contrib/tzcode/tz-link.htm

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   6 <title>Sources for time zone and daylight saving time data</title>
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  10  content="Sources for time zone and daylight saving time data">
  11 <meta name="DC.Creator" content="Eggert, Paul">
  12 <meta name="DC.Contributor" content="Olson, Arthur David">
  13 <meta name="DC.Date" content="2017-02-28">
  14 <meta name="DC.Description"
  15  content="Sources of information about time zones and daylight saving time">
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  17  content="https://www.iana.org/time-zones/repository/tz-link.htm">
  18 <meta name="Keywords"
  19  content="database,daylight saving,DST,time zone,timezone,tz,zoneinfo">
  20 </head>
  21 <body>
  22 <h1>Sources for time zone and daylight saving time data</h1>
  23 <p>
  24 Time zone and daylight saving rules are controlled by individual
  25 governments. They are sometimes changed with little notice, and their
  26 histories and planned futures are often recorded only fitfully. Here
  27 is a summary of attempts to organize and record relevant data in this
  28 area.
  29 </p>
  30 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
  31 <p>
  32 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
  33 time zone database contains code and data
  34 that represent the history of local time
  35 for many representative locations around the globe.
  36 It is updated periodically to reflect changes made by political bodies
  37 to <a href="https://en.wikipedia.org/wiki/Time_zone">time zone</a>
  38 boundaries and
  39 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
  40 rules.
  41 This database (often called <code>zoneinfo</code> or
  42 <code><abbr>tz</abbr></code>)
  43 is used by several implementations,
  44 including
  45 <a href="https://www.gnu.org/software/libc/">the
  46 <abbr title="GNU's Not Unix">GNU</abbr>
  47 C Library</a> (used in
  48 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
  49 <a href="https://www.android.com">Android</a>,
  50 <a href="https://developer.mozilla.org/en-US/docs/Mozilla/B2G_OS">B2G
  51 <abbr title="Operating System">OS</abbr></a>,
  52 <a href="https://www.freebsd.org">Free<abbr
  53 title="Berkeley Software Distribution">BSD</abbr></a>,
  54 <a href="http://netbsd.org">Net<abbr>BSD</abbr></a>,
  55 <a href="http://www.openbsd.org">Open<abbr>BSD</abbr></a>,
  56 <a href="http://www.chromium.org/chromium-os">Chromium OS</a>,
  57 <a href="https://cygwin.com">Cygwin</a>,
  58 <a href="http://www.delorie.com/djgpp/"><abbr
  59 title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
  60 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
  61 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
  62 title="Web Operating System">webOS</abbr></a>,
  63 <a href="http://ibm.com/aix"><abbr
  64 title="Advanced Interactive eXecutive">AIX</abbr></a>,
  65 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
  66 <a href="http://www.apple.com/ios/"><abbr
  67 title="iPhone OS">iOS</abbr></a>,
  68 <a href="http://www.apple.com/macos/">macOS</a>,
  69 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
  70 <a href="https://www.hpe.com/info/openvms">Open<abbr
  71 title="Virtual Memory System">VMS</abbr></a>,
  72 <a href="https://www.oracle.com/database/index.html">Oracle Database</a>, and
  73 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
  74 <p>
  75 Each location in the database represents a region where all
  76 clocks keeping local time have agreed since 1970.
  77 Locations are identified by continent or ocean and then by the name of
  78 the location, which is typically the largest city within the region.
  79 For example, <code>America/New_York</code>
  80 represents most of the <abbr title="United States">US</abbr> eastern time zone;
  81 <code>America/Phoenix</code> represents most of Arizona, which
  82 uses mountain time without daylight saving time (<abbr
  83 title="daylight saving time">DST</abbr>);
  84 <code>America/Detroit</code> represents most of Michigan, which uses
  85 eastern time but with different <abbr>DST</abbr> rules in 1975;
  86 and other entries represent smaller regions like Starke County,
  87 Indiana, which switched from central to eastern time in 1991
  88 and switched back in 2006.
