commands/zoneinfo/tz-link.htm

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   7 <title>Sources for Time Zone and Daylight Saving Time Data</title>
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  10 <meta name="DC.Creator" content="Eggert, Paul" />
  11 <meta name="DC.Contributor" content="Olson, Arthur David" />
  12 <meta name="DC.Date" content="2006-01-20" />
  13 <meta name="DC.Description"
  14  content="Sources of information about time zones and daylight saving time" />
  15 <meta name="DC.Identifier" content="http://www.twinsun.com/tz/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 <address>
  22 @(#)tz-link.htm 7.54
  23 </address>
  24 <p>
  25 Please send corrections to this web page to the
  26 <a href="mailto:tz@elsie.nci.nih.gov">time zone mailing list</a>.</p>
  27 <h2>The <code>tz</code> database</h2>
  28 <p>
  29 The public-domain time zone database contains code and data
  30 that represent the history of local time
  31 for many representative locations around the globe.
  32 It is updated periodically to reflect changes made by political bodies
  33 to <a href="http://en.wikipedia.org/wiki/Time_zone">time zone</a>
  34 boundaries, <a
  35 href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
  36 title="Coordinated Universal Time">UTC</abbr></a> offsets, and
  37 <a href="http://en.wikipedia.org/wiki/Daylight_saving">daylight-saving</a>
  38 rules.
  39 This database (often called <code>tz</code> or <code>zoneinfo</code>)
  40 is used by several implementations,
  41 including
  42 <a href="http://www.gnu.org/software/libc/">the
  43 <abbr title="GNU's Not Unix">GNU</abbr>
  44 C Library</a> used in
  45 <a href="http://www.linux.org/"><abbr>GNU</abbr>/Linux</a>,
  46 <a href="http://www.freebsd.org/">FreeBSD</a>,
  47 <a href="http://www.netbsd.org/">NetBSD</a>,
  48 <a href="http://www.openbsd.org/">OpenBSD</a>,
  49 <a href="http://www.cygwin.com/">Cygwin</a>,
  50 <a href="http://www.delorie.com/djgpp/"><abbr
  51 title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
  52 <a href="http://www.hp.com/products1/unix/operating/">HP-UX</a>,
  53 <a href="http://www.sgi.com/developers/technology/irix/">IRIX</a>,
  54 <a href="http://www.apple.com/macosx/">Mac OS X</a>,
  55 <a href="http://h71000.www7.hp.com/">OpenVMS</a>,
  56 <a href="http://www.sun.com/software/solaris/">Solaris</a>,
  57 <a href="http://h30097.www3.hp.com/">Tru64</a>, and
  58 <a href="http://www.sco.com/products/unixware/">UnixWare</a>.</p>
  59 <p>
  60 Each location in the database represents a national region where all
  61 clocks keeping local time have agreed since 1970.
  62 Locations are identified by continent or ocean and then by the name of
  63 the location, which is typically the largest city within the region.
  64 For example, <code>America/New_York</code>
  65 represents most of the <abbr title="United States">US</abbr> eastern time zone;
  66 <code>America/Phoenix</code> represents most of Arizona, which
  67 uses mountain time without daylight saving time (<abbr
  68 title="daylight saving time">DST</abbr>);
  69 <code>America/Detroit</code> represents most of Michigan, which uses
  70 eastern time but with different <abbr>DST</abbr> rules in 1975;
  71 and other entries represent smaller regions like Starke County,
  72 Indiana, which switched from central to eastern time in 1991
  73 and switched back in 2006.
  74 To use the database on an extended <a
  75 href="http://en.wikipedia.org/wiki/POSIX"><abbr
  76 title="Portable Operating System Interface">POSIX</abbr></a>
  77 implementation set the <code>TZ</code> environment variable to
  78 the location's full name, e.g., <code>TZ="America/New_York"</code>.</p>
  79 <p>
  80 In the <code>tz</code> database's
  81 <a href="ftp://elsie.nci.nih.gov/pub/"><abbr
  82 title="File Transfer Protocol">FTP</abbr> distribution</a>
  83 the code is in the file <code>tzcode<var>C</var>.tar.gz</code>,
  84 where <code><var>C</var></code> is the code's version;
  85 similarly, the data are in <code>tzdata<var>D</var>.tar.gz</code>,
  86 where <code><var>D</var></code> is the data's version.
