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="2013-09-05">
  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>tz</code> database</h2>
  22 <p>
  23 The <a href="http://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="http://en.wikipedia.org/wiki/Time_zone">time zone</a>
  29 boundaries, <a
  30 href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
  31 title="Coordinated Universal Time">UTC</abbr></a> offsets, and
  32 <a href="http://en.wikipedia.org/wiki/Daylight_saving">daylight-saving</a>
  33 rules.
  34 This database (often called <code>zoneinfo</code> or <code>tz</code>)
  35 is used by several implementations,
  36 including
  37 <a href="http://www.gnu.org/software/libc/">the
  38 <abbr title="GNU's Not Unix">GNU</abbr>
  39 C Library</a> (used in
  40 <a href="http://www.linux.org/"><abbr>GNU</abbr>/Linux</a>),
  41 <a href="http://www.android.com/">Android</a>,
  42 <a href="https://developer.mozilla.org/en-US/docs/Mozilla/Firefox_OS">Firefox
  43 OS</a>,
  44 <a href="http://www.freebsd.org/">FreeBSD</a>,
  45 <a href="http://netbsd.org/">NetBSD</a>,
  46 <a href="http://openbsd.org/">OpenBSD</a>,
  47 <a href="http://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="http://en.wikipedia.org/wiki/WebOS">webOS</a>,
  51 <a href="http://ibm.com/aix">AIX</a>,
  52 <a href="http://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
  53 <a href="http://www.apple.com/ios/">iOS</a>,
  54 <a href="http://h71000.www7.hp.com/">OpenVMS</a>,
  55 <a href="http://oracle.com/database">Oracle Database</a>,
  56 <a href="http://www.apple.com/osx/">OS X</a>,
  57 <a href="http://oracle.com/solaris">Solaris</a>,
  58 <a href="http://h30097.www3.hp.com/">Tru64</a>, and
  59 <a href="http://sco.com/products/unixware">UnixWare</a>.</p>
  60 <p>
  61 Each location in the database represents a national region where all
  62 clocks keeping local time have agreed since 1970.
  63 Locations are identified by continent or ocean and then by the name of
  64 the location, which is typically the largest city within the region.
  65 For example, <code>America/New_York</code>
  66 represents most of the <abbr title="United States">US</abbr> eastern time zone;
  67 <code>America/Phoenix</code> represents most of Arizona, which
  68 uses mountain time without daylight saving time (<abbr
  69 title="daylight saving time">DST</abbr>);
  70 <code>America/Detroit</code> represents most of Michigan, which uses
  71 eastern time but with different <abbr>DST</abbr> rules in 1975;
  72 and other entries represent smaller regions like Starke County,
  73 Indiana, which switched from central to eastern time in 1991
  74 and switched back in 2006.
  75 To use the database on an extended <a
  76 href="http://en.wikipedia.org/wiki/POSIX"><abbr
  77 title="Portable Operating System Interface">POSIX</abbr></a>
  78 implementation set the <code>TZ</code> environment variable to
  79 the location's full name, e.g., <code>TZ="America/New_York"</code>.</p>
  80 <p>
  81 In the <code>tz</code> database's
  82 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
  83 title="File Transfer Protocol">FTP</abbr> releases</a>
  84 the code is in the file <code>tzcode<var>C</var>.tar.gz</code>,
  85 where <code><var>C</var></code> is the code's version;
  86 similarly, the data are in <code>tzdata<var>D</var>.tar.gz</code>,
  87 where <code><var>D</var></code> is the data's version.
  88 Each version is a four-digit year followed by lower-case letters
  89 (a through z, then za through zz, then zza through zzz, and so on).
  90 Convenience links to
  91 the <a href="ftp://ftp.iana.org/tz/tzcode-latest.tar.gz">latest
  92 code</a> and
  93 <a href="ftp://ftp.iana.org/tz/tzdata-latest.tar.gz">latest data</a> revisions
  94 are also available.
  95 The following <a
  96 href="http://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
  97 these files to a <abbr>GNU</abbr>/Linux or similar host;
  98 see the downloaded
  99 <code>README</code> file for what to do next.</p>
 100 <pre style="margin-left: 2em"><code>mkdir tz
 101 cd tz
 102 <a href="http://www.gnu.org/software/wget/">wget</a> --retr-symlinks 'ftp://ftp.iana.org/tz/tz*-latest.tar.gz'
 103 <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 -
 104 gzip -dc tzdata-latest.tar.gz | tar -xf -
 105 </code></pre>
 106 <p>
 107 The code and data files can also be obtained from the
 108 <a href="http://www.iana.org/time-zones">Time Zone Database website</a>
 109 of the <a href="http://www.iana.org">Internet Assigned Numbers
 110 Authority (IANA)</a>.
 111 An <a href="https://github.com/eggert/tz">unofficial development
 112 repository</a> of the code and data is available
 113 in <a href="http://git-scm.com/">Git</a> form
 114 from <a href="https://github.com/">GitHub</a>; be careful, as this
 115 repository is less well tested and probably contains more errors.