  89 To use the database on an extended <a
  90 href="https://en.wikipedia.org/wiki/POSIX"><abbr
  91 title="Portable Operating System Interface">POSIX</abbr></a>
  92 implementation set the <code><abbr>TZ</abbr></code>
  93 environment variable to the location's full name,
  94 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
  95 <p>
  96 Associated with each region is a history of offsets from
  97 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
  98 Time</a> (<abbr>UT</abbr>), which is <a
  99 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
 100 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
 101 for time stamps after 1960 this is more precisely <a
 102 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
 103 Universal Time</a> (<abbr>UTC</abbr>).
 104 The database also records when daylight saving time was in use,
 105 along with some time zone abbreviations such as <abbr>EST</abbr>
 106 for Eastern Standard Time in the <abbr>US</abbr>.</p>
 107 <p>
 108 The following <a
 109 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
 110 the latest release's two
 111 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
 112 to a <abbr>GNU</abbr>/Linux or similar host.</p>
 113 <pre style="margin-left: 2em"><code>mkdir tzdb
 114 cd tzdb
 115 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
 116 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
 117 <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 -
 118 gzip -dc tzdata-latest.tar.gz | tar -xf -
 119 </code></pre>
 120 <p>Alternatively, the following shell commands download the same
 121 release in a single-tarball format containing extra data
 122 useful for regression testing:</p>
 123 <pre style="margin-left: 2em"><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>
 124 <a href="http://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
 125 </code></pre>
 126 <p>These commands use convenience links to the latest release
 127 of the <code><abbr>tz</abbr></code> database hosted by the
 128 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
 129 of the <a href="https://www.iana.org">Internet Assigned Numbers
 130 Authority (IANA)</a>.
 131 Older releases are in files named
 132 <code>tzcode<var>V</var>.tar.gz</code>,
 133 <code>tzdata<var>V</var>.tar.gz</code>, and
 134 <code>tzdb-<var>V</var>.tar.lz</code>,
 135 where <code><var>V</var></code> is the version.
 136 Since 1996, each version has been a four-digit year followed by
 137 lower-case letter (<samp>a</samp> through <samp>z</samp>,
 138 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
 139 through <samp>zzz</samp>, and so on).
 140 Since version 2016h, each release has contained a text file named
 141 "<samp>version</samp>" whose first (and currently only) line is the version.
 142 The releases are also available in an
 143 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
 144 title="File Transfer Protocol">FTP</abbr> directory</a> via a
 145 less-secure protocol .</p>
 146 <p>Alternatively, a development repository of code and data can be
 147 retrieved from <a href="https://github.com">GitHub</a> via the shell
 148 command:</p>
 149 <pre style="margin-left: 2em"><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
 150 </code></pre>
 151 <p>
 152 Since version 2012e, each release has been tagged in development repositories.
 153 Untagged commits are less well tested and probably contain
 154 more errors.</p>
 155 <p>
 156 After obtaining the code and data files, see the
 157 <code>README</code> file for what to do next.
 158 The code lets you compile the <code><abbr>tz</abbr></code> source files into
 159 machine-readable binary files, one for each location. It also lets
 160 you read a <code><abbr>tz</abbr></code> binary file and interpret time stamps for that
 161 location.</p>
 162 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
 163 <p>
 164 The <code><abbr>tz</abbr></code> code and data
 165 are by no means authoritative. If you find errors, please
 166 send changes to the <a href="mailto:tz@iana.org">time zone
 167 mailing list</a>. You can also <a
 168 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
 169 and browse the <a
 170 href="https://mm.icann.org/pipermail/tz/">archive of old
 171 messages</a>.</p>
 172 <p>
 173 If your government plans to change its time zone boundaries or
 174 daylight saving rules, let the mailing list know well in advance. With
 175 less than a year's notice there is a good chance that some
 176 computer-based clocks will operate incorrectly after the change, due
 177 to delays in propagating updates to software and data. The shorter
 178 the notice, the more likely clock problems will arise.
 179 </p>
 180 <p>Sources for the <code><abbr>tz</abbr></code> database are
 181 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
 182 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
 183 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
 184 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
 185 title="linefeed">LF</abbr></a>,
 186 which can be modified by common text editors such
 187 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
 188 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
 189 <a href="http://www.vim.org">vim</a>. One
 190 editor has a package to simplify editing further:</p>
 191 <ul>
 192 <li><a href="https://packagecontrol.io/packages/zoneinfo">Sublime
 193 zoneinfo</a> is a <a href="http://www.sublimetext.com">Sublime
 194 Text</a> package for syntax highlighting <code><abbr>tz</abbr></code>
 195 source files.</li>
 196 </ul>
 197 <p>
 198 For further information about updates, please see
 199 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
 200 Maintaining the Time Zone Database</a> (Internet <abbr
 201 title="Request For Comments">RFC</abbr> 6557).</p>
 202 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
 203 <ul>
 204 <li>The article
 205 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
 206 an encyclopedic summary.</li>
 207 <li><a href="tz-how-to.html">How to Read the
 208 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
 209 database format.</li>
 210 <li><a
 211 href="http://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
 212 the Timing of Time Zone Changes</a> gives examples of problems caused
 213 by inadequate notice by governments of time zone and daylight saving
 214 rule changes.</li>
 215 <li><a
 216 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
 217 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
 218 database's style.</li>
 219 </ul>
 220 <h2 id="web">Web sites using recent versions of the
 221 <code><abbr>tz</abbr></code> database</h2>
 222 <p>
 223 These are listed roughly in ascending order of complexity and fanciness.
 224 </p>
 225 <ul>
 226 <li><a href="http://time.is">Time.is</a> shows locations'
 227 time and zones in a simple format.</li>
 228 <li><a href="https://www.timejones.com">TimeJones.com</a> is a simple
 229 time zone converter.</li>
 230 <li><a
 231 href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
 232 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
 233 <li><a
 234 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
 235 Time in 1000 Places</a> uses descriptions of the values.</li>
 236 <li><a href="http://www.timezoneconverter.com/cgi-bin/tzc.tzc">Time Zone
 237 Converter</a>
 238 uses a pulldown menu.</li>
 239 <li><a href="http://home.kpn.nl/vanadovv/time/TZworld.html">Complete
 240 timezone information for all countries</a> displays tables of DST rules.