  87 The following shell commands download
  88 these files to a <abbr>GNU</abbr>/Linux or similar host;
  89 see the downloaded
  90 <code>README</code> file for what to do next.</p>
  91 <pre style="margin-left: 2em"><code><a href="http://www.gnu.org/software/wget/">wget</a> 'ftp://elsie.nci.nih.gov/pub/tz*.tar.gz'
  92 <a href="http://www.gnu.org/software/gzip/">gzip</a> -dc tzcode*.tar.gz | <a href="http://www.gnu.org/software/tar/">tar</a> -xf -
  93 gzip -dc tzdata*.tar.gz | tar -xf -
  94 </code></pre>
  95 <p>
  96 The code lets you compile the <code>tz</code> source files into
  97 machine-readable binary files, one for each location. It also lets
  98 you read a <code>tz</code> binary file and interpret time stamps for that
  99 location.</p>
 100 <p>
 101 The data are by no means authoritative. If you find errors, please
 102 send changes to the <a href="mailto:tz@elsie.nci.nih.gov">time zone
 103 mailing list</a>. You can also <a
 104 href="mailto:tz-request@elsie.nci.nih.gov">subscribe</a> to the
 105 mailing list, retrieve the <a
 106 href="ftp://elsie.nci.nih.gov/pub/tzarchive.gz">archive of old
 107 messages</a> (in gzip compressed format), or retrieve <a
 108 href="ftp://munnari.oz.au/pub/oldtz/">archived older versions of code
 109 and data</a>; there is also a smaller <a
 110 href="http://public.planetmirror.com/pub/timezone/"><abbr
 111 title="Hypertext Transfer Protocol">HTTP</abbr>
 112 mirror</a>.</p>
 113 <p>
 114 The Web has several other sources for time zone and daylight saving time data.
 115 Here are some recent links that may be of interest.
 116 </p>
 117 <h2>Web pages using recent versions of the <code>tz</code> database</h2>
 118 <ul>
 119 <li><a href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
 120 is a text-based point-and-click interface to tables of current time
 121 throughout the world.</li>
 122 <li>Fancier web interfaces, roughly in ascending order of complexity, include:
 123 <ul>
 124 <li><a
 125 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
 126 Time in 1000 Places</a></li>
 127 <li><a href="http://timezoneconverter.com/">Time Zone Converter</a></li>
 128 </ul></li>
 129 <li><a href="http://www.timeanddate.com/worldclock/">The World Clock -
 130 Time Zones</a>
 131 is a web interface to a time zone database derived from
 132 <code>tz</code>'s.</li>
 133 </ul>
 134 <h2>Other time zone database formats</h2>
 135 <ul>
 136 <li>The <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2445.txt">
 137 Internet Calendaring and Scheduling Core Object Specification
 138 (iCalendar)</a> specification published by the (now-concluded) <a
 139 href="http://www.ietf.org/html.charters/OLD/calsch-charter.html"><abbr
 140 title="Internet Engineering Task Force">IETF</abbr>
 141 Calendaring and Scheduling Working Group (<abbr
 142 title="Calendaring and Scheduling Working Group">calsch</abbr>)</a>
 143 covers time zone
 144 data; see its VTIMEZONE calendar component.