 116 <p>
 117 The code lets you compile the <code>tz</code> source files into
 118 machine-readable binary files, one for each location. It also lets
 119 you read a <code>tz</code> binary file and interpret time stamps for that
 120 location.</p>
 121 <p>
 122 The data are by no means authoritative. If you find errors, please
 123 send changes to the <a href="mailto:tz@iana.org">time zone
 124 mailing list</a>. You can also <a
 125 href="http://news.gmane.org/gmane.comp.time.tz">browse recent
 126 messages</a> sent to the mailing list, <a
 127 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it,
 128 and browse the <a
 129 href="http://mm.icann.org/pipermail/tz/">archive of old
 130 messages</a>. For further information about updates, please see
 131 <a href="http://tools.ietf.org/html/rfc6557">Procedures for
 132 Maintaining the Time Zone Database</a> (Internet <abbr title="Request
 133 For Comments">RFC</abbr> 6557).</p>
 134 <p>
 135 The Web has several other sources for time zone and daylight saving time data.
 136 Here are some links that may be of interest.
 137 </p>
 138 <h2>Commentary on the <code>tz</code> database</h2>
 139 <ul>
 140 <li>The article
 141 <a href="http://en.wikipedia.org/wiki/Tz_database">tz database</a> is
 142 an encyclopedic summary.</li>
 143 <li><a href="http://www.cstdbill.com/tzdb/tz-how-to.html">How to Read the
 144 tz Database Source Files</a> explains the tz database format.</li>
 145 <li><a href="http://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
 146 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
 147 database's style.</li>
 148 </ul>
 149 <h2>Web sites using recent versions of the <code>tz</code> database</h2>
 150 <p>
 151 These are listed roughly in ascending order of complexity and fanciness.
 152 </p>
 153 <ul>
 154 <li><a href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
 155 lets you see the <code>TZ</code> values directly.</li>
 156 <li><a
 157 href="http://convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
 158 Time in 1000 Places</a> uses descriptions of the values.</li>
 159 <li><a href="http://www.timezoneconverter.com/cgi-bin/tzc.tzc">Time Zone Converter</a>
 160 uses a pulldown menu.</li>
 161 <li><a href="http://home.tiscali.nl/~t876506/TZworld.html">Complete
 162 timezone information for all countries</a> displays tables of DST rules.
 163 <li><a href="http://timeanddate.com/worldclock/">The World Clock &ndash;
 164 Time Zones</a> lets you sort zone names and convert times.</li>
 165 <li><a href="http://permatime.com/">Permatime</a> generates and views
 166 links that refer to a particular point in time and can be displayed in
 167 multiple timezones.</li>
 168 <li><a href="http://daylight-savings-time.info/">Daylight Saving Time info</a>
 169 shows a graph of time
 170 difference versus time for any pair of locations.</li>
 171 <li>The <a href="http://worldtimeengine.com/">World Time Engine</a>
 172 also contains data about time zone boundaries; it supports queries via place
 173 names and shows location maps.</li>
 174 <li><a href="http://simpletimerclocks.mozdev.org/">Simple Timer + Clocks</a>
 175 is a Firefox add-on which uses a timezone data file generated from the
 176 tz data files.</li>
 177 </ul>
 178 <h2>Other time zone database formats</h2>
 179 <ul>
 180 <li>The <a href="http://tools.ietf.org/html/rfc5545">
 181 Internet Calendaring and Scheduling Core Object Specification
 182 (iCalendar)</a>, Internet <abbr>RFC</abbr> 5445
 183 covers time zone
 184 data; see its VTIMEZONE calendar component.
