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<h1>Sources for Time Zone and Daylight Saving Time Data</h1>
<address>
@(#)tz-link.htm 7.42
</address>
<p>
Please send corrections to this web page to the
<a href="mailto:tz@elsie.nci.nih.gov">time zone mailing list</a>.
</p>
<h2>The <code>tz</code> database</h2>
<p>
The public-domain time zone database contains code and data
that represent the history of local time
for many representative locations around the globe.
It is updated periodically to reflect changes made by political bodies
to UTC offsets and daylight-saving rules.
This database (often called <code>tz</code> or <code>zoneinfo</code>)
is used by several implementations,
including
<a href="http://www.gnu.org/software/libc/">the GNU C Library</a> used in
<a href="http://www.linux.org/">GNU/Linux</a>,
<a href="http://www.freebsd.org/">FreeBSD</a>,
<a href="http://www.netbsd.org/">NetBSD</a>,
<a href="http://www.openbsd.org/">OpenBSD</a>,
<a href="http://www.cygwin.com/">Cygwin</a>,
<a href="http://www.delorie.com/djgpp/">DJGPP</a>,
<a href="http://www.hp.com/products1/unix/operating/">HP-UX</a>,
<a href="http://www.sgi.com/developers/technology/irix/">IRIX</a>,
<a href="http://www.apple.com/macosx/">Mac OS X</a>,
<a href="http://h71000.www7.hp.com/">OpenVMS</a>,
<a href="http://wwws.sun.com/software/solaris/">Solaris</a>,
<a href="http://www.tru64unix.compaq.com/">Tru64</a>, and
<a href="http://www.sco.com/products/unixware/">UnixWare</a>.</p>
<p>
Each location in the database represents a national region where all
clocks keeping local time have agreed since 1970.
Locations are identified by continent or ocean and then by the name of
the location, which is typically the largest city within the region.
For example, <code>America/New_York</code>
represents most of the US eastern time zone;
<code>America/Indianapolis</code> represents most of Indiana, which
uses eastern time without daylight saving time (DST);
<code>America/Detroit</code> represents most of Michigan, which uses
eastern time but with different DST rules in 1975;
and other entries represent smaller regions like Starke County,
Kentucky, which switched from central to eastern time in 1991.
To use the database, set the <code>TZ</code> environment variable to
the location's full name, e.g., <code>TZ="America/New_York"</code>.</p>
<p>
In the <code>tz</code> database's
<a href="ftp://elsie.nci.nih.gov/pub/">FTP distribution</a>,
the code is in the file <code>tzcode<var>C</var>.tar.gz</code>,
where <code><var>C</var></code> is the code's version;
similarly, the data are in <code>tzdata<var>D</var>.tar.gz</code>,
where <code><var>D</var></code> is the data's version.
The following shell commands download
these files to a GNU/Linux or similar host; see the downloaded
<code>README</code> file for what to do next.</p>
<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'
<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 -
gzip -dc tzdata*.tar.gz | tar -xf -
</code></pre>
<p>
The code lets you compile the <code>tz</code> source files into
machine-readable binary files, one for each location. It also lets
you read a <code>tz</code> binary file and interpret time stamps for that
location.</p>
<p>
The data are by no means authoritative. If you find errors, please
send changes to the <a href="mailto:tz@elsie.nci.nih.gov">time zone
mailing list</a>. You can also <a
href="mailto:tz-request@elsie.nci.nih.gov">subscribe</a> to the
mailing list, retrieve the <a
href="ftp://elsie.nci.nih.gov/pub/tzarchive.gz">archive of old
messages</a> (in gzip compressed format), or retrieve <a
href="ftp://munnari.oz.au/pub/oldtz/">archived older versions of code
and data</a>.</p>
<p>
The Web has several other sources for time zone and daylight saving time data.
Here are some recent links that may be of interest.