 145 The <a href="http://www.calconnect.org/">Calendaring and Scheduling
 146 Consortium</a> is promoting further work in this area. <a
 147 href="ftp://ftp.rfc-editor.org/internet-drafts/draft-royer-ical-basic-04.txt">iCalendar
 148 Basic</a> is a draft simplified definition that omits VTIMEZONE due to
 149 its complexity, but this removal is expected to appear as an
 150 independent specification later.</li>
 151 <li><a
 152 href="ftp://ftp.rfc-editor.org/internet-drafts/draft-royer-timezone-registry-03.txt">Time
 153 Zone Registry</a> proposes an <a href="http://www.iana.org/"><abbr
 154 title="Internet Assigned Numbers Authority">IANA</abbr></a> time zone
 155 registration process that would establish unique names for each
 156 version of each <code>tz</code> zone, along with a polygonal
 157 representation of the geographical area corresponding to the
 158 zone.</li>
 159 <li>The <a
 160 href="http://lists.w3.org/Archives/Public/www-rdf-calendar/">www-rdf-calendar</a>
 161 list discusses <a
 162 href="http://www.w3.org/RDF/"><abbr
 163 title="Resource Description Framework">RDF</abbr></a>-based calendar
 164 and group scheduling systems, and has a <a
 165 href="http://www.w3.org/2002/12/cal/#tzd">workspace on time zone
 166 data</a> converted from <code>tz</code>. An earlier <a
 167 href="http://www.w3.org/2000/01/foo">schema</a> was sketched out.</li>
 168 <li><a
 169 href="ftp://ftp.rfc-editor.org/internet-drafts/draft-royer-calsch-xcal-03.txt"><abbr
 170 title="iCalendar in XML Format">xCal-Basic</abbr></a>
 171 is a draft <a
 172 href="http://www.w3.org/XML/"><abbr
 173 title="Extensible Markup Language">XML</abbr></a> document type
 174 definition that corresponds to iCalendar.</li>
 175 </ul>
 176 <h2>Other <code>tz</code> compilers</h2>
 177 <ul>
 178 <li><a href="http://dialspace.dial.pipex.com/prod/dialspace/town/pipexdsl/s/asbm26/vzic/">Vzic iCalendar
 179 Timezone Converter</a> describes a program Vzic that compiles
 180 <code>tz</code> source into iCalendar-compatible VTIMEZONE files.
 181 Vzic is freely
 182 available under the <a
 183 href="http://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
 184 General Public License (<abbr
 185 title="General Public License">GPL</abbr>)</a>.</li>
 186 <li><a
 187 href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
 188 contains a script <code>parse_olson</code> that compiles
 189 <code>tz</code> source into <a href="http://www.perl.org/">Perl</a>
 190 modules. It is part of the Perl <a
 191 href="http://datetime.perl.org/">DateTime Project</a>, which is freely
 192 available under both the <abbr>GPL</abbr> and the Perl Artistic
 193 License. DateTime::TimeZone also contains a script
 194 <code>tests_from_zdump</code> that generates test cases for each clock
 195 transition in the <code>tz</code> database.</li>
 196 <li><a href="http://www-306.ibm.com/software/globalization/icu/"><abbr
 197 title="International Components for Unicode">ICU</abbr></a>
 198 contains a C/C++ library for internationalization that
 199 has a compiler from <code>tz</code> source
 200 into an <abbr>ICU</abbr>-specific format.
 201 <abbr>ICU</abbr> is freely available under a
 202 <abbr title="Berkeley Software Distribution">BSD</abbr>-style license.</li>
 203 <li><a href="http://joda-time.sourceforge.net/">Joda Time - Java date
 204 and time <abbr title="Application Program Interface">API</abbr></a>
 205 contains a class
 206 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
 207 <code>tz</code> source into a Joda-specific binary format. Joda Time
 208 is freely available under a <abbr>BSD</abbr>-style license.</li>
 209 <li><a href="http://sourceforge.net/projects/pytz/">PyTZ - Python Time
 210 Zone Library</a> compiles <code>tz</code> source into
 211 <a href="http://www.python.org/">Python</a>.
 212 It is freely available under a <abbr>BSD</abbr>-style license.</li>
 213 <li><a href="http://tzinfo.rubyforge.org/">TZInfo - Ruby Timezone Library</a>
 214 compiles <code>tz</code> source into
 215 <a href="http://www.ruby-lang.org/">Ruby</a>.
 216 It is freely available under the <abbr
 217 title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
 218 </ul>
 219 <h2>Other <code>tz</code> binary file readers</h2>
 220 <ul>
 221 <li>The <a
 222 href="http://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
 223 Library</a>
 224 has an independent, thread-safe implementation of
 225 a <code>tz</code> binary file reader.
