lib/libc/time/tz-link.htm

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   6 <title>Sources for Time Zone and Daylight Saving Time Data</title>
   7 <link rel="schema.DC" href="http://purl.org/DC/elements/1.1/">
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   9 <meta name="DC.Creator" content="Eggert, Paul">
  10 <meta name="DC.Contributor" content="Olson, Arthur David">
  11 <meta name="DC.Date" content="2015-10-01">
  12 <meta name="DC.Description"
  13  content="Sources of information about time zones and daylight saving time">
  14 <meta name="DC.Identifier"
  15  content="http://www.iana.org/time-zones/repository/tz-link.htm">
  16 <meta name="Keywords"
  17  content="database,daylight saving,DST,time zone,timezone,tz,zoneinfo">
  18 </head>
  19 <body>
  20 <h1>Sources for Time Zone and Daylight Saving Time Data</h1>
  21 <h2>The <code><abbr title="time zone">tz</abbr></code> database</h2>
  22 <p>
  23 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
  24 time zone database contains code and data
  25 that represent the history of local time
  26 for many representative locations around the globe.
  27 It is updated periodically to reflect changes made by political bodies
  28 to <a href="https://en.wikipedia.org/wiki/Time_zone">time zone</a>
  29 boundaries and
  30 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
  31 rules.
  32 This database (often called <code>zoneinfo</code> or
  33 <code><abbr>tz</abbr></code>)
  34 is used by several implementations,
  35 including
  36 <a href="http://www.gnu.org/software/libc/">the
  37 <abbr title="GNU's Not Unix">GNU</abbr>
  38 C Library</a> (used in
  39 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
  40 <a href="http://www.android.com">Android</a>,
  41 <a href="https://developer.mozilla.org/en-US/Firefox_OS">Firefox
  42 <abbr title="Operating System">OS</abbr></a>,
  43 <a href="https://www.freebsd.org">Free<abbr
  44 title="Berkeley Software Distribution">BSD</abbr></a>,
  45 <a href="http://netbsd.org">Net<abbr>BSD</abbr></a>,
  46 <a href="http://www.openbsd.org">Open<abbr>BSD</abbr></a>,
  47 <a href="https://cygwin.com">Cygwin</a>,
  48 <a href="http://www.delorie.com/djgpp/"><abbr
  49 title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
  50 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
  51 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
  52 title="Web Operating System">webOS</abbr></a>,
  53 <a href="http://ibm.com/aix"><abbr
  54 title="Advanced Interactive eXecutive">AIX</abbr></a>,
  55 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
  56 <a href="http://www.apple.com/ios/"><abbr
  57 title="iPhone OS">iOS</abbr></a>,
  58 <a href="http://windows.microsoft.com">Microsoft Windows</a>,
  59 <a href="http://www8.hp.com/us/en/products/servers/openvms/overview.html">Open<abbr
  60 title="Virtual Memory System">VMS</abbr></a>,
  61 <a href="https://www.oracle.com/database/index.html">Oracle Database</a>,
  62 <a href="http://oracle.com/solaris">Oracle Solaris</a>, and
  63 <a href="http://www.apple.com/osx/"><abbr title="Operating System Ten">OS
  64 X</abbr></a>.</p>
  65 <p>
  66 Each location in the database represents a region where all
  67 clocks keeping local time have agreed since 1970.
  68 Locations are identified by continent or ocean and then by the name of
  69 the location, which is typically the largest city within the region.
  70 For example, <code>America/New_York</code>
  71 represents most of the <abbr title="United States">US</abbr> eastern time zone;
  72 <code>America/Phoenix</code> represents most of Arizona, which
  73 uses mountain time without daylight saving time (<abbr
  74 title="daylight saving time">DST</abbr>);
  75 <code>America/Detroit</code> represents most of Michigan, which uses
  76 eastern time but with different <abbr>DST</abbr> rules in 1975;
  77 and other entries represent smaller regions like Starke County,
  78 Indiana, which switched from central to eastern time in 1991
  79 and switched back in 2006.
  80 To use the database on an extended <a
  81 href="https://en.wikipedia.org/wiki/POSIX"><abbr
  82 title="Portable Operating System Interface">POSIX</abbr></a>
  83 implementation set the <code><abbr>TZ</abbr></code>
  84 environment variable to the location's full name,
  85 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
  86 <p>
  87 Associated with each region is a history of offsets from
  88 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
  89 Time</a> (<abbr>UT</abbr>), which is <a
  90 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
  91 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
  92 for time stamps after 1960 this is more precisely <a
  93 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
  94 Universal Time</a> (<abbr>UTC</abbr>).
  95 The database also records when daylight saving time was in use,
  96 along with alphabetic time zone abbreviations such as <abbr>EST</abbr>
  97 for Eastern Standard Time in the <abbr>US</abbr>.</p>
  98 <p>
  99 In the <code><abbr>tz</abbr></code> database's
 100 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
 101 title="File Transfer Protocol">FTP</abbr> releases</a>
 102 the code is in the file <code>tzcode<var>C</var>.tar.gz</code>,
 103 where <code><var>C</var></code> is the code's version;
 104 similarly, the data entries are in <code>tzdata<var>D</var>.tar.gz</code>,
 105 where <code><var>D</var></code> is the data's version.