 185 <a href="http://calconnect.org/">CalConnect, The Calendaring and Scheduling
 186 Consortium</a> is promoting further work in this area. <a
 187 href="http://calconnect.org/publications/icalendartimezoneproblemsandrecommendationsv1.0.pdf">iCalendar
 188 TIMEZONE Problems and Recommendations</a> offers guidelines and
 189 recommendations for the use of VTIMEZONE and <code>tz</code>.</li>
 190 <li><a href="http://calconnect.org/dstlinks.shtml">Extended Daylight
 191 Saving Time Links, Advisories and Changes</a> lists vendor material on 2007
 192 U.S. daylight saving time changes.</li>
 193 <li><a
 194 href="http://calconnect.org/publications/timezoneregistryandservicerecommendationsv1.0.pdf">Timezone
 195 Registry and Service Recommendations</a> discusses a
 196 strategy for defining and deploying a time zone
 197 registration process that would establish unique names for each
 198 version of each <code>tz</code> zone, along with a polygonal
 199 representation of the geographical area corresponding to the
 200 zone.</li>
 201 <li>The <a
 202 href="http://lists.w3.org/Archives/Public/www-rdf-calendar/">www-rdf-calendar</a>
 203 list discusses <a
 204 href="http://www.w3.org/RDF/"><abbr
 205 title="Resource Description Framework">RDF</abbr></a>-based calendar
 206 and group scheduling systems, and has a <a
 207 href="http://www.w3.org/2002/12/cal/#tzd">workspace on time zone
 208 data</a> converted from <code>tz</code>. An earlier <a
 209 href="http://www.w3.org/2000/01/foo">schema</a> was sketched out.</li>
 210 </ul>
 211 <h2>Other <code>tz</code> compilers</h2>
 212 <ul>
 213 <li><a href="http://sourceforge.net/projects/vzic/">Vzic iCalendar
 214 Timezone Converter</a> describes a <a
 215 href="http://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
 216 program that compiles
 217 <code>tz</code> source into iCalendar-compatible VTIMEZONE files.
 218 Vzic is freely
 219 available under the <a
 220 href="http://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
 221 General Public License (<abbr
 222 title="General Public License">GPL</abbr>)</a>.</li>
 223 <li><a href="http://sourceforge.net/projects/tzical">tziCal - tz
 224 database conversion utility</a> is like Vzic, except for the <a
 225 href="http://msdn.microsoft.com/netframework">.NET framework</a>.</li>
 226 <li><a
 227 href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
 228 contains a script <code>parse_olson</code> that compiles
 229 <code>tz</code> source into <a href="http://www.perl.org/">Perl</a>
 230 modules. It is part of the Perl <a
 231 href="http://datetime.perl.org/">DateTime Project</a>, which is freely
 232 available under both the <abbr>GPL</abbr> and the Perl Artistic
 233 License. DateTime::TimeZone also contains a script
 234 <code>tests_from_zdump</code> that generates test cases for each clock
 235 transition in the <code>tz</code> database.</li>
 236 <li><a href="http://icu-project.org/">International Components for
 237 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
 238 href="http://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
 239 libraries for internationalization that
 240 has a compiler from <code>tz</code> source
 241 into an <abbr>ICU</abbr>-specific format.
 242 <abbr>ICU</abbr> is freely available under a
 243 <abbr title="Berkeley Software Distribution">BSD</abbr>-style license.</li>
 244 <li><a href="http://joda-time.sourceforge.net/">Joda Time - Java date
 245 and time <abbr title="Application Program Interface">API</abbr></a>
 246 contains a class
 247 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
 248 <code>tz</code> source into a Joda-specific binary format. Joda Time
 249 is freely available under a <abbr>BSD</abbr>-style license.</li>
 250 <li><a href="http://pytz.sourceforge.net">pytz - World Timezone
 251 Definitions for Python</a> compiles <code>tz</code> source into
 252 <a href="http://python.org/">Python</a>.
 253 It is freely available under a <abbr>BSD</abbr>-style license.</li>
 254 <li><a href="http://tzinfo.rubyforge.org/">TZInfo - Ruby Timezone Library</a>
 255 compiles <code>tz</code> source into
 256 <a href="http://www.ruby-lang.org/en/">Ruby</a>.
 257 It is freely available under the <abbr
 258 title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
 259 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
 260 Library</a> is
 261 a <a href="http://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
 262 library that compiles <code>tz</code> source into a time zone repository whose format
 263 is either proprietary or an <a href="http://www.w3.org/XML/"><abbr
 264 title="Extensible Markup Language">XML</abbr></a>-encoded
 265 representation.</li>
 266 <li>Starting with version 8.5, <a href="http://tcl.tk/">Tcl</a>
 267 contains a developer-oriented parser that compiles <samp>tz</samp>
 268 source into text files, along with a runtime that can read those
 269 files. Tcl is freely available under a <abbr>BSD</abbr>-style
 270 license.</li>
 271 </ul>
 272 <h2>Other <code>tz</code> binary file readers</h2>
 273 <ul>
 274 <li>The <a
 275 href="http://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
 276 Library</a>
 277 has an independent, thread-safe implementation of
 278 a <code>tz</code> binary file reader.