 226 This library is freely available under the
 227 <a href="http://www.gnu.org/copyleft/lesser.html">
 228 <abbr>GNU</abbr> Lesser General Public License
 229 (<abbr title="Lesser General Public License">LGPL</abbr>)</a>,
 230 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
 231 <li><a href="http://www.bmsi.com/java/#TZ">ZoneInfo.java</a>
 232 is a <code>tz</code> binary file reader written in Java.
 233 It is freely available under the <abbr>LGPL</abbr>.</li>
 234 <li><a href="http://s.keim.free.fr/tz/doc.html">Python time zones</a>
 235 is a <code>tz</code> binary file reader written in Python.
 236 It is freely available under a <abbr>BSD</abbr>-style license.</li>
 237 </ul>
 238 <h2>Other <code>tz</code>-based time zone software</h2>
 239 <ul>
 240 <li><a
 241 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
 242 clock (intclock)</a> is a multi-timezone clock for
 243 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
 244 under the <abbr>GPL</abbr>.</li>
 245 <li><a href="http://java.sun.com/">Sun Java</a> releases since 1.4
 246 contain a copy of a subset of a recent <code>tz</code> database in a
 247 Java-specific format.</li>
 248 <li><a
 249 href="http://www1.tip.nl/~t876506/AboutTimeZonesHC.html">HyperCard
 250 time zones calculator</a> was a HyperCard stack.</li>
 251 <li><a
 252 href="http://www.veladg.com/velaterra.html">VelaTerra</a> is
 253 a Mac OS X program. Its developers
 254 <a href="http://www.veladg.com/tzoffer.html">offer free
 255 licenses</a> to <code>tz</code> contributors.</li>
 256 <li><a
 257 href="http://www.cimmyt.org/timezone/">World Time Explorer</a> is a
 258 Microsoft Windows program.</li>
 259 </ul>
 260 <h2>Other time zone databases</h2>
 261 <ul>
 262 <li><a href="http://www.astro.com/cgi/aq.cgi?lang=e">Atlas Query</a>
 263 is Astrodienst's Web version of Shanks's
 264 excellent time zone history atlases published in both <a
 265 href="http://astrocom.com/products/software.php?software_id=ibmwboth">computer</a>
 266 and book form (<a
 267 href="http://astrocom.com/products/book.php?book_id=b110x">one volume
 268 for the USA</a>, and <a
 269 href="http://astrocom.com/products/book.php?book_id=b112x">one for
 270 other locations</a>) by <a
 271 href="http://astrocom.com/">Astro Communications Services</a>.</li>
 272 <li><a href="http://worldtime.com/">WORLDTIME: interactive atlas,
 273 time info, public holidays</a>
 274 contains information on local time, sunrise and sunset,
 275 and public holidays in several hundred cities around the world.</li>
 276 <li><a href="http://www.worldtimeserver.com/">World Time Server</a>
 277 is another time zone database.</li>
 278 <li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a>
 279 contains data from the Time Service Department of the
 280 <abbr>US</abbr> Naval Observatory, used as the source
 281 for the <code>usno*</code> files in the <code>tz</code> distribution.</li>
 282 <li>The <a href="http://www.iata.org/ps/publications/9179.htm">Standard
 283 Schedules Information Manual</a> of the
 284 <a href="http://www.iata.org/index.htm">International Air Transport
 285 Association</a>
 286 gives current time zone rules for airports served by commercial aviation.</li>
 287 </ul>
 288 <h2>Maps</h2>
 289 <ul>
 290 <li>The <a href="http://www.odci.gov/">United States Central
 291 Intelligence Agency (<abbr
 292 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
 293 href="http://www.odci.gov/cia/publications/factbook/reference_maps/pdf/time_zones.pdf">time
 294 zone map</a>; the
 295 <a
 296 href="http://www.lib.utexas.edu/maps/world.html">Perry-Casta&ntilde;eda
 297 Library Map Collection</a>
 298 of the University of Texas at Austin has copies of
 299 recent editions.