 106 Since 1996, each version has been a four-digit year followed by
 107 lower-case letter (<samp>a</samp> through <samp>z</samp>,
 108 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
 109 through <samp>zzz</samp>, and so on).
 110 Convenience links to
 111 the <a href="ftp://ftp.iana.org/tz/tzcode-latest.tar.gz">latest
 112 code</a> and
 113 <a href="ftp://ftp.iana.org/tz/tzdata-latest.tar.gz">latest data</a> revisions
 114 are also available.
 115 The following <a
 116 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
 117 these files to a <abbr>GNU</abbr>/Linux or similar host;
 118 see the downloaded
 119 <code>README</code> file for what to do next.</p>
 120 <pre style="margin-left: 2em"><code>mkdir tz
 121 cd tz
 122 <a href="http://www.gnu.org/software/wget/">wget</a> --retr-symlinks 'ftp://ftp.iana.org/tz/tz*-latest.tar.gz'
 123 <a href="http://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="http://www.gnu.org/software/tar/">tar</a> -xf -
 124 gzip -dc tzdata-latest.tar.gz | tar -xf -
 125 </code></pre>
 126 <p>
 127 The code and data files can also be obtained from the
 128 <a href="http://www.iana.org/time-zones">Time Zone Database website</a>
 129 of the <a href="http://www.iana.org">Internet Assigned Numbers
 130 Authority (IANA)</a>.
 131 An <a href="https://github.com/eggert/tz">unofficial development
 132 repository</a> of the code and data is available
 133 in <a href="http://git-scm.com">Git</a> form
 134 from <a href="https://github.com">GitHub</a>; be careful, as this
 135 repository is less well tested and probably contains more errors.
 136 <p>
 137 The code lets you compile the <code><abbr>tz</abbr></code> source files into
 138 machine-readable binary files, one for each location. It also lets
 139 you read a <code><abbr>tz</abbr></code> binary file and interpret time stamps for that
 140 location.</p>
 141 <p>
 142 The files are by no means authoritative. If you find errors, please
 143 send changes to the <a href="mailto:tz@iana.org">time zone
 144 mailing list</a>. You can also <a
 145 href="http://news.gmane.org/gmane.comp.time.tz">browse recent
 146 messages</a> sent to the mailing list, <a
 147 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it,
 148 and browse the <a
 149 href="http://mm.icann.org/pipermail/tz/">archive of old
 150 messages</a>. For further information about updates, please see
 151 <a href="http://tools.ietf.org/html/rfc6557">Procedures for
 152 Maintaining the Time Zone Database</a> (Internet <abbr
 153 title="Request For Comments">RFC</abbr> 6557).</p>
 154 <p>
 155 The Web has several other sources for time zone and daylight saving time data.
 156 Here are some links that may be of interest.
 157 </p>
 158 <h2>Commentary on the <code><abbr>tz</abbr></code> database</h2>
 159 <ul>
 160 <li>The article
 161 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
 162 an encyclopedic summary.</li>
 163 <li><a href="https://web.archive.org/web/20140221080916/http://www.cstdbill.com/tzdb/tz-how-to.html">How to Read the
 164 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
 165 database format.</li>
 166 <li><a
 167 href="http://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
 168 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
 169 database's style.</li>
 170 </ul>
 171 <h2>Web sites using recent versions of the <code><abbr>tz</abbr></code> database</h2>
 172 <p>
 173 These are listed roughly in ascending order of complexity and fanciness.
 174 </p>
 175 <ul>
 176 <li><a href="http://time.is">Time.is</a> shows locations'
 177 time and zones in a simple format.</li>
 178 <li><a
 179 href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
 180 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
 181 <li><a
 182 href="http://convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
 183 Time in 1000 Places</a> uses descriptions of the values.</li>
 184 <li><a href="http://www.timezoneconverter.com/cgi-bin/tzc.tzc">Time Zone
 185 Converter</a>
 186 uses a pulldown menu.</li>
 187 <li><a href="http://home.kpn.nl/vanadovv/time/TZworld.html">Complete
 188 timezone information for all countries</a> displays tables of DST rules.