 279 This library is freely available under the
 280 <a href="http://www.gnu.org/copyleft/lesser.html">
 281 <abbr>GNU</abbr> Lesser General Public License
 282 (<abbr title="Lesser General Public License">LGPL</abbr>)</a>,
 283 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
 284 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
 285 is a <code>tz</code> binary file reader written in Java.
 286 It is freely available under the <abbr>LGPL</abbr>.</li>
 287 <li>Tcl, mentioned above, also contains a
 288 <code>tz</code> binary file reader.</li>
 289 <li><a href="http://search.cpan.org/perldoc?DateTime::TimeZone::Tzfile">
 290 DateTime::TimeZone::Tzfile</a>
 291 is a <code>tz</code> binary file reader written in Perl.
 292 It is freely available under the same terms as Perl
 293 (dual <abbr>GPL</abbr> and Artistic license).</li>
 294 <li>The
 295 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
 296 library contains a Python tool that converts <code>tz</code> binary data into
 297 <a href="http://www.json.org/">JSON</a>-format data suitable for use
 298 in its <a href="http://en.wikipedia.org/wiki/JavaScript">JavaScript</a> library for time zone conversion. Dates before 1970
 299 are not supported.</li>
 300 </ul>
 301 <h2>Other <code>tz</code>-based time zone software</h2>
 302 <ul>
 303 <li><a href="http://foxclocks.org/">FoxClocks</a>
 304 is an extension for <a href="http://google.com/chrome">Google
 305 Chrome</a> and for <a
 306 href="http://developer.mozilla.org/en/docs/Toolkit_API">Mozilla
 307 Toolkit</a> applications like <a
 308 href="http://mozilla.com/firefox">Firefox</a> and <a
 309 href="http://mozilla.com/thunderbird">Thunderbird</a>.
 310 It displays multiple clocks in the application window, and has a mapping
 311 interface to <a href="http://earth.google.com/">Google Earth</a>.
 312 It is freely available under the <abbr>GPL</abbr>.</li>
 313 <li><a
 314 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
 315 clock (intclock)</a> is a multi-timezone clock for
 316 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
 317 under the <abbr>GPL</abbr>.</li>
 318 <li><a href="http://www.oracle.com/us/technologies/java/overview/index.html">Oracle
 319 Java</a> releases since 1.4
 320 contain a copy of a subset of a recent <code>tz</code> database in a
 321 Java-specific format.</li>
 322 <li><a href="http://kimmo.suominen.com/sw/timezone/">Time Zone</a> is
 323 a <a href="http://wordpress.org/">WordPress</a> plugin. It is freely
 324 available under a <abbr>BSD</abbr>-style license.</li>
 325 <li><a href="http://www.relativedata.com/time-zone-master">Time Zone
 326 Master</a> is a Microsoft Windows clock program that can automatically
 327 download, compile and use the <code>tzdata<var>D</var>.tar.gz</code>
 328 files as they are released. The Basic version is free.</li>
 329 <li><a
 330 href="http://veladg.com/velaterra.html">VelaTerra</a> is
 331 a Mac OS X program. Its developers
 332 <a href="http://veladg.com/tzoffer.html">offer free
 333 licenses</a> to <code>tz</code> contributors.</li>
 334 <li><a
 335 href="http://worldtimeexplorer.com/">World Time Explorer</a> is a
 336 Microsoft Windows program.</li>
 337 </ul>
 338 <h2>Other time zone databases</h2>
 339 <ul>
 340 <li><a href="http://www.astro.com/cgi/aq.cgi">Atlas Query</a>
 341 is Astrodienst's Web version of Shanks and Pottenger's
 342 time zone history atlases published in both <a
 343 href="http://astrocom.com/astrology-products/software/acs-atlas-software">computer</a>
 344 and book form (<a
 345 href="http://www.astrocom.com/astrology/books/american-atlas">one volume
 346 for the U.S.</a>, and <a
 347 href="http://www.astrocom.com/astrology/books/international-atlas">one for
 348 other locations</a>) by <a
 349 href="http://astrocom.com/">Astro Computing Services</a>.</li>
 350 <li><a href="http://worldtime.com/">WORLDTIME: interactive atlas,
 351 time info, public holidays</a>
 352 contains information on local time, sunrise and sunset,
 353 and public holidays in several hundred cities around the world.</li>
 354 <li><a href="http://worldtimeserver.com/">World Time Server</a>
 355 is another time zone database.</li>
 356 <li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a>
 357 contains data from the Time Service Department of the
 358 <abbr>US</abbr> Naval Observatory, used as the source
 359 for the <code>usno*</code> files in the <code>tz</code> distribution.</li>
 360 <li>The <a href="http://www.iata.org/publications/Pages/ssim.aspx">Standard
 361 Schedules Information Manual</a> of the