 300 The pictorial quality is good,
 301 but the maps do not indicate summer time,
 302 and parts of the data are a few years out of date.</li>
 303 <li><a href="http://worldtimezone.com/">Current time around the world
 304 and standard time zones map of the world</a>
 305 has several fancy time zone maps; it covers Russia particularly well.
 306 The maps' pictorial quality is not quite as good as the
 307 <abbr>CIA</abbr>'s
 308 but the maps are more up to date.</li>
 309 </ul>
 310 <h2>Time zone boundaries</h2>
 311 <ul>
 312 <li><a href="http://www.statoids.com/statoids.html">Administrative Divisions
 313 of Countries ("Statoids")</a> contains detailed lists of
 314 <code>tz</code>-related zone subdivision data.</li>
 315 <li><a href="http://home-4.tiscali.nl/~t876506/Multizones.html">Time
 316 zone boundaries for multizone countries</a> summarizes legal
 317 boundaries between time zones within countries.</li>
 318 <li>Manifold.net's <a
 319 href="http://www.manifold.net/download/freemaps.html">Free Maps and
 320 <abbr title="Geographic Information Systems">GIS</abbr>
 321 Data</a> includes a Manifold-format map of
 322 world time zone boundaries distributed under the
 323 <abbr>GPL</abbr>.</li>
 324 <li>The <abbr>US</abbr> Geological Survey's National Atlas of
 325 the United States
 326 publishes the <a href="http://nationalatlas.gov/mld/timeznp.html">Time
 327 Zones of the United States</a> in the public domain.</li>
 328 <li>The GeoCommunity lists several commercial sources for <a
 329 href="http://spatialnews.geocomm.com/features/timezones/">International
 330 Time Zones and Time Zone Data</a>.</li>
 331 </ul>
 332 <h2>Civil time concepts and history</h2>
 333 <ul>
 334 <li><a href="http://physics.nist.gov/GenInt/Time/time.html">A
 335 Walk through Time</a>
 336 surveys the evolution of timekeeping.</li>
 337 <li><a href="http://webexhibits.org/daylightsaving/">About Daylight
 338 Saving Time - History, rationale, laws &amp; dates</a>
 339 is an overall history of <abbr>DST</abbr>.</li>
 340 <li><a href="http://www.energy.ca.gov/daylightsaving.html">Saving Time,
 341 Saving Energy</a> discusses a primary justification for <abbr>DST</abbr>.</li>
 342 <li><a href="http://www.seizethedaylight.com/dst/">Who Knew? A Brief
 343 History of Daylight Saving Time</a> summarizes some of the contentious
 344 history of <abbr>DST</abbr>.</li>
 345 <li><a href="http://toi.iriti.cnr.it/">The
 346 Time of Internet</a>
 347 describes time zones and daylight saving time,
 348 with diagrams.
 349 The time zone map is out of date, however.</li>
 350 <li><a href="http://www.phys.uu.nl/~vgent/idl/idl.htm">A History of
 351 the International Date Line</a> tells the story of the most important
 352 time zone boundary.</li>
 353 <li><a href="http://www.statoids.com/tconcept.html">Basic Time
 354 Zone Concepts</a> discusses terminological issues behind time zones.</li>
 355 </ul>
 356 <h2>National histories of legal time</h2>
 357 <dl>
 358 <dt>Australia</dt>
 359 <dd>The Bureau of Metrology publishes a list of
 360 <a href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation Dates of Daylight Savings Time within Australia</a>.</dd>
 361 <dt>Austria</dt>
 362 <dd>The Federal Office of Metrology and Surveying publishes a
 363 table of <a href="http://www.metrologie.at/pdf/sommerzeit.pdf"
 364 hreflang="de">daylight saving time in Austria (in German)</a>.</dd>
 365 <dt>Belgium</dt>
 366 <dd>The Royal Observatory of Belgium maintains a table of <a