 189 <li><a href="http://www.timeanddate.com/worldclock/">The World Clock &ndash;
 190 Time Zones</a> lets you sort zone names and convert times.</li>
 191 <li><a href="http://www.zeitverschiebung.net/en/">Time Difference</a>
 192 calculates the current time difference between locations.</li>
 193 <li><a href="http://www.wx-now.com">Weather Now</a> lists the weather too.</li>
 194 <li><a href="http://www.thetimenow.com">The Time Now</a> also lists weather.</li>
 195 <li><a href="http://worldtime.io">worldtime.io</a>
 196 also contains data about time zone boundaries; it supports queries via place
 197 names and shows location maps.</li>
 198 </ul>
 199 <h2>Network protocols for <code><abbr>tz</abbr></code> data</h2>
 200 <ul>
 201 <li>The <a href="http://www.ietf.org">Internet Engineering Task Force</a>'s
 202 <a href="http://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
 203 Distribution Service (tzdist) working group</a> is developing a scalable
 204 protocol for distributing time zone data to clients. It has generated a
 205 a <a href="http://datatracker.ietf.org/doc/draft-ietf-tzdist-service/">draft
 206 protocol for transferring time zone data</a> and a <a
 207 href="http://datatracker.ietf.org/doc/draft-ietf-tzdist-caldav-timezone-ref/">draft
 208 protocol for transferring time zone data by reference</a>. This work is based
 209 on the iCalendar and CalConnect efforts described below.</li>
 210 <li>The <a href="http://tools.ietf.org/html/rfc5545">
 211 Internet Calendaring and Scheduling Core Object Specification
 212 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
 213 covers time zone
 214 data; see its VTIMEZONE calendar component.
 215 The iCalendar format requires specialized parsers and generators; a
 216 variant <a href="http://tools.ietf.org/html/rfc6321">xCal</a>
 217 (Internet <abbr>RFC</abbr> 6321) uses
 218 <a href="http://www.w3.org/XML/"><abbr
 219 title="Extensible Markup Language">XML</abbr></a> format, and a variant
 220 <a href="http://tools.ietf.org/html/rfc7265">jCal</a>
 221 (Internet <abbr>RFC</abbr> 7265)
 222 uses <a href="http://www.json.org"><abbr
 223 title="JavaScript Object Notation">JSON</abbr></a> format.
 224 <a href="http://calconnect.org">CalConnect, The Calendaring and Scheduling
 225 Consortium</a> is promoting further work in this area. <a
 226 href="http://calconnect.org/publications/icalendartimezoneproblemsandrecommendationsv1.0.pdf">iCalendar
 227 TIMEZONE Problems and Recommendations</a> offers guidelines and
 228 recommendations for the use of VTIMEZONE and <code><abbr>tz</abbr></code>.</li>
 229 <li>The <a
 230 href="http://calconnect.org/publications/timezoneregistryandservicerecommendationsv1.0.pdf">Timezone
 231 Registry and Service Recommendations</a> of CalConnect's
 232 <a href="http://calconnect.org/tc-timezone.shtml">TIMEZONE
 233 Technical Committee</a> discusses a
 234 strategy for defining and deploying a time zone
 235 registration process that would establish unique names for each
 236 version of each <code><abbr>tz</abbr></code> zone, along with a polygonal
 237 representation of the geographical area corresponding to the
 238 zone.</li>
 239 <li>The <a
 240 href="http://lists.w3.org/Archives/Public/www-rdf-calendar/">www-rdf-calendar</a>
 241 list discusses <a
 242 href="http://www.w3.org/RDF/"><abbr
 243 title="Resource Description Framework">RDF</abbr></a>-based calendar
 244 and group scheduling systems, and has a <a
 245 href="http://www.w3.org/2002/12/cal/#tzd">workspace on time zone
 246 data</a> converted from <code><abbr>tz</abbr></code>.</li>
 247 </ul>
 248 <h2>Other <code><abbr>tz</abbr></code> compilers</h2>
 249 <ul>
 250 <li><a href="http://sourceforge.net/projects/vzic/">Vzic</a> is a <a
 251 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
 252 program that compiles
 253 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
 254 Vzic is freely
 255 available under the <a
 256 href="http://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
 257 General Public License (<abbr
 258 title="General Public License">GPL</abbr>)</a>.</li>
 259 <li><a href="http://sourceforge.net/projects/tzical/">tziCal &ndash; tz
 260 database conversion utility</a> is like Vzic, except for the <a
 261 href="https://msdn.microsoft.com/netframework">.NET framework</a>
 262 and with a <abbr>BSD</abbr>-style license.</li>
 263 <li><a
 264 href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
 265 contains a script <code>parse_olson</code> that compiles
 266 <code><abbr>tz</abbr></code> source into <a href="http://www.perl.org">Perl</a>
 267 modules. It is part of the Perl <a
 268 href="http://datetime.perl.org">DateTime Project</a>, which is freely
 269 available under both the <abbr>GPL</abbr> and the Perl Artistic
 270 License. DateTime::TimeZone also contains a script
 271 <code>tests_from_zdump</code> that generates test cases for each clock
 272 transition in the <code><abbr>tz</abbr></code> database.</li>
 273 <li>The <a href="http://howardhinnant.github.io/tz.html">Time Zone
 274 Database Parser</a> is a
 275 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
 276 runtime library. It is freely available under the
 277 <a href="http://creativecommons.org/licenses/by/4.0/">Creative Commons
 278 Attribution 4.0 International Public License</a>.</li>
 279 <li><a href="http://site.icu-project.org">International Components for
 280 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
 281 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
 282 libraries for internationalization that
 283 has a compiler from <code><abbr>tz</abbr></code> source
 284 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
 285 (mentioned below)
 286 into an <abbr>ICU</abbr>-specific format.