 362 <a href="http://iata.org/index.htm">International Air Transport
 363 Association</a>
 364 gives current time zone rules for airports served by commercial aviation.</li>
 365 <li>Some Microsoft Windows versions contain time zone information in
 366 an undocumented format, with IDs that can be mapped to <code>TZ</code>
 367 values using the <a
 368 href="http://unicode.org/cldr/charts/supplemental/zone_tzid.html">Zone
 369 &rarr; Tzid table</a> maintained by the <abbr
 370 title="Common Locale Data Repository">CLDR</abbr> data mentioned
 371 below.</li>
 372 </ul>
 373 <h2>Maps</h2>
 374 <ul>
 375 <li>The <a href="https://www.cia.gov/">United States Central
 376 Intelligence Agency (<abbr
 377 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
 378 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
 379 zone map</a>; the
 380 <a
 381 href="http://www.lib.utexas.edu/maps/world.html">Perry-Casta&ntilde;eda
 382 Library Map Collection</a>
 383 of the University of Texas at Austin has copies of
 384 recent editions.
 385 The pictorial quality is good,
 386 but the maps do not indicate summer time,
 387 and parts of the data are a few years out of date.</li>
 388 <li><a href="http://worldtimezone.com/">Current time around the world
 389 and standard time zones map of the world</a>
 390 has several fancy time zone maps; it covers Russia particularly well.
 391 The maps' pictorial quality is not quite as good as the
 392 <abbr>CIA</abbr>'s
 393 but the maps are more up to date.</li>
 394 </ul>
 395 <h2>Time zone boundaries</h2>
 396 <ul>
 397 <li><a href="http://efele.net/maps/tz/">TZ timezones maps</a> contains <a
 398 href="http://en.wikipedia.org/wiki/Shapefile">shapefiles</a> of
 399 sets of <code>tz</code> regions.</li>
 400 <li><a href="http://derickrethans.nl/what-time-is-it.html">What Time
 401 is It Here?</a> applies <a href="http://www.mongodb.org/">MongoDB</a>
 402 geospatial query operators to shapefiles' data.</li>
 403 <li><a href="http://statoids.com/statoids.html">Administrative
 404 Divisions of Countries ("Statoids")</a> contains lists of
 405 political subdivision data related to time zones.</li>
 406 <li><a href="http://home.tiscali.nl/~t876506/Multizones.html">Time
 407 zone boundaries for multizone countries</a> summarizes legal
 408 boundaries between time zones within countries.</li>
 409 <li>Manifold.net's <a
 410 href="http://manifold.net/info/freestuff.shtml">Free Stuff for
 411 Manifold System Users</a> includes a Manifold-format map of
 412 world time zone boundaries distributed under the
 413 <abbr>GPL</abbr>.</li>
 414 <li>The <abbr>US</abbr> Geological Survey's National Atlas of
 415 the United States
 416 publishes the <a href="http://nationalatlas.gov/mld/timeznp.html">Time
 417 Zones of the United States</a> in the public domain.</li>
 418 <li>The GeoCommunity lists several commercial sources for <a
 419 href="http://spatialnews.geocomm.com/features/timezones/">International
 420 Time Zones and Time Zone Data</a>.</li>
 421 <li>A ship within the <a
 422 href="http://en.wikipedia.org/wiki/Territorial_waters">territorial
 423 waters</a> of any nation uses that nation's time. In international
 424 waters, time zone boundaries are meridians 15&deg; apart, except that
 425 UTC&minus;12 and UTC+12 are each 7.5&deg; wide and are separated by
 426 the 180&deg; meridian (not by the International Date Line, which is
 427 for land and territorial waters only). A captain can change ship's
 428 clocks any time after entering a new time zone; midnight changes are
 429 common.</li>
 430 </ul>
 431 <h2>Civil time concepts and history</h2>
 432 <ul>
 433 <li><a href="http://physics.nist.gov/GenInt/Time/time.html">A
 434 Walk through Time</a>
 435 surveys the evolution of timekeeping.</li>
 436 <li><a href="http://webexhibits.org/daylightsaving/">About Daylight
 437 Saving Time - History, rationale, laws &amp; dates</a>
 438 is an overall history of <abbr>DST</abbr>.</li>
 439 <li><a href="http://www.w3.org/TR/timezone/">Working with Time Zones</a>
 440 contains guidelines and best practices for software applications that
 441 deal with civil time.</li>
 442 <li><a href="http://energy.ca.gov/daylightsaving.html">Saving Time,
 443 Saving Energy</a> discusses a primary justification for <abbr>DST</abbr>.</li>
 444 <li><a href="http://seizethedaylight.com/dst/">A Brief
 445 History of Daylight Saving Time</a> summarizes some of the contentious
 446 history of <abbr>DST</abbr>.</li>
 447 <li><a href="http://toi.inrim.it/uk/toi.html">The
 448 Time of Internet</a>
 449 describes time zones and daylight saving time,
 450 with diagrams.