 367 href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
 368 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
 369 <dt>Brazil</dt>
 370 <dd>The Time Service Department of the National Observatory
 371 records <a href="http://pcdsh01.on.br/DecHV.html"
 372 hreflang="pt-BR">Brazil's daylight saving time decrees (in
 373 Portuguese)</a>.</dd>
 374 <dt>Canada</dt>
 375 <dd>The Institute for National Measurement Standards publishes current
 376 and some older information about <a
 377 href="http://inms-ienm.nrc-cnrc.gc.ca/en/time_services/daylight_saving_e.php">Time
 378 Zones &amp; Daylight Saving Time</a>.</dd>
 379 <dt>Chile</dt>
 380 <dd>WebExhibits publishes a <a
 381 href="http://webexhibits.org/daylightsaving/chile.html"
 382 hreflang="es">history of official time (in Spanish)</a> originally
 383 written by the Chilean Hydrographic and Oceanographic Service.</dd>
 384 <dt>Germany</dt>
 385 <dd>The National Institute for Science and Technology maintains the <a
 386 href="http://www.ptb.de/en/org/4/44/441/dars_e.htm">Realisation of
 387 Legal Time in Germany</a>.</dd>
 388 <dt>Israel</dt>
 389 <dd>The Interior Ministry periodically issues <a
 390 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements/"
 391 hreflang="he">announcements (in Hebrew)</a>.</dd>
 392 <dt>Mexico</dt>
 393 <dd>The Investigation and Analysis Service of the Mexican Library of
 394 Congress has published a <a
 395 href="http://www.cddhcu.gob.mx/bibliot/publica/inveyana/polisoc/horver/"
 396 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
 397 <dt>Malaysia</dt>
 398 <dd>See Singapore below.</dd>
 399 <dt>Netherlands</dt>
 400 <dd><a href="http://www.phys.uu.nl/~vgent/wettijd/wettijd.htm"
 401 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
 402 covers the history of local time in the Netherlands from ancient times.</dd>
 403 <dt>New Zealand</dt>
 404 <dd>The Department of Internal Affairs maintains a brief history <a
 405 href="http://www.dia.govt.nz/diawebsite.nsf/wpg_URL/Resource-material-Information-We-Provide-About-Daylight-Saving">About
 406 Daylight Saving</a>. The privately-maintained <a
 407 href="http://www.astrologyschool.com/nztime.html">History of New Zealand
 408 time</a> has more details.</dd>
 409 <dt>Norway</dt>
 410 <dd>The Norwegian Meteorological Institute lists
 411 <a href="http://met.no/met/met_lex/q_u/sommertid.html" hreflang="no">Summer
 412 time in Norway (in Norwegian)</a>, citing the
 413 Institute of Theoretical Astrophysics, Oslo.</dd>
 414 <dt>Singapore</dt>
 415 <dd><a
 416 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
 417 is Singapore in the "Wrong" Time Zone?</a> details the
 418 history of legal time in Singapore and Malaysia.</dd>
 419 <dt>United Kingdom</dt>
 420 <dd><a
 421 href="http://www.srcf.ucam.org/~jsm28/british-time/">History of
 422 legal time in Britain</a> discusses in detail the country
 423 with perhaps the best-documented history of clock adjustments.
 424 The National Physical Laboratory also maintains an <a
 425 href="http://www.npl.co.uk/time/summer_time_archive.html">Archive
 426 of Summer time dates</a>.</dd>
 427 </dl>
 428 <h2>Precision timekeeping</h2>
 429 <ul>
 430 <li><a
 431 href="http://literature.agilent.com/litweb/pdf/5965-7984E.pdf">The
 432 Science of Timekeeping</a> is a thorough introduction
 433 to the theory and practice of precision timekeeping.</li>
 434 <li><a href="http://www.ntp.org/"><abbr
 435 title="Network Time Protocol">NTP</abbr>: The Network
 436 Time Protocol</a>