 287 <abbr>ICU</abbr> is freely available under a
 288 <abbr>BSD</abbr>-style license.</li>
 289 <li>The <a
 290 href="http://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
 291 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
 292 Oracle Java.</li>
 293 <li><a href="http://www.joda.org/joda-time/">Joda-Time &ndash; Java date
 294 and time <abbr title="Application Program Interface">API</abbr></a>
 295 contains a class
 296 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
 297 <code><abbr>tz</abbr></code> source into a Joda-specific binary format. Joda Time
 298 is freely available under a <abbr>BSD</abbr>-style license.</li>
 299 <li><a href="http://nodatime.org">Noda Time &ndash; Date and time API for
 300 .NET</a> and <a href="http://www.babiej.demon.nl/Tz4Net/main.htm">TZ4Net</a>
 301 are similar to Joda Time, but for the .NET framework instead of
 302 Java. They are freely available under the
 303 <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License</a>
 304 and a <abbr>BSD</abbr>-style license, respectively.</li>
 305 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
 306 compilers and libraries include:
 307 <ul>
 308 <li><a href="http://momentjs.com/timezone/">Moment Timezone</a> is a
 309 plugin for the <a href="http://momentjs.com">Moment.js</a> date
 310 manipulation library. It is freely available under
 311 the <abbr title="Massachusetts Institute of Technology">MIT</abbr>
 312 license.</li>
 313 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
 314 <abbr>API</abbr> is upward compatible with standard JavaScript
 315 Dates. It is freely available under the Apache License.</li>
 316 <li><a href="https://github.com/sproutsocial/walltime-js">Walltime-js</a>
 317 translates <abbr>UT</abbr> to local time. It is freely available under
 318 the <abbr>MIT</abbr> license.</li>
 319 </ul>
 320 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
 321 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
 322 <a href="https://www.python.org">Python</a>.
 323 It is freely available under a <abbr>BSD</abbr>-style license.</li>
 324 <li><a href="http://tzinfo.github.io">TZInfo &ndash;
 325 Ruby Timezone Library</a>
 326 compiles <code><abbr>tz</abbr></code> source into
 327 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
 328 It is freely available under the <abbr>MIT</abbr> license.</li>
 329 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
 330 Library</a> is
 331 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
 332 library that compiles <code><abbr>tz</abbr></code> source into a time
 333 zone repository whose format
 334 is either proprietary or an <abbr>XML</abbr>-encoded
 335 representation.</li>
 336 <li><a href="http://tcl.tk">Tcl</a>
 337 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
 338 source into text files, along with a runtime that can read those
 339 files. Tcl is freely available under a <abbr>BSD</abbr>-style
 340 license.</li>
 341 </ul>
 342 <h2>Other <code><abbr>tz</abbr></code> binary file readers</h2>
 343 <ul>
 344 <li>The <a
 345 href="http://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
 346 Library</a>
 347 has an independent, thread-safe implementation of
 348 a <code><abbr>tz</abbr></code> binary file reader.
 349 This library is freely available under the
 350 <a href="http://www.gnu.org/copyleft/lesser.html">
 351 <abbr>GNU</abbr> Lesser General Public License
 352 (<abbr title="Lesser General Public License">LGPL</abbr>)</a>,
 353 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
 354 <li><a href="https://www.gnome.org">GNOME</a>'s Glib has
 355 a <code><abbr>tz</abbr></code> binary file reader written in C that
 356 creates a <code>GTimeZone</code> object representing sets
 357 of <abbr>UT</abbr> offsets.
 358 It is freely available under the <abbr>LGPL</abbr>.</li>
 359 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
 360 is a <code><abbr>tz</abbr></code> binary file reader written in Java.
 361 It is freely available under the <abbr>LGPL</abbr>.</li>
 362 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
 363 JavaScript library that supports date arithmetic that is time zone
 364 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
 365 <li>Tcl, mentioned above, also contains a
 366 <code><abbr>tz</abbr></code> binary file reader.</li>
 367 <li><a href="http://search.cpan.org/perldoc?DateTime::TimeZone::Tzfile">
 368 DateTime::TimeZone::Tzfile</a>
 369 is a <code><abbr>tz</abbr></code> binary file reader written in Perl.