 451 The time zone map is out of date, however.</li>
 452 <li><a href="http://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
 453 the International Date Line</a> tells the story of the most important
 454 time zone boundary.</li>
 455 <li><a href="http://statoids.com/tconcept.html">Basic Time
 456 Zone Concepts</a> discusses terminological issues behind time zones.</li>
 457 </ul>
 458 <h2>National histories of legal time</h2>
 459 <dl>
 460 <dt>Australia</dt>
 461 <dd>The Parliamentary Library has commissioned <a
 462 href="http://www.aph.gov.au/binaries/library/pubs/rn/2006-07/07rn13.pdf">research
 463 note on daylight saving time in Australia</a>.
 464 The Bureau of Meteorology publishes a list of
 465 <a href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation Dates of Daylight Savings Time within Australia</a>.</dd>
 466 <dt>Belgium</dt>
 467 <dd>The Royal Observatory of Belgium maintains a table of <a
 468 href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
 469 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
 470 <dt>Brazil</dt>
 471 <dd>The Time Service Department of the National Observatory
 472 records <a href="http://pcdsh01.on.br/DecHV.html"
 473 hreflang="pt-BR">Brazil's daylight saving time decrees (in
 474 Portuguese)</a>.</dd>
 475 <dt>Canada</dt>
 476 <dd>National Research Council Canada publishes current
 477 and some older information about <a
 478 href="http://www.nrc-cnrc.gc.ca/eng/services/time/time_zones.html">time
 479 zones &amp; daylight saving time</a>.</dd>
 480 <dt>Chile</dt>
 481 <dd>The Chilean Hydrographic and Oceanographic Service publishes a <a
 482 href="http://www.horaoficial.cl/horaof.htm" hreflang="es"> history of
 483 official time (in Spanish)</a>.</dd>
 484 <dt>Germany</dt>
 485 <dd>The National Institute for Science and Technology maintains the <a
 486 href="http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation of
 487 Legal Time in Germany</a>.</dd>
 488 <dt>Israel</dt>
 489 <dd>The Interior Ministry periodically issues <a
 490 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
 491 hreflang="he">announcements (in Hebrew)</a>.</dd>
 492 <dt>Mexico</dt>
 493 <dd>The Investigation and Analysis Service of the Mexican Library of
 494 Congress has published a <a
 495 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/"
 496 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
 497 <dt>Malaysia</dt>
 498 <dd>See Singapore below.</dd>
 499 <dt>Netherlands</dt>
 500 <dd><a href="http://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
 501 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
 502 covers the history of local time in the Netherlands from ancient times.</dd>
 503 <dt>New Zealand</dt>
 504 <dd>The Department of Internal Affairs maintains a brief <a
 505 href="http://www.dia.govt.nz/Daylight-Saving-History">History of
 506 Daylight Saving</a>. The privately-maintained <a
 507 href="http://astrologyschool.com/nztime.html">History of New Zealand
 508 time</a> has more details.</dd>
 509 <dt>Singapore</dt>
 510 <dd><a
 511 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
 512 is Singapore in the "Wrong" Time Zone?</a> details the
 513 history of legal time in Singapore and Malaysia.</dd>
 514 <dt>United Kingdom</dt>
 515 <dd><a
 516 href="http://www.polyomino.org.uk/british-time/">History of
 517 legal time in Britain</a> discusses in detail the country
 518 with perhaps the best-documented history of clock adjustments.