 437 discusses how to synchronize clocks of
 438 Internet hosts.</li>
 439 <li><a href="http://gauss.gge.unb.ca/GMT.UT.and.the.RGO.html">A Few
 440 Facts Concerning <abbr title="Greenwich Mean Time">GMT</abbr>, <abbr
 441 title="Universal Time">UT</abbr>, and
 442 the <abbr title="Royal Greenwich Observatory">RGO</abbr></a>
 443 answers questions like "What is the
 444 difference between <abbr>GMT</abbr> and <abbr>UTC</abbr>?"</li>
 445 <li><a
 446 href="http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
 447 Times</a> explains more abstruse astronomical time scales like
 448 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
 449 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
 450 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
 451 <a href="http://www.ucolick.org/~sla/leapsecs/timescales.html">Time
 452 Scales</a> goes into more detail, particularly for historical variants.</li>
 453 <li>The <a href="http://www.iau.org/"><abbr
 454 title="International Astronomical Union">IAU</abbr></a>'s <a
 455 href="http://www.iau-sofa.rl.ac.uk/"><abbr
 456 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
 457 initiative publishes Fortran
 458 code for converting among time scales like
 459 <abbr title="International Atomic Time">TAI</abbr>,
 460 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
 461 <abbr>UTC</abbr>.</li>
 462 <li><a href="http://www.jpl.nasa.gov/basics/bsf2-3.htm">Basics of
 463 Space Flight - Reference Systems - Time Conventions</a>
 464 briefly explains interplanetary space flight timekeeping.</li>
 465 <li><a
 466 href="http://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
 467 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a> briefly
 468 describes Mars Coordinated Time (<abbr
 469 title="Mars Coordinated Time">MTC</abbr>) and the
 470 diverse local time
 471 scales used by each landed mission on Mars.</li>
 472 <li><a href="http://www.leapsecond.com/">LeapSecond.com</a> is
 473 dedicated not only to leap seconds but to precise time and frequency
 474 in general. It covers the state of the art in amateur timekeeping, and
 475 how the art has progressed over the past few decades.</li>
 476 <li><a
 477 href="http://hpiers.obspm.fr/eop-pc/products/bulletins/bulletins.html">Bulletins
 478 maintained by the
 479 <abbr title="International Earth Rotation Service">IERS</abbr>
 480 <abbr title="Earth Orientation Parameters">EOP</abbr>
 481 (<abbr title="Product Center">PC</abbr>)</a> contains official publications of
 482 the Earth Orientation Parameters Product Center of the
 483 International Earth Rotation Service, the committee that decides
 484 when leap seconds occur.</li>
 485 <li>The <a
 486 href="http://www.mail-archive.com/leapsecs@rom.usno.navy.mil/">Leap
 487 Second Discussion List</a> and <a
 488 href="http://rom.usno.navy.mil/archives/leapsecs.html">archive</a> covers <a
 489 href="http://gauss.gge.unb.ca/papers.pdf/gpsworld.november99.pdf">McCarthy
 490 and Klepczynski's proposal to discontinue leap seconds</a>, published in <a
 491 href="http://www.gpsworld.com/gpsworld/"><abbr
 492 title="Global Positioning System">GPS</abbr> World</a>
 493 <strong>10</strong>, 11
 494 (1999-11), 50&ndash;57 and discussed further in R. A. Nelson et al.,
 495 <a href="http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
 496 leap second: its history and possible future</a>,
 497 <a href="http://www.bipm.fr/metrologia/index.html">Metrologia</a>
 498 <strong>38</strong> (2001), 509&ndash;529.