 370 It is freely available under the same terms as Perl
 371 (dual <abbr>GPL</abbr> and Artistic license).</li>
 372 <li>The
 373 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
 374 library contains a Python tool that
 375 converts <code><abbr>tz</abbr></code> binary data into
 376 <abbr>JSON</abbr>-format data suitable for use
 377 in its JavaScript library for time zone conversion. Dates before 1970
 378 are not supported.</li>
 379 <li>The <a
 380 href="http://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
 381 package contains <a href="http://haskell.org">Haskell</a> code that
 382 parses and uses <code><abbr>tz</abbr></code> binary data. It is freely
 383 available under a BSD-style license.</li>
 384 </ul>
 385 <h2>Other <code><abbr>tz</abbr></code>-based time zone software</h2>
 386 <ul>
 387 <li><a href="https://foxclocks.org">FoxClocks</a>
 388 is an extension for <a href="http://www.google.com/chrome/">Google
 389 Chrome</a> and for <a
 390 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
 391 Toolkit</a> applications like <a
 392 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
 393 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
 394 It displays multiple clocks in the application window, and has a mapping
 395 interface to <a href="http://www.google.com/earth/">Google Earth</a>.
 396 It is freely available under the <abbr>GPL</abbr>.</li>
 397 <li><a href="http://golang.org">Go programming language</a>
 398 implementations contain a copy of a 32-bit subset of a recent
 399 <code><abbr>tz</abbr></code> database in a
 400 Go-specific format.</li>
 401 <li><a
 402 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
 403 clock (intclock)</a> is a clock that displays multiple time zones on
 404 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
 405 under the <abbr>GPL</abbr>.</li>
 406 <li>Microsoft Windows 8.1
 407 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
 408 data (mentioned below) used by
 409 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a>
 410 classes such as <a
 411 href="https://msdn.microsoft.com/en-us/library/windows/apps/windows.globalization.datetimeformatting.datetimeformatter.aspx"><code>DateTimeFormatter</code></a>.
 412 <a
 413 href="http://blogs.msdn.com/b/bclteam/archive/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free.aspx">Exploring
 414 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
 415 the older, proprietary method of Microsoft Windows 2000 and later,
 416 which stores time zone data in the
 417 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
 418 <a
 419 href="http://www.unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
 420 Tzid table</a> or <a
 421 href="http://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
 422 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
 423 to <code><abbr>tz</abbr></code> names.
 424 <li><a
 425 href="https://www.oracle.com/java/index.html">Oracle
 426 Java</a> contains a copy of a subset of a recent
 427 <code><abbr>tz</abbr></code> database in a
 428 Java-specific format.</li>
 429 <li><a href="https://kimmo.suominen.com/sw/timezone/">Time Zone</a> is
 430 a <a href="https://wordpress.org">WordPress</a> plugin. It is freely
 431 available under a <abbr>BSD</abbr>-style license.</li>
 432 <li><a href="http://www.relativedata.com/time-zone-master">Time Zone
 433 Master</a> is a Microsoft Windows clock program that can automatically
 434 download, compile and use the <code>tzdata<var>D</var>.tar.gz</code>
 435 files as they are released. The Basic version is free.</li>
 436 <li><a
 437 href="http://veladg.com/velaterra.html">VelaTerra</a> is
 438 an <abbr>OS X</abbr> program. Its developers
 439 <a href="http://veladg.com/tzoffer.html">offer free
 440 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
 441 </ul>
 442 <h2>Other time zone databases</h2>
 443 <ul>
 444 <li><a href="http://www.astro.com/cgi/aq.cgi">Atlas Query</a>
 445 is Astrodienst's Web version of Shanks and Pottenger's
 446 time zone history atlases published in both <a
 447 href="http://astrocom.com/astrology-products/software/acs-atlas-software">computer</a>
 448 and book form (<a
 449 href="http://www.astrocom.com/astrology/books/american-atlas">one volume
 450 for the <abbr>US</abbr></a>, and <a
 451 href="http://www.astrocom.com/astrology/books/international-atlas">one for
 452 other locations</a>) by <a
 453 href="http://astrocom.com">Astro Computing Services</a>.
 454 These atlases are extensive but unreliable, as Shanks appears to have
 455 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
 456 sources and do not indicate which entries are guesswork.</li>
 457 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
 458 its own <code>tztab</code>(4) format.</li>
 459 <li>Microsoft Windows has proprietary data mentioned above.</li>
 460 <li><a href="http://www.worldtimeserver.com">World Time Server</a>
 461 is another time zone database.</li>
 462 <li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a>
 463 contains data from the Time Service Department of the
 464 <abbr>US</abbr> Naval Observatory.</li>
 465 <li>The <a href="http://www.iata.org/publications/Pages/ssim.aspx">Standard
 466 Schedules Information Manual</a> of the
 467 International Air Transport Association
 468 gives current time zone rules for airports served by commercial aviation.</li>
 469 </ul>
 470 <h2>Maps</h2>
 471 <ul>
 472 <li>The <a href="https://www.cia.gov/index.html">United States Central
 473 Intelligence Agency (<abbr
 474 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
 475 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
 476 zone map</a>; the
 477 <a
 478 href="http://www.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
 479 Library Map Collection</a>
 480 of the University of Texas at Austin has copies of
 481 recent editions.