 241 <li><a href="http://www.timeanddate.com/worldclock/">The World Clock &ndash;
 242 Worldwide</a> lets you sort zone names and convert times.</li>
 243 <li><a href="http://www.zeitverschiebung.net/en/">Time Difference</a>
 244 calculates the current time difference between locations.</li>
 245 <li><a href="http://www.wx-now.com">Weather Now</a> lists the weather too.</li>
 246 <li><a href="http://www.thetimenow.com">The Time Now</a> also lists weather.</li>
 247 </ul>
 248 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
 249 <ul>
 250 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
 251 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
 252 Distribution Service (tzdist) working group</a> defined <a
 253 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
 254 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
 255 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
 256 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
 257 transferring time zone data by reference. The draft <a name="TZDIST-Geolocate"
 258 href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate-00">TZDIST
 259 Geolocate Extension</a> lets a client determine its time zone region
 260 from its geographic location using a <a
 261 href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li>
 262 <li>The <a href="https://tools.ietf.org/html/rfc5545">
 263 Internet Calendaring and Scheduling Core Object Specification
 264 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
 265 covers time zone
 266 data; see its VTIMEZONE calendar component.
 267 The iCalendar format requires specialized parsers and generators; a
 268 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
 269 (Internet <abbr>RFC</abbr> 6321) uses
 270 <a href="https://www.w3.org/XML/"><abbr
 271 title="Extensible Markup Language">XML</abbr></a> format, and a variant
 272 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
 273 (Internet <abbr>RFC</abbr> 7265)
 274 uses <a href="http://www.json.org"><abbr
 275 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
 276 </ul>
 277 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
 278 <ul>
 279 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
 280 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
 281 program that compiles
 282 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
 283 Vzic is freely
 284 available under the <a
 285 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
 286 General Public License (<abbr
 287 title="General Public License">GPL</abbr>)</a>.</li>
 288 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
 289 database conversion utility</a> is like Vzic, except for the <a
 290 href="https://www.microsoft.com/net">.NET framework</a>
 291 and with a <abbr>BSD</abbr>-style license.</li>
 292 <li><a
 293 href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
 294 contains a script <code>parse_olson</code> that compiles
 295 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
 296 modules. It is part of the Perl <a
 297 href="http://datetime.perl.org">DateTime Project</a>, which is freely
 298 available under both the <abbr>GPL</abbr> and the Perl Artistic
 299 License. DateTime::TimeZone also contains a script
 300 <code>tests_from_zdump</code> that generates test cases for each clock
 301 transition in the <code><abbr>tz</abbr></code> database.</li>
 302 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
 303 Database Parser</a> is a
 304 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
 305 runtime library. It is freely available under the
 306 <a href="http://creativecommons.org/licenses/by/4.0/">Creative Commons
 307 Attribution 4.0 International Public License</a>.</li>
 308 <li><a name="ICU" href="http://site.icu-project.org">International Components for
 309 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
 310 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
 311 libraries for internationalization that
 312 has a compiler from <code><abbr>tz</abbr></code> source
 313 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
 314 (mentioned <a href="#CLDR">below</a>)
 315 into an <abbr>ICU</abbr>-specific format.
 316 <abbr>ICU</abbr> is freely available under a
 317 <abbr>BSD</abbr>-style license.</li>
 318 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
 319 the <a href="http://elixir-lang.org">Elixir</a> language downloads
 320 and compiles tz source and exposes <abbr
 321 title="Application Program Interface">API</abbr>s for use. It is
 322 freely available under the <abbr
 323 title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
 324 <li>Java-based compilers and libraries include:
 325 <ul>
 326 <li>The <a
 327 href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
 328 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
 329 Oracle Java.</li>
 330 <li>The <a
 331 href="http://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
 332 8 <code>java.time</code> <abbr>API</abbr></a> can be supplemented by <a
 333 href="http://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
 334 which is freely available under a <abbr>BSD</abbr>-style license.</li>
 335 <li><a href="http://www.joda.org/joda-time/">Joda-Time &ndash; Java date
 336 and time <abbr>API</abbr></a> contains a class
 337 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
 338 <code><abbr>tz</abbr></code> source into a binary format. It inspired
 339 Java 8 <code>java.time</code>, which its users should migrate to once
 340 they can assume Java 8 or later. It is available under the <a
 341 href="http://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
 342 <li><a href="https://github.com/MenoData/Time4J/">Time4J &ndash;
 343 Advanced date, time and interval library for Java</a> contains a class
 344 <code>net.time4j.tool.TimezoneRepositoryCompiler</code> that compiles
 345 <code><abbr>tz</abbr></code> source into a binary format. Time4J is
 346 available under the <a
 347 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
 348 General Public License (<abbr title="Lesser General Public
 349 License">LGPL</abbr>)</a>.</li>
 350 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
 351 Java-based libraries.</li>
 352 </ul>
 353 <li><a href="http://nodatime.org">Noda Time &ndash; Date and
 354 time <abbr>API</abbr> for .NET</a>
 355 and <a href="http://www.babiej.demon.nl/Tz4Net/main.htm">TZ4Net</a>
 356 are similar to Joda-Time and Time4J, but for the .NET framework instead of
 357 Java. They are freely available under the
 358 Apache License
 359 and a <abbr>BSD</abbr>-style license, respectively.</li>
 360 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
 361 compilers and libraries include:
 362 <ul>
 363 <li><a href="http://momentjs.com/timezone/">Moment Timezone</a> is a
 364 plugin for the <a href="http://momentjs.com">Moment.js</a> date
 365 manipulation library. It is freely available under the <abbr>MIT</abbr>
 366 license.</li>
 367 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
 368 <abbr>API</abbr> is upward compatible with standard JavaScript
 369 Dates. It is freely available under the Apache License.</li>
 370 <li><a href="https://github.com/sproutsocial/walltime-js">Walltime-js</a>
 371 translates <abbr>UT</abbr> to local time. It is freely available under
 372 the <abbr>MIT</abbr> license.</li>
 373 </ul>
 374 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
 375 compiler from <code><abbr>tz</abbr></code> source into
 376 <a href="http://julialang.org/">Julia</a>. It is freely available
 377 under the <abbr>MIT</abbr> license.</li>
 378 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
 379 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
 380 <a href="https://www.python.org">Python</a>.