 499 <a href="http://www.ucolick.org/~sla/leapsecs/">The
 500 Future of Leap Seconds</a> covers this
 501 contentious issue.</li>
 502 </ul>
 503 <h2>Time notation</h2>
 504 <ul>
 505 <li>
 506 <a href="http://www.cl.cam.ac.uk/~mgk25/iso-time.html">A Summary of
 507 the International Standard Date and Time Notation</a> is a good
 508 summary of
 509 <a
 510 href="http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=40874"><abbr
 511 title="International Organization for Standardization">ISO</abbr>
 512 8601:2004 -- Data elements and interchange formats -- Information
 513 interchange -- Representation of dates and times</a>.</li>
 514 <li>
 515 <a href="http://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
 516 Schema: Datatypes - dateTime</a> specifies a format inspired by
 517 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
 518 <li>
 519 Section 3.3 of <a
 520 href="ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt">Internet
 521 <abbr title="Request For Comments">RFC</abbr> 2822</a>
 522 specifies the time notation used in email and <a
 523 href="ftp://ftp.rfc-editor.org/in-notes/rfc2616.txt"><abbr>HTTP</abbr></a>
 524 headers.</li>
 525 <li>
 526 <a href="ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt">Internet
 527 <abbr>RFC</abbr> 3339</a> specifies an <abbr>ISO</abbr> 8601
 528 profile for use in new Internet
 529 protocols.</li>
 530 <li>
 531 <a href="http://www.hackcraft.net/web/datetime/">Date &amp; Time
 532 Formats on the Web</a> surveys web- and Internet-oriented date and time
 533 formats.</li>
 534 <li>
 535 <a href="http://www.exit109.com/~ghealton/y2k/yrexamples.html">The
 536 Best of Dates, the Worst of Dates</a> covers many problems encountered
 537 by software developers when handling dates and time stamps.</li>
 538 <li><a
 539 href="http://www-306.ibm.com/software/globalization/icu/"><abbr>ICU</abbr></a>
 540 contains a mechanism for localizing time zone
 541 labels and abbreviations; for example, one can use it to specify
 542 Russian translations for "Eastern European Summer Time",
 543 "<abbr title="Eastern European Summer Time">EEST</abbr>",
 544 and <code>Europe/Bucharest</code>.
 545 This mechanism is part of the
 546 <a href="http://unicode.org/cldr/">Unicode
 547 <abbr title="Common Locale Data Repository">CLDR</abbr> Project</a>;
 548 for example, the <a
 549 href="http://unicode.org/cldr/data/diff/by_type/dates_timeZoneNames.html"><abbr>CLDR</abbr> Sideways Data for dates_timeZoneNames</a>
 550 shows values for time zone names in many locales.</li>
 551 <li>Alphabetic time zone abbreviations should not be used as unique
 552 identifiers for <abbr>UTC</abbr> offsets as they are ambiguous in
 553 practice. For example, "<abbr>EST</abbr>" denotes 5 hours behind
 554 <abbr>UTC</abbr> in English-speaking North America, but it denotes 10
 555 or 11 hours ahead of <abbr>UTC</abbr> in Australia; and
 556 French-speaking North Americans prefer
 557 "<abbr title="Heure Normale de l'Est">HNE</abbr>" to
 558 "<abbr>EST</abbr>". For <abbr>POSIX</abbr> the <code>tz</code>
 559 database contains English abbreviations for all time stamps but in
 560 many cases these are merely inventions of the database
 561 maintainers.</li>
 562 <li>Numeric time zone abbreviations typically count hours east of
 563 <abbr>UTC</abbr>, e.g., <code>+09</code> for Japan and
 564 <code>-10</code> for Hawaii. However, the <abbr>POSIX</abbr>
 565 <code>TZ</code> environment variable uses the opposite convention. For
 566 example, one might use <code>TZ="JST-9"</code> and
 567 <code>TZ="HST10"</code> for Japan and Hawaii, respectively. If the
 568 <code>tz</code> database is available, it is usually better to use
 569 settings like <code>TZ="Asia/Tokyo"</code> and
 570 <code>TZ="Pacific/Honolulu"</code> instead, as this should avoid
 571 confusion, handle old time stamps better, and insulate you better from
 572 any future changes to the rules. One should never set
 573 <abbr>POSIX</abbr> <code>TZ</code> to a value like
 574 <code>"GMT-9"</code>, though, since this would falsely claim that
 575 local time is nine hours ahead of <abbr>UTC</abbr> and the time zone
 576 is called "<abbr>GMT</abbr>".</li>
 577 </ul>
 578 <h2>Related indexes</h2>
 579 <ul>
 580 <li><a href="tz-art.htm">Time and the Arts</a></li>
 581 <li><a href="http://dmoz.org/Reference/Time/">Open Directory -
 582 Reference: Time</a></li>
 583 <li><a href="http://directory.google.com/Top/Reference/Time/">Google Directory
 584 - Reference &gt; Time</a></li>
 585 <li><a href="http://dir.yahoo.com/Science/Measurements_and_Units/Time/">Yahoo!
 586 Directory &gt; Science &gt; Measurements and Units &gt; Time</a></li>
 587 </ul>
 588 </body>
 589 </html>