
1022 1.1 christos Earth Rotation and Reference Systems Service, which decides when leap
1023 1.4 christos seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1024 1.9 christos via an optional "<code>right</code>" configuration where a computer's internal
1025 1.9 christos <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1026 1.9 christos as opposed to the default "<code>posix</code>" configuration
1027 1.9 christos where the internal clock ignores leap seconds.
1028 1.9 christos The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1029 1.9 christos <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1030 1.9 christos timestamps starting with <code>time_t</code> 78 796 800,
1031 1.9 christos which corresponds to the first leap second
1032 1.9 christos 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1033 1.9 christos and to
1034 1.9 christos 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1035 1.9 christos In practice the two configurations also agree for timestamps before
1036 1.9 christos 1972 even though the historical situation is messy, partly because
1037 1.9 christos neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1038 1.9 christos is well-defined for sufficiently-old timestamps.</li>
1039 1.1 christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1040 1.1 christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1041 1.1 christos leap second so that they disagree with <abbr>UTC</abbr> by at most a
1042 1.1 christos half second, even though every <abbr>POSIX</abbr> minute has exactly
1043 1.4 christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1044 1.1 christos "<code>posix</code>" configuration, is <a
1045 1.1 christos href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1046 1.1 christos the <abbr>NTP</abbr> reference implementation, and is used by major
1047 1.7 christos cloud service providers. However, according to
1048 1.7 christos <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of
1049 1.7 christos Network Time Protocol Best Current Practices</a>
1050 1.7 christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1051 1.9 christos applications requiring accurate <abbr>UTC</abbr> or civil time,
1052 1.7 christos and is intended for use only in single, well-controlled environments.</li>
1053 1.1 christos <li>The <a
1054 1.1 christos href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1055 1.1 christos Second Discussion List</a> covers <a
1056 1.1 christos href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1057 1.1 christos and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1058 1.1 christos discussed further in
1059 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1060 1.1 christos leap second: its history and possible future</a>.
1061 1.1 christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1062 1.1 christos might be redefined
1063 1.1 christos without Leap Seconds</a> gives pointers on this
1064 1.1 christos contentious issue, which was active until 2015 and could become active
1065 1.1 christos again.</li>
1066 1.1 christos </ul>
1067 1.6 christos </section>
1068 1.6 christos
1069 1.6 christos <section>
1070 1.1 christos <h2 id="notation">Time notation</h2>
1071 1.1 christos <ul>
1072 1.1 christos <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
1073 1.1 christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1074 1.1 christos zone names, abbreviations, identifiers, and formats. For example, it
1075 1.1 christos contains French translations for "Eastern European Summer Time",
1076 1.1 christos "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1077 1.1 christos "Bucharest". Its
1078 1.1 christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1079 1.1 christos charts</a> show these values for many locales. Data values are available in
1080 1.1 christos both <abbr title="Locale Data Markup Language">LDML</abbr>
1081 1.1 christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1082 1.1 christos <li>
1083 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1084 1.6 christos the international standard date and time notation</a> covers
1085 1.1 christos <a
1086 1.6 christos href="https://www.iso.org/standard/70907.html"><em><abbr
1087 1.6 christos title="International Organization for Standardization">ISO</abbr>
1088 1.6 christos 8601-1:2019 – Date and time – Representations for information
1089 1.6 christos interchange – Part 1: Basic rules</em></a>.</li>
1090 1.1 christos <li>
1091 1.6 christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1092 1.1 christos Schema: Datatypes – dateTime</a> specifies a format inspired by
1093 1.1 christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1094 1.1 christos <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of
1095 1.1 christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1096 1.1 christos specifies the time notation used in email and <a
1097 1.1 christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1098 1.1 christos headers.</li>
1099 1.1 christos <li>
1100 1.1 christos <a href="https://tools.ietf.org/html/rfc3339">Date and Time
1101 1.1 christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1102 1.1 christos specifies an <abbr>ISO</abbr> 8601
1103 1.1 christos profile for use in new Internet
1104 1.1 christos protocols.</li>
1105 1.1 christos <li>
1106 1.7 christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1107 1.1 christos Formats on the Web</a> surveys web- and Internet-oriented date and time
1108 1.1 christos formats.</li>
1109 1.1 christos <li>Alphabetic time zone abbreviations should not be used as unique
1110 1.1 christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1111 1.1 christos practice. For example, in English-speaking North America
1112 1.1 christos "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1113 1.1 christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1114 1.1 christos and French-speaking North Americans prefer
1115 1.1 christos "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1116 1.1 christos "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1117 1.2 christos database contains English abbreviations for many timestamps;
1118 1.1 christos unfortunately some of these abbreviations were merely the database maintainers'
1119 1.1 christos inventions, and these have been removed when possible.</li>
1120 1.1 christos <li>Numeric time zone abbreviations typically count hours east of
1121 1.1 christos <abbr>UT</abbr>, e.g., +09 for Japan and
1122 1.1 christos −10 for Hawaii. However, the <abbr>POSIX</abbr>
1123 1.1 christos <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1124 1.1 christos For example, one might use <code><abbr>TZ</abbr>="<abbr
1125 1.1 christos title="Japan Standard Time">JST</abbr>-9"</code> and
1126 1.1 christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1127 1.1 christos for Japan and Hawaii, respectively. If the
1128 1.1 christos <code><abbr>tz</abbr></code> database is available, it is usually better to use
1129 1.1 christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1130 1.1 christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1131 1.2 christos confusion, handle old timestamps better, and insulate you better from
1132 1.1 christos any future changes to the rules. One should never set
1133 1.1 christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1134 1.1 christos <code>"GMT-9"</code>, though, since this would incorrectly imply that
1135 1.1 christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1136 1.1 christos is called "<abbr>GMT</abbr>".</li>
1137 1.1 christos </ul>
1138 1.6 christos </section>
1139 1.6 christos
1140 1.6 christos <section>
1141 1.1 christos <h2 id="see-also">See also</h2>
1142 1.1 christos <ul>
1143 1.7 christos <li><a href="theory.html">Theory and pragmatics of the
1144 1.7 christos <code><abbr>tz</abbr></code> code and data</a></li>
1145 1.1 christos <li><a href="tz-art.html">Time and the Arts</a></li>
1146 1.1 christos </ul>
1147 1.6 christos </section>
1148 1.6 christos
1149 1.6 christos <footer>
1150 1.1 christos <hr>
1151 1.1 christos This web page is in the public domain, so clarified as of
1152 1.1 christos 2009-05-17 by Arthur David Olson.
1153 1.1 christos <br>
1154 1.1 christos Please send corrections to this web page to the
1155 1.1 christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1156 1.6 christos </footer>
1157 1.1 christos </body>
1158 1.1 christos </html>
1159