 381 It is freely available under a <abbr>BSD</abbr>-style license.</li>
 382 <li><a href="http://tzinfo.github.io">TZInfo &ndash;
 383 Ruby Timezone Library</a>
 384 compiles <code><abbr>tz</abbr></code> source into
 385 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
 386 It is freely available under the <abbr>MIT</abbr> license.</li>
 387 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
 388 Library</a> is
 389 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
 390 library that compiles <code><abbr>tz</abbr></code> source into a time
 391 zone repository whose format
 392 is either proprietary or an <abbr>XML</abbr>-encoded
 393 representation.</li>
 394 <li><a name="Tcl" href="http://tcl.tk">Tcl</a>
 395 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
 396 source into text files, along with a runtime that can read those
 397 files. Tcl is freely available under a <abbr>BSD</abbr>-style
 398 license.</li>
 399 </ul>
 400 <h2 id="binary">Other <code><abbr>tz</abbr></code> binary file readers</h2>
 401 <ul>
 402 <li>The <a
 403 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
 404 Library</a>
 405 has an independent, thread-safe implementation of
 406 a <code><abbr>tz</abbr></code> binary file reader.
 407 This library is freely available under the LGPL
 408 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
 409 <li><a href="https://www.gnome.org">GNOME</a>'s
 410 <a href="https://developer.gnome.org/glib/">GLib</a> has
 411 a <code><abbr>tz</abbr></code> binary file reader written in C that
 412 creates a <code>GTimeZone</code> object representing sets
 413 of <abbr>UT</abbr> offsets.
 414 It is freely available under the <abbr>LGPL</abbr>.</li>
 415 <li>The
 416 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
 417 <code>baltzo::TimeZoneUtil</code> component contains a C++
 418 implementation of a binary file reader. It is freely available under
 419 the Apache License.</li>
 420 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
 421 library that translates between <abbr>UTC</abbr> and civil time and
 422 can read binary files. It is freely available under the Apache
 423 License.</li>
 424 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
 425 is a <code><abbr>tz</abbr></code> binary file reader written in Java.
 426 It is freely available under the <abbr>LGPL</abbr>.</li>
 427 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
 428 JavaScript library that supports date arithmetic that is time zone
 429 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
 430 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
 431 <code><abbr>tz</abbr></code> binary file reader.</li>
 432 <li><a href="http://search.cpan.org/perldoc?DateTime::TimeZone::Tzfile">
 433 DateTime::TimeZone::Tzfile</a>
 434 is a <code><abbr>tz</abbr></code> binary file reader written in Perl.
 435 It is freely available under the same terms as Perl
 436 (dual <abbr>GPL</abbr> and Artistic license).</li>
 437 <li>The
 438 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
 439 library contains a Python tool that
 440 converts <code><abbr>tz</abbr></code> binary data into
 441 <abbr>JSON</abbr>-format data suitable for use
 442 in its JavaScript library for time zone conversion. Dates before 1970
 443 are not supported.</li>
 444 <li>The <a
 445 href="http://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
 446 package contains <a href="https://www.haskell.org">Haskell</a> code that
 447 parses and uses <code><abbr>tz</abbr></code> binary data. It is freely
 448 available under a <abbr>BSD</abbr>-style license.</li>
 449 </ul>
 450 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
 451 <ul>
 452 <li><a href="https://foxclocks.org">FoxClocks</a>
 453 is an extension for <a href="https://www.google.com/chrome/">Google
 454 Chrome</a> and for <a
 455 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
 456 Toolkit</a> applications like <a
 457 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
 458 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
 459 It displays multiple clocks in the application window, and has a mapping
 460 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
 461 It is freely available under the <abbr>GPL</abbr>.</li>
 462 <li><a href="https://golang.org">Go programming language</a>
 463 implementations contain a copy of a 32-bit subset of a recent
 464 <code><abbr>tz</abbr></code> database in a
 465 Go-specific format.</li>
 466 <li><a
 467 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
 468 clock (intclock)</a> is a clock that displays multiple time zones on
 469 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
 470 under the <abbr>GPL</abbr>.</li>
 471 <li>Microsoft Windows 8.1
 472 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
 473 data (mentioned <a href="#CLDR">below</a>) used by
 474 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a>
 475 classes such as <a
 476 href="https://msdn.microsoft.com/en-us/library/windows/apps/windows.globalization.datetimeformatting.datetimeformatter.aspx"><code>DateTimeFormatter</code></a>.