 482 The pictorial quality is good,
 483 but the maps do not indicate summer time,
 484 and parts of the data are a few years out of date.</li>
 485 <li><a href="http://www.worldtimezone.com">Current time around the world
 486 and standard time zones map of the world</a>
 487 has several fancy time zone maps; it covers Russia particularly well.
 488 The maps' pictorial quality is not quite as good as the
 489 <abbr>CIA</abbr>'s
 490 but the maps are more up to date.</li>
 491 <li><a
 492 href="http://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
 493 much is time wrong around the world?</a> maps the difference between
 494 mean solar and standard time, highlighting areas such as western China
 495 where the two differ greatly. It's a bit out of date, unfortunately.</li>
 496 </ul>
 497 <h2>Time zone boundaries</h2>
 498 <ul>
 499 <li><a href="http://efele.net/maps/tz/"><abbr>TZ</abbr> timezones
 500 maps</a> contains <a
 501 href="https://en.wikipedia.org/wiki/Shapefile">shapefiles</a> of
 502 sets of <code><abbr>tz</abbr></code> regions.</li>
 503 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
 504 maps geographical coordinates to a <code><abbr>tz</abbr></code> region.
 505 It is written in Go and is freely available under the Apache License.</li>
 506 <li><a href="http://derickrethans.nl/what-time-is-it.html">What Time
 507 is It Here?</a> applies <a href="http://www.mongodb.org">MongoDB</a>
 508 geospatial query operators to shapefiles' data.</li>
 509 <li><a href="http://statoids.com/statoids.html">Administrative
 510 Divisions of Countries ("Statoids")</a> contains lists of
 511 political subdivision data related to time zones.</li>
 512 <li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time
 513 zone boundaries for multizone countries</a> summarizes legal
 514 boundaries between time zones within countries.</li>
 515 <li>Manifold.net's <a
 516 href="http://manifold.net/info/freestuff.shtml">Free Stuff for
 517 Manifold System Users</a> includes a Manifold-format map of
 518 world time zone boundaries distributed under the
 519 <abbr>GPL</abbr>.</li>
 520 <li>The GeoCommunity lists several commercial sources for <a
 521 href="http://spatialnews.geocomm.com/features/timezones/">International
 522 Time Zones and Time Zone Data</a>.</li>
 523 <li>A ship within the <a
 524 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
 525 waters</a> of any nation uses that nation's time. In international
 526 waters, time zone boundaries are meridians 15&deg; apart, except that
 527 <abbr>UTC</abbr>&minus;12 and <abbr>UTC</abbr>+12 are each 7.5&deg;
 528 wide and are separated by
 529 the 180&deg; meridian (not by the International Date Line, which is
 530 for land and territorial waters only). A captain can change ship's
 531 clocks any time after entering a new time zone; midnight changes are
 532 common.</li>
 533 </ul>
 534 <h2>Civil time concepts and history</h2>
 535 <ul>
 536 <li><a href="http://www.nist.gov/pml/general/time/index.cfm">A
 537 Walk through Time</a>
 538 surveys the evolution of timekeeping.</li>
 539 <li><a href="http://www.webexhibits.org/daylightsaving/">About Daylight
 540 Saving Time &ndash; History, rationale, laws &amp; dates</a>
 541 is an overall history of <abbr>DST</abbr>.</li>
 542 <li><a href="http://www.w3.org/TR/timezone/">Working with Time Zones</a>
 543 contains guidelines and best practices for software applications that
 544 deal with civil time.</li>
 545 <li><a href="http://energy.ca.gov/daylightsaving.html">Saving Time,
 546 Saving Energy</a> discusses a primary justification for <abbr>DST</abbr>.</li>
 547 <li><a href="http://seizethedaylight.com/dst/">A Brief
 548 History of Daylight Saving Time</a> summarizes some of the contentious
 549 history of <abbr>DST</abbr>.</li>
 550 <li><a href="http://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
 551 the International Date Line</a> tells the story of the most important
 552 time zone boundary.</li>
 553 <li><a href="http://statoids.com/tconcept.html">Basic Time
 554 Zone Concepts</a> discusses terminological issues behind time zones.</li>
 555 </ul>
 556 <h2>National histories of legal time</h2>
 557 <dl>
 558 <dt>Australia</dt>
 559 <dd>The Parliamentary Library has commissioned a <a
 560 href="http://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
 561 paper on daylight saving time in Australia</a>.