 519 The National Physical Laboratory also maintains an <a
 520 href="http://www.npl.co.uk/educate-explore/what-is-time/archive-of-summer-time-dates-1916-2006">Archive
 521 of Summer time dates</a>.</dd>
 522 </dl>
 523 <h2>Precision timekeeping</h2>
 524 <ul>
 525 <li><a
 526 href="http://literature.agilent.com/litweb/pdf/5965-7984E.pdf">The
 527 Science of Timekeeping</a> is a thorough introduction
 528 to the theory and practice of precision timekeeping.</li>
 529 <li><a href="http://www.ntp.org/"><abbr
 530 title="Network Time Protocol">NTP</abbr>: The Network
 531 Time Protocol</a>
 532 discusses how to synchronize clocks of
 533 Internet hosts.</li>
 534 <li><a
 535 href="http://tools.ietf.org/html/rfc4833">Timezone
 536 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
 537 (Internet <abbr>RFC</abbr> 4833)
 538 specifies a <a
 539 href="http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol">DHCP</a>
 540 option for a server to configure
 541 a client's time zone and daylight saving settings automatically.</li>
 542 <li><a href="http://gauss.gge.unb.ca/GMT.UT.and.the.RGO.html">A Few
 543 Facts Concerning <abbr title="Greenwich Mean Time">GMT</abbr>, <abbr
 544 title="Universal Time">UT</abbr>, and
 545 the <abbr title="Royal Greenwich Observatory">RGO</abbr></a>
 546 answers questions like "What is the
 547 difference between <abbr>GMT</abbr> and <abbr>UTC</abbr>?"</li>
 548 <li><a
 549 href="http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
 550 Times</a> explains more abstruse astronomical time scales like
 551 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
 552 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
 553 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
 554 <a href="http://www.ucolick.org/~sla/leapsecs/timescales.html">Time
 555 Scales</a> goes into more detail, particularly for historical variants.</li>
 556 <li>The <a href="http://iau.org/"><abbr
 557 title="International Astronomical Union">IAU</abbr></a>'s <a
 558 href="http://www.iausofa.org/"><abbr
 559 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
 560 initiative publishes Fortran
 561 code for converting among time scales like
 562 <abbr title="International Atomic Time">TAI</abbr>,
 563 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
 564 <abbr>UTC</abbr>.</li>
 565 <li><a href="http://www2.jpl.nasa.gov/basics/bsf2-3.php">Basics of
 566 Space Flight - Reference Systems - Time Conventions</a>
 567 briefly explains interplanetary space flight timekeeping.</li>
 568 <li><a
 569 href="http://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
 570 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a> briefly
 571 describes Mars Coordinated Time (<abbr
 572 title="Mars Coordinated Time">MTC</abbr>) and the
 573 diverse local time
 574 scales used by each landed mission on Mars.</li>
 575 <li><a href="http://leapsecond.com/">LeapSecond.com</a> is
 576 dedicated not only to leap seconds but to precise time and frequency
 577 in general. It covers the state of the art in amateur timekeeping, and
 578 how the art has progressed over the past few decades.</li>
 579 <li><a
 580 href="http://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html">IERS
 581 Bulletins</a> contains official publications of the International
 582 Earth Rotation and Reference Systems Service, which decides
 583 when leap seconds occur.</li>
 584 <li>The <a
 585 href="http://six.pairlist.net/mailman/listinfo/leapsecs">Leap
 586 Second Discussion List</a> covers <a
 587 href="http://gauss.gge.unb.ca/papers.pdf/gpsworld.november99.pdf">McCarthy
 588 and Klepczynski's proposal to discontinue leap seconds</a>,
 589 discussed further in
 590 <a href="http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
 591 leap second: its history and possible future</a>.