 477 <a name="System.TimeZoneInfo"
 478 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
 479 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
 480 the older, proprietary method of Microsoft Windows 2000 and later,
 481 which stores time zone data in the
 482 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
 483 <a
 484 href="http://www.unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
 485 Tzid table</a> or <a
 486 href="http://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
 487 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
 488 to <code><abbr>tz</abbr></code> names.
 489 <li><a
 490 href="https://www.oracle.com/java/index.html">Oracle
 491 Java</a> contains a copy of a subset of a recent
 492 <code><abbr>tz</abbr></code> database in a
 493 Java-specific format.</li>
 494 <li><a href="https://kimmo.suominen.com/sw/timezone/">Time Zone</a> is
 495 a <a href="https://wordpress.org">WordPress</a> plugin. It is freely
 496 available under a <abbr>BSD</abbr>-style license.</li>
 497 <li><a href="http://www.relativedata.com/time-zone-master">Time Zone
 498 Master</a> is a Microsoft Windows clock program that can automatically
 499 download, compile and use <code>tz</code> releases. The Basic version
 500 is free.</li>
 501 <li><a
 502 href="http://veladg.com/velaterra.html">VelaTerra</a> is
 503 a macOS program. Its developers
 504 <a href="http://veladg.com/tzoffer.html">offer free
 505 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
 506 </ul>
 507 <h2 id="other-dbs">Other time zone databases</h2>
 508 <ul>
 509 <li><a href="http://www.astro.com/atlas">Time-zone Atlas</a>
 510 is Astrodienst's Web version of Shanks and Pottenger's
 511 time zone history atlases also published in <a
 512 href="http://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
 513 form by <a href="http://astrocom.com">ACS-Starcrafts</a>.
 514 These atlases are extensive but unreliable, as Shanks appears to have
 515 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
 516 sources and do not indicate which entries are guesswork.</li>
 517 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
 518 its own <code>tztab</code>(4) format.</li>
 519 <li>Microsoft Windows has proprietary data mentioned
 520 <a href="#System.TimeZoneInfo">above</a>.</li>
 521 <li><a href="http://www.worldtimeserver.com">World Time Server</a>
 522 is another time zone database.</li>
 523 <li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a>
 524 contains data from the Time Service Department of the
 525 <abbr>US</abbr> Naval Observatory.</li>
 526 <li>The <a
 527 href="http://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
 528 Schedules Information Manual</a> of the
 529 International Air Transport Association
 530 gives current time zone rules for airports served by commercial aviation.</li>
 531 </ul>
 532 <h2 id="maps">Maps</h2>
 533 <ul>
 534 <li>The <a href="https://www.cia.gov/index.html">United States Central
 535 Intelligence Agency (<abbr
 536 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
 537 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
 538 zone map</a>; the
 539 <a
 540 href="http://www.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
 541 Library Map Collection</a>
 542 of the University of Texas at Austin has copies of
 543 recent editions.
 544 The pictorial quality is good,
 545 but the maps do not indicate summer time,
 546 and parts of the data are a few years out of date.</li>
 547 <li><a href="http://www.worldtimezone.com">Current time around the world
 548 and standard time zones map of the world</a>
 549 has several fancy time zone maps; it covers Russia particularly well.
 550 The maps' pictorial quality is not quite as good as the
 551 <abbr>CIA</abbr>'s
 552 but the maps are more up to date.</li>
 553 <li><a
 554 href="http://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
 555 much is time wrong around the world?</a> maps the difference between
 556 mean solar and standard time, highlighting areas such as western China
 557 where the two differ greatly. It's a bit out of date, unfortunately.</li>
 558 </ul>
 559 <h2 id="boundaries">Time zone boundaries</h2>
 560 <p>Geographical boundaries between time zone regions are available
 561 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
 562 services and other sources.</p>
 563 <ul>
 564 <li>Databases of time zone boundaries include:
 565 <ul>
 566 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
 567 Boundary Builder</a> extracts
 568 <a href="http://www.openstreetmap.org">Open Street Map</a> data to build
 569 boundaries of <code><abbr>tz</abbr></code> regions.