 562 The Bureau of Meteorology publishes a list of <a
 563 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
 564 Dates of Daylight Savings Time within Australia</a>.</dd>
 565 <dt>Belgium</dt>
 566 <dd>The Royal Observatory of Belgium maintains a table of <a
 567 href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
 568 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
 569 <dt>Brazil</dt>
 570 <dd>The Time Service Department of the National Observatory
 571 records <a href="http://pcdsh01.on.br/DecHV.html"
 572 hreflang="pt-BR">Brazil's daylight saving time decrees (in
 573 Portuguese)</a>.</dd>
 574 <dt>Canada</dt>
 575 <dd>National Research Council Canada publishes current
 576 and some older information about <a
 577 href="http://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time
 578 zones &amp; daylight saving time</a>.</dd>
 579 <dt>Chile</dt>
 580 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
 581 <a href="http://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
 582 Chile's official time (in Spanish)</a>.</dd>
 583 <dt>Germany</dt>
 584 <dd>The National Institute for Science and Technology maintains the <a
 585 href="http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
 586 of Legal Time in Germany</a>.</dd>
 587 <dt>Israel</dt>
 588 <dd>The Interior Ministry periodically issues <a
 589 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
 590 hreflang="he">announcements (in Hebrew)</a>.</dd>
 591 <dt>Mexico</dt>
 592 <dd>The Investigation and Analysis Service of the Mexican Library of
 593 Congress has published a <a
 594 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
 595 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
 596 <dt>Malaysia</dt>
 597 <dd>See Singapore below.</dd>
 598 <dt>Netherlands</dt>
 599 <dd><a href="http://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
 600 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
 601 covers the history of local time in the Netherlands from ancient times.</dd>
 602 <dt>New Zealand</dt>
 603 <dd>The Department of Internal Affairs maintains a brief <a
 604 href="http://www.dia.govt.nz/Daylight-Saving-History">History of
 605 Daylight Saving</a>. The privately-maintained <a
 606 href="http://astrologyschool.com/nztime.html">History of New Zealand
 607 time</a> has more details.</dd>
 608 <dt>Singapore</dt>
 609 <dd><a
 610 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
 611 is Singapore in the "Wrong" Time Zone?</a> details the
 612 history of legal time in Singapore and Malaysia.</dd>
 613 <dt>United Kingdom</dt>
 614 <dd><a
 615 href="http://www.polyomino.org.uk/british-time/">History of
 616 legal time in Britain</a> discusses in detail the country
 617 with perhaps the best-documented history of clock adjustments.
 618 The National Physical Laboratory also maintains an <a
 619 href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates-1916-2006">Archive
 620 of Summer time dates</a>.</dd>
 621 <dt>United States</dt>
 622 <dd>The Department of Transportation's <a
 623 href="http://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
 624 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
 625 </dl>
 626 <h2>Precision timekeeping</h2>
 627 <ul>
 628 <li><a
 629 href="http://literature.agilent.com/litweb/pdf/5965-7984E.pdf">The
 630 Science of Timekeeping</a> is a thorough introduction
 631 to the theory and practice of precision timekeeping.</li>
 632 <li><a href="http://www.ntp.org"><abbr
 633 title="Network Time Protocol">NTP</abbr>: The Network
 634 Time Protocol</a>
 635 discusses how to synchronize clocks of
 636 Internet hosts.</li>
 637 <li>The <a href="http://www.nist.gov/el/isd/ieee/ieee1588.cfm">Precision
 638 Time Protocol</a> (<abbr
 639 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
 640 can achieve submicrosecond clock accuracy on a local area network.</li>
 641 <li><a
 642 href="http://tools.ietf.org/html/rfc4833">Timezone
 643 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
 644 (Internet <abbr>RFC</abbr> 4833)
 645 specifies a <a
 646 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
 647 option for a server to configure
 648 a client's time zone and daylight saving settings automatically.</li>
 649 <li><a href="http://gauss.gge.unb.ca/GMT.UT.and.the.RGO.html">A Few Facts
 650 Concerning <abbr>GMT</abbr>, <abbr>UT</abbr>, and
 651 the <abbr title="Royal Greenwich Observatory">RGO</abbr></a>
 652 answers questions like "What is the
 653 difference between <abbr>GMT</abbr> and <abbr>UTC</abbr>?"</li>
 654 <li><a
 655 href="http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
 656 Times</a> explains more abstruse astronomical time scales like
 657 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
 658 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
 659 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
 660 <a href="http://www.ucolick.org/~sla/leapsecs/timescales.html">Time
 661 Scales</a> goes into more detail, particularly for historical variants.</li>
 662 <li>The <a href="http://iau.org"><abbr
 663 title="International Astronomical Union">IAU</abbr></a>'s <a
 664 href="http://www.iausofa.org"><abbr
 665 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
 666 collection contains C and <a
 667 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
 668 code for converting among time scales like
 669 <abbr title="International Atomic Time">TAI</abbr>,
 670 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
 671 <abbr>UTC</abbr>.</li>
 672 <li><a href="http://www2.jpl.nasa.gov/basics/bsf2-3.php">Basics of
 673 Space Flight &ndash; Reference Systems &ndash; Time Conventions</a>
 674 briefly explains interplanetary space flight timekeeping.</li>
 675 <li><a
 676 href="http://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
 677 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a> briefly
 678 describes Mars Coordinated Time (<abbr
 679 title="Mars Coordinated Time">MTC</abbr>) and the
 680 diverse local time
 681 scales used by each landed mission on Mars.</li>
 682 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
 683 dedicated not only to leap seconds but to precise time and frequency
 684 in general. It covers the state of the art in amateur timekeeping, and
 685 how the art has progressed over the past few decades.</li>
 686 <li><a
 687 href="http://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
 688 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
 689 Bulletins</a> contains official publications of the International
 690 Earth Rotation and Reference Systems Service, which decides
 691 when leap seconds occur.</li>
 692 <li>The <a
 693 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
 694 Second Discussion List</a> covers <a
 695 href="http://gauss.gge.unb.ca/papers.pdf/gpsworld.november99.pdf">McCarthy
 696 and Klepczynski's proposal to discontinue leap seconds</a>,
 697 discussed further in
 698 <a href="http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
 699 leap second: its history and possible future</a>.