 592 <a href="http://www.ucolick.org/~sla/leapsecs/">UTC might be redefined
 593 without Leap Seconds</a> gives pointers on this
 594 contentious issue.</li>
 595 </ul>
 596 <h2>Time notation</h2>
 597 <ul>
 598 <li>
 599 <a href="http://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
 600 the international standard date and time notation</a> is a good
 601 summary of
 602 <a
 603 href="http://www.iso.org/iso/catalogue_detail?csnumber=40874"><abbr
 604 title="International Organization for Standardization">ISO</abbr>
 605 8601:2004 -- Data elements and interchange formats -- Information
 606 interchange -- Representation of dates and times</a>.</li>
 607 <li>
 608 <a href="http://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
 609 Schema: Datatypes - dateTime</a> specifies a format inspired by
 610 <abbr>ISO</abbr> 8601 that is in common use in XML data.</li>
 611 <li>
 612 <a href="http://tools.ietf.org/html/rfc5322">Internet
 613 Message Format</a> (Internet <abbr>RFC</abbr> 5322) &sect;3.3
 614 specifies the time notation used in email and <a
 615 href="http://tools.ietf.org/html/rfc2616"><abbr>HTTP</abbr></a>
 616 headers.</li>
 617 <li>
 618 <a href="http://tools.ietf.org/html/rfc3339">Date and Time
 619 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
 620 specifies an <abbr>ISO</abbr> 8601
 621 profile for use in new Internet
 622 protocols.</li>
 623 <li>
 624 <a href="http://www.hackcraft.net/web/datetime/">Date &amp; Time
 625 Formats on the Web</a> surveys web- and Internet-oriented date and time
 626 formats.</li>
 627 <li>
 628 <a href="http://exit109.com/~ghealton/y2k/yrexamples.html">The
 629 Best of Dates, the Worst of Dates</a> covers many problems encountered
 630 by software developers when handling dates and time stamps.</li>
 631 <li>The <a
 632 href="http://unicode.org/cldr/">Unicode Common Locale Data Repository
 633 (<abbr>CLDR</abbr>) Project</a> has localizations for time zone names,
 634 abbreviations, identifiers, and formats. For example, it contains
 635 French translations for "Eastern European Summer Time", "<abbr
 636 title="Eastern European Summer Time">EEST</abbr>", and
 637 "Bucharest". <a
 638 href="http://unicode.org/cldr/charts/by_type/index.html">By-Type
 639 Chart</a> shows these values for many locales.
 640 <abbr>ICU</abbr> contains a mechanism for using this data.</li>
 641 <li>Alphabetic time zone abbreviations should not be used as unique
 642 identifiers for <abbr>UTC</abbr> offsets as they are ambiguous in
 643 practice. For example, "<abbr>EST</abbr>" denotes 5 hours behind
 644 <abbr>UTC</abbr> in English-speaking North America, but it denotes 10
 645 or 11 hours ahead of <abbr>UTC</abbr> in Australia; and
 646 French-speaking North Americans prefer
 647 "<abbr title="Heure Normale de l'Est">HNE</abbr>" to
 648 "<abbr>EST</abbr>". For <abbr>POSIX</abbr> the <code>tz</code>
 649 database contains English abbreviations for all time stamps but in
 650 many cases these are merely inventions of the database
 651 maintainers.</li>
 652 <li>Numeric time zone abbreviations typically count hours east of
 653 <abbr>UTC</abbr>, e.g., <code>+09</code> for Japan and
 654 <code>-10</code> for Hawaii. However, the <abbr>POSIX</abbr>
 655 <code>TZ</code> environment variable uses the opposite convention. For
 656 example, one might use <code>TZ="JST-9"</code> and
 657 <code>TZ="HST10"</code> for Japan and Hawaii, respectively. If the
 658 <code>tz</code> database is available, it is usually better to use
 659 settings like <code>TZ="Asia/Tokyo"</code> and
 660 <code>TZ="Pacific/Honolulu"</code> instead, as this should avoid
 661 confusion, handle old time stamps better, and insulate you better from
 662 any future changes to the rules. One should never set
 663 <abbr>POSIX</abbr> <code>TZ</code> to a value like
 664 <code>"GMT-9"</code>, though, since this would falsely claim that
 665 local time is nine hours ahead of <abbr>UTC</abbr> and the time zone
 666 is called "<abbr>GMT</abbr>".</li>
 667 </ul>
 668 <h2>Related indexes</h2>
 669 <ul>
 670 <li><a href="tz-art.htm">Time and the Arts</a></li>
 671 <li><a href="http://www.dmoz.org/Reference/Time/">Open Directory -
 672 Reference: Time</a></li>
 673 <li><a href="http://dir.yahoo.com/Science/Measurements_and_Units/Time">Yahoo!
 674 Directory &gt; Science &gt; Measurements and Units &gt; Time</a></li>
 675 </ul>
 676 <hr>
 677 <address>
 678 This file is in the public domain, so clarified as of
 679 2009-05-17 by Arthur David Olson.
 680 <br>
 681 Please send corrections to this web page to the
 682 <a href="mailto:tz@iana.org">time zone mailing list</a>.
 683 </address>
 684 </body>
 685 </html>