 570 Its code is freely available under the <abbr>MIT</abbr> license, and
 571 its data entries are freely available under the
 572 <a href="http://opendatacommons.org/licenses/odbl/">Open Data Commons
 573 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
 574 <li><a href="http://efele.net/maps/tz/"><abbr>TZ</abbr> timezones
 575 maps</a> contains <a
 576 href="https://en.wikipedia.org/wiki/Shapefile">shapefiles</a> of
 577 sets of <code><abbr>tz</abbr></code> regions. This includes
 578 <a href="http://efele.net/maps/tz/world/">tz_world</a>, a shapefile
 579 for all the world's regions</li>
 580 <li><a
 581 href="https://github.com/straup/whereonearth-timezone">Whereonearth-timezone</a>
 582 is in <a href="https://tools.ietf.org/html/rfc7946">GeoJSON</a> format
 583 (Internet <abbr>RFC</abbr> 7946), and combines the
 584 the tz_world shapefiles with the
 585 <a href="https://developer.yahoo.com/geo/geoplanet/">GeoPlanet</a>
 586 dataset.</li>
 587 </ul></li>
 588 <li>Programmatic interfaces that map geographical coordinates via tz_world to
 589 <code><abbr>tz</abbr></code> regions include:
 590 <ul>
 591 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
 592 written in <a
 593 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
 594 and is freely available under the <abbr>MIT</abbr> license.</li>
 595 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
 596 is written in Go and is freely available under the Apache License.</li>
 597 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
 598 in both Java and
 599 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
 600 form, is freely available under the MIT license.</li>
 601 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
 602 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
 603 is freely available under the MIT license, and
 604 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
 605 is in the public domain.</li>
 606 <li>The <a
 607 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
 608 library for Python is freely available under the MIT license.
 609 <li>The <a
 610 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
 611 library for Ruby is freely available under the MIT license.</li>
 612 <li><a href="https://derickrethans.nl/what-time-is-it.html">What Time
 613 is It Here?</a> applies <a href="https://www.mongodb.com">MongoDB</a>
 614 geospatial query operators to shapefiles' data.</li>
 615 </ul></li>
 616 <li>Free access via a network API, if you register a key, is provided by
 617 the <a href="http://www.geonames.org/export/web-services.html#timezone">GeoNames Timezone web service</a>,
 618 the <a href="https://developers.google.com/maps/documentation/timezone/intro">Google Maps Time Zone API</a>, and
 619 the <a href="https://timezonedb.com">Time Zone Database &amp; API</a>.
 620 Commercial network API access is provided
 621 by <a href="https://askgeo.com">AskGeo</a>
 622 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
 623 Also, an <a
 624 href="https://mm.icann.org/pipermail/tz/2016-October/024309.html">experimental
 625 server</a> is available for the TZDIST Geolocate Extension mentioned
 626 <a href="#TZDIST-Geolocate">above</a>.</li>
 627 <li>"<a
 628 href="http://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
 629 to get a time zone from a location using latitude and longitude
 630 coordinates?</a>" discusses other geolocation possibilities.</li>
 631 <li><a href="http://statoids.com/statoids.html">Administrative
 632 Divisions of Countries ("Statoids")</a> lists
 633 political subdivision data related to time zones.</li>
 634 <li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time
 635 zone boundaries for multizone countries</a> summarizes legal
 636 boundaries between time zones within countries.</li>
 637 <li>Manifold.net's <a
 638 href="http://manifold.net/info/freestuff.shtml">Free Stuff for
 639 Manifold System Users</a> includes a Manifold-format map of
 640 world time zone boundaries distributed under the
 641 <abbr>GPL</abbr>.</li>
 642 <li>The GeoCommunity lists several commercial sources for <a
 643 href="http://spatialnews.geocomm.com/features/timezones/">International
 644 Time Zones and Time Zone Data</a>.</li>
 645 <li>A ship within the <a
 646 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
 647 waters</a> of any nation uses that nation's time. In international
 648 waters, time zone boundaries are meridians 15&deg; apart, except that
 649 <abbr>UTC</abbr>&minus;12 and <abbr>UTC</abbr>+12 are each 7.5&deg;
 650 wide and are separated by
 651 the 180&deg; meridian (not by the International Date Line, which is
 652 for land and territorial waters only). A captain can change ship's
 653 clocks any time after entering a new time zone; midnight changes are
 654 common.</li>
 655 </ul>
 656 <h2 id="civil">Civil time concepts and history</h2>
 657 <ul>
 658 <li><a href="http://physics.nist.gov/time">A
 659 Walk through Time</a>
 660 surveys the evolution of timekeeping.</li>
 661 <li><a href="http://www.webexhibits.org/daylightsaving/">About Daylight
 662 Saving Time &ndash; History, rationale, laws &amp; dates</a>
 663 is an overall history of <abbr>DST</abbr>.</li>
 664 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
 665 contains guidelines and best practices for software applications that
 666 deal with civil time.</li>
 667 <li><a href="http://seizethedaylight.com/dst/">A Brief
 668 History of Daylight Saving Time</a> summarizes some of the contentious
 669 history of <abbr>DST</abbr>.</li>
 670 <li><a href="http://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
 671 the International Date Line</a> tells the story of the most important
 672 time zone boundary.</li>
 673 <li><a href="http://statoids.com/tconcept.html">Basic Time
 674 Zone Concepts</a> discusses terminological issues behind time zones.</li>
 675 </ul>
 676 <h2 id="national">National histories of legal time</h2>
 677 <dl>
 678 <dt>Australia</dt>
 679 <dd>The Parliamentary Library has commissioned a <a
 680 href="http://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
 681 paper on daylight saving time in Australia</a>.