 700 <a href="http://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
 701 might be redefined
 702 without Leap Seconds</a> gives pointers on this
 703 contentious issue.</li>
 704 </ul>
 705 <h2>Time notation</h2>
 706 <ul>
 707 <li>The <a href="http://cldr.unicode.org">Unicode Common Locale Data
 708 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
 709 zone names, abbreviations, identifiers, and formats. For example, it
 710 contains French translations for "Eastern European Summer Time",
 711 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
 712 "Bucharest". Its
 713 <a href="http://unicode.org/cldr/charts/by_type/index.html">by-type
 714 charts</a> show these values for many locales. Data values are available in
 715 both <abbr title="Locale Data Markup Language">LDML</abbr>
 716 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
 717 <li>
 718 <a href="http://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
 719 the international standard date and time notation</a> is a good
 720 summary of
 721 <a
 722 href="http://www.iso.org/iso/catalogue_detail?csnumber=40874"><abbr
 723 title="International Organization for Standardization">ISO</abbr>
 724 8601:2004 &ndash; Data elements and interchange formats &ndash; Information
 725 interchange &ndash; Representation of dates and times</a>.</li>
 726 <li>
 727 <a href="http://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
 728 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
 729 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
 730 <li>
 731 <a href="http://tools.ietf.org/html/rfc5322">Internet
 732 Message Format</a> (Internet <abbr>RFC</abbr> 5322) &sect;3.3
 733 specifies the time notation used in email and <a
 734 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
 735 headers.</li>
 736 <li>
 737 <a href="http://tools.ietf.org/html/rfc3339">Date and Time
 738 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
 739 specifies an <abbr>ISO</abbr> 8601
 740 profile for use in new Internet
 741 protocols.</li>
 742 <li>
 743 <a href="http://www.hackcraft.net/web/datetime/">Date &amp; Time
 744 Formats on the Web</a> surveys web- and Internet-oriented date and time
 745 formats.</li>
 746 <li>
 747 <a href="http://exit109.com/~ghealton/y2k/yrexamples.html">The
 748 Best of Dates, the Worst of Dates</a> covers many problems encountered
 749 by software developers when handling dates and time stamps.</li>
 750 <li>Alphabetic time zone abbreviations should not be used as unique
 751 identifiers for <abbr>UTC</abbr> offsets as they are ambiguous in
 752 practice. For example, in English-speaking North America
 753 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UTC</abbr>,
 754 but in China it denotes 8 hours ahead of <abbr>UTC</abbr>,
 755 and French-speaking North Americans prefer
 756 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
 757 "<abbr>CST</abbr>". For <abbr>POSIX</abbr> the <code><abbr>tz</abbr></code>
 758 database contains English abbreviations for all time stamps but in
 759 many cases these are merely inventions of the database
 760 maintainers.</li>
 761 <li>Numeric time zone abbreviations typically count hours east of
 762 <abbr>UTC</abbr>, e.g., +09 for Japan and
 763 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
 764 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
 765 For example, one might use <code><abbr>TZ</abbr>="<abbr
 766 title="Japan Standard Time">JST</abbr>-9"</code> and
 767 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
 768 for Japan and Hawaii, respectively. If the
 769 <code><abbr>tz</abbr></code> database is available, it is usually better to use
 770 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
 771 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
 772 confusion, handle old time stamps better, and insulate you better from
 773 any future changes to the rules. One should never set
 774 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
 775 <code>"GMT-9"</code>, though, since this would falsely claim that
 776 local time is nine hours ahead of <abbr>UTC</abbr> and the time zone
 777 is called "<abbr>GMT</abbr>".</li>
 778 </ul>
 779 <h2>See also</h2>
 780 <ul>
 781 <li><a href="tz-art.htm">Time and the Arts</a></li>
 782 </ul>
 783 <hr>
 784 <address>
 785 This web page is in the public domain, so clarified as of
 786 2009-05-17 by Arthur David Olson.
 787 <br>
 788 Please send corrections to this web page to the
 789 <a href="mailto:tz@iana.org">time zone mailing list</a>.
 790 </address>
 791 </body>
 792 </html>