 682 The Bureau of Meteorology publishes a list of <a
 683 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
 684 Dates of Daylight Savings Time within Australia</a>.</dd>
 685 <dt>Belgium</dt>
 686 <dd>The Royal Observatory of Belgium maintains a table of <a
 687 href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
 688 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
 689 <dt>Brazil</dt>
 690 <dd>The Time Service Department of the National Observatory
 691 records <a href="http://pcdsh01.on.br/DecHV.html"
 692 hreflang="pt-BR">Brazil's daylight saving time decrees (in
 693 Portuguese)</a>.</dd>
 694 <dt>Canada</dt>
 695 <dd>National Research Council Canada publishes current
 696 and some older information about <a
 697 href="http://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time
 698 zones &amp; daylight saving time</a>.</dd>
 699 <dt>Chile</dt>
 700 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
 701 <a href="http://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
 702 Chile's official time (in Spanish)</a>.</dd>
 703 <dt>Germany</dt>
 704 <dd>The National Institute for Science and Technology maintains the <a
 705 href="http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
 706 of Legal Time in Germany</a>.</dd>
 707 <dt>Israel</dt>
 708 <dd>The Interior Ministry periodically issues <a
 709 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
 710 hreflang="he">announcements (in Hebrew)</a>.</dd>
 711 <dt>Italy</dt>
 712 <dd>The National Institute of Metrological Research maintains a
 713 <a href="http://www.inrim.it/res/tf/ora_legale_i.shtml">table of civil time
 714 (in Italian)</a>.</dd>
 715 <dt>Mexico</dt>
 716 <dd>The Investigation and Analysis Service of the Mexican Library of
 717 Congress has published a <a
 718 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
 719 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
 720 <dt>Malaysia</dt>
 721 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
 722 <dt>Netherlands</dt>
 723 <dd><a href="http://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
 724 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
 725 covers the history of local time in the Netherlands from ancient times.</dd>
 726 <dt>New Zealand</dt>
 727 <dd>The Department of Internal Affairs maintains a brief <a
 728 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
 729 Daylight Saving</a>. The privately-maintained <a
 730 href="http://astrologyschool.com/nztime.html">History of New Zealand
 731 time</a> has more details.</dd>
 732 <dt>Singapore</dt>
 733 <dd><a name="Singapore"
 734 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
 735 is Singapore in the "Wrong" Time Zone?</a> details the
 736 history of legal time in Singapore and Malaysia.</dd>
 737 <dt>United Kingdom</dt>
 738 <dd><a
 739 href="https://www.polyomino.org.uk/british-time/">History of
 740 legal time in Britain</a> discusses in detail the country
 741 with perhaps the best-documented history of clock adjustments.
 742 The National Physical Laboratory also maintains an <a
 743 href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates">Archive
 744 of Summer time dates</a>.</dd>
 745 <dt>United States</dt>
 746 <dd>The Department of Transportation's <a
 747 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
 748 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
 749 </dl>
 750 <h2 id="precision">Precision timekeeping</h2>
 751 <ul>
 752 <li><a
 753 href="http://literature.agilent.com/litweb/pdf/5965-7984E.pdf">The
 754 Science of Timekeeping</a> is a thorough introduction
 755 to the theory and practice of precision timekeeping.</li>
 756 <li><a href="http://www.ntp.org"><abbr
 757 title="Network Time Protocol">NTP</abbr>: The Network
 758 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
 759 discusses how to synchronize clocks of
 760 Internet hosts.</li>
 761 <li>The <a
 762 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
 763 Time Protocol</a> (<abbr
 764 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
 765 can achieve submicrosecond clock accuracy on a local area network.</li>
 766 <li><a
 767 href="https://tools.ietf.org/html/rfc4833">Timezone
 768 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
 769 (Internet <abbr>RFC</abbr> 4833)
 770 specifies a <a
 771 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
 772 option for a server to configure
 773 a client's time zone and daylight saving settings automatically.</li>
 774 <li><a
 775 href="http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
 776 Times</a> explains more abstruse astronomical time scales like
 777 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
 778 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
 779 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
 780 <a href="http://www.ucolick.org/~sla/leapsecs/timescales.html">Time
 781 Scales</a> goes into more detail, particularly for historical variants.</li>
 782 <li>The <a href="https://www.iau.org"><abbr
 783 title="International Astronomical Union">IAU</abbr></a>'s <a
 784 href="http://www.iausofa.org"><abbr
 785 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
 786 collection contains C and <a
 787 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
 788 code for converting among time scales like
 789 <abbr title="International Atomic Time">TAI</abbr>,
 790 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
 791 <abbr>UTC</abbr>.</li>
 792 <li><a
 793 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
 794 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
 795 diverse local time
 796 scales used by each landed mission on Mars.</li>
 797 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
 798 dedicated not only to leap seconds but to precise time and frequency
 799 in general. It covers the state of the art in amateur timekeeping, and
 800 how the art has progressed over the past few decades.</li>
 801 <li><a
 802 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
 803 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
 804 Bulletins</a> contains official publications of the International
 805 Earth Rotation and Reference Systems Service, which decides when leap
 806 seconds occur. The <code>tz</code> code and data support leap seconds
 807 via an optional "<code>right</code>" configuration, as opposed to the
 808 default "<code>posix</code>" configuration.</li>
 809 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
 810 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
 811 leap second so that they disagree with <abbr>UTC</abbr> by at most a
 812 half second, even though every <abbr>POSIX</abbr> minute has exactly
 813 sixty seconds. This approach works with the default <code>tz</code>
 814 "<code>posix</code>" configuration, is <a
 815 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
 816 the <abbr>NTP</abbr> reference implementation, and is used by major
 817 cloud service providers.</li>
 818 <li>The <a
 819 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
 820 Second Discussion List</a> covers <a
 821 href="http://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
 822 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
 823 discussed further in
 824 <a href="http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
 825 leap second: its history and possible future</a>.
 826 <a href="http://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
 827 might be redefined
 828 without Leap Seconds</a> gives pointers on this
 829 contentious issue, which was active until 2015 and could become active
 830 again.</li>
 831 </ul>
 832 <h2 id="notation">Time notation</h2>
 833 <ul>
 834 <li>The <a name="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
 835 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
 836 zone names, abbreviations, identifiers, and formats. For example, it
 837 contains French translations for "Eastern European Summer Time",
 838 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
 839 "Bucharest". Its
 840 <a href="http://www.unicode.org/cldr/charts/latest/by_type/">by-type
 841 charts</a> show these values for many locales. Data values are available in
 842 both <abbr title="Locale Data Markup Language">LDML</abbr>
 843 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
 844 <li>
 845 <a href="http://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
 846 the international standard date and time notation</a> is a good
 847 summary of
 848 <a
 849 href="https://www.iso.org/standard/40874.html"><abbr
 850 title="International Organization for Standardization">ISO</abbr>
 851 8601:2004 &ndash; Data elements and interchange formats &ndash; Information
 852 interchange &ndash; Representation of dates and times</a>.</li>
 853 <li>
 854 <a href="https://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
 855 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
 856 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
 857 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
 858 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
 859 specifies the time notation used in email and <a
 860 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
 861 headers.</li>
 862 <li>
 863 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
 864 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
 865 specifies an <abbr>ISO</abbr> 8601
 866 profile for use in new Internet
 867 protocols.</li>
 868 <li>
 869 <a href="https://www.hackcraft.net/web/datetime/">Date &amp; Time
 870 Formats on the Web</a> surveys web- and Internet-oriented date and time
 871 formats.</li>
 872 <li>
 873 <a href="http://exit109.com/~ghealton/y2k/yrexamples.html">The
 874 Best of Dates, the Worst of Dates</a> covers many problems encountered
 875 by software developers when handling dates and time stamps.</li>
 876 <li>Alphabetic time zone abbreviations should not be used as unique
 877 identifiers for <abbr>UTC</abbr> offsets as they are ambiguous in
 878 practice. For example, in English-speaking North America
 879 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UTC</abbr>,
 880 but in China it denotes 8 hours ahead of <abbr>UTC</abbr>,
 881 and French-speaking North Americans prefer
 882 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
 883 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
 884 database contains English abbreviations for many time stamps;
 885 unfortunately some of these abbreviations were merely the database maintainers'
 886 inventions, and are gradually being removed.</li>
 887 <li>Numeric time zone abbreviations typically count hours east of
 888 <abbr>UTC</abbr>, e.g., +09 for Japan and
 889 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
 890 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
 891 For example, one might use <code><abbr>TZ</abbr>="<abbr
 892 title="Japan Standard Time">JST</abbr>-9"</code> and
 893 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
 894 for Japan and Hawaii, respectively. If the
 895 <code><abbr>tz</abbr></code> database is available, it is usually better to use
 896 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
 897 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
 898 confusion, handle old time stamps better, and insulate you better from
 899 any future changes to the rules. One should never set
 900 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
 901 <code>"GMT-9"</code>, though, since this would incorrectly imply that
 902 local time is nine hours ahead of <abbr>UTC</abbr> and the time zone
 903 is called "<abbr>GMT</abbr>".</li>
 904 </ul>
 905 <h2 id="see-also">See also</h2>
 906 <ul>
 907 <li><a href="tz-art.htm">Time and the Arts</a></li>
 908 </ul>
 909 <hr>
 910 <address>
 911 This web page is in the public domain, so clarified as of
 912 2009-05-17 by Arthur David Olson.
 913 <br>
 914 Please send corrections to this web page to the
 915 <a href="mailto:tz@iana.org">time zone mailing list</a>.
 916 </address>
 917 </body>
 918 </html>