</p>
<h2>Web pages using recent versions of the <code>tz</code> database</h2>
<ul>
<li><a href="http://twiki.org/cgi-bin/xtra/tzdate">Date and Time Gateway</a>
is a text-based point-and-click interface to tables of current time
throughout the world.</li>
<li>Fancier web interfaces, roughly in ascending order of complexity, include:
<ul>
<li><a href="http://www.hilink.com.au/times/">Local Times Around the
World</a></li>
<li><a href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current Time in 1000 Places</a></li>
<li><a href="http://timezoneconverter.com/">Time Zone Converter</a></li>
</ul></li>
<li><a href="http://www.holidayfestival.com/">The Worldwide Holiday
& Festival Site</a> lists DST-related clock changes along with
holidays.</li>
<li><a href="http://www.timeanddate.com/worldclock/">The World Clock -
Time Zones</a>
is a web interface to a time zone database derived from
<code>tz</code>'s.</li>
</ul>
<h2>Other time zone database formats</h2>
<ul>
<li>The <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2445.txt">
Internet Calendaring and Scheduling Core Object Specification
(iCalendar)</a> specification published by the <a
href="http://www.ietf.org/html.charters/calsch-charter.html">IETF
Calendaring and Scheduling Working Group (calsch)</a> covers time zone
data; see its VTIMEZONE calendar component.</li>
<li>The <a
href="http://lists.w3.org/Archives/Public/www-rdf-calendar/">www-rdf-calendar</a>
list discusses <a href="http://www.w3.org/RDF/">RDF</a>-based calendar
and group scheduling systems, and has a <a
href="http://www.w3.org/2002/12/cal/#tzd">workspace on time zone
data</a> converted from <code>tz</code>. An earlier <a
href="http://www.w3.org/2000/01/foo">schema</a> was sketched out by <a
href="http://www.w3.org/People/Berners-Lee/">Tim Berners-Lee</a>.</li>
<li><a
href="http://www.calsch.org/ietf/archives/draft-ietf-calsch-many-xcal-02.txt">XCal</a>
was a draft <a href="http://www.w3.org/XML/">XML</a> document type
definition that corresponded to iCalendar.</li>
</ul>
<h2>Other <code>tz</code> compilers</h2>
<ul>
<li><a href="http://www.dachaplin.dsl.pipex.com/vzic">Vzic iCalendar
Timezone Converter</a> describes a program Vzic that compiles
<code>tz</code> source into iCalendar-compatible VTIMEZONE files.
Vzic is freely
available under the <a href="http://www.gnu.org/copyleft/gpl.html">GNU
General Public License (GPL)</a>.</li>
<li><a
href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
contains a script <code>parse_olson</code> that compiles
<code>tz</code> source into <a href="http://www.perl.org/">Perl</a>
modules. It is part of the Perl <a
href="http://datetime.perl.org/">DateTime Project</a>, which is freely
available under both the GPL and the Perl <a
href="http://www.perl.com/language/misc/Artistic.html">Artistic
License</a>. DateTime::TimeZone also contains a script
<code>tests_from_zdump</code> that generates test cases for each clock
transition in the <code>tz</code> database.</li>
<li><a href="http://oss.software.ibm.com/icu/">International Components for
Unicode (ICU)</a> contains a C/C++ library for internationalization that
has a compiler from <samp>tz</samp> source into an ICU-specific format.
ICU is freely available under a BSD-style license.</li>
<li><a href="http://joda-time.sourceforge.net/">Joda Time - Java date
and time API</a> contains a class
<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
<code>tz</code> source into a Joda-specific binary format. Joda Time
is freely available under a BSD-style license.</li>
</ul>
<h2>Other <code>tz</code> binary file readers</h2>
<ul>
<li>The <a href="http://www.gnu.org/software/libc/">GNU C Library</a>
has an independent, thread-safe implementation of
a <code>tz</code> binary file reader.
This library is freely available under the
<a href="http://www.gnu.org/copyleft/lesser.html">
GNU Lesser General Public License (LGPL)</a>,
and is widely used in GNU/Linux systems.</li>
<li><a href="http://www.bmsi.com/java/#TZ">ZoneInfo.java</a>
is a <code>tz</code> binary file reader written in Java.
It is freely available under the GNU LGPL.</li>
<li><a href="http://s.keim.free.fr/tz/doc.html">Python time zones</a>
is a <code>tz</code> binary file reader written in <a
href="http://www.python.org/">Python</a>. It is freely available
under a BSD-style license.</li>
</ul>
<h2>Other <code>tz</code>-based time zone conversion software</h2>
<ul>
<li><a href="http://java.sun.com/">Sun Java</a> releases since 1.4
contain a copy of a recent <samp>tz</samp> database in a Java-specific
format.</li>
<li><a
href="http://www1.tip.nl/~t876506/AboutTimeZonesHC.html">HyperCard
time zones calculator</a> is a HyperCard stack.</li>
<li><a
href="http://www.cimmyt.org/timezone/">World Time Explorer</a> is a
Microsoft Windows program.</li>
</ul>
<h2>Other time zone databases</h2>
<ul>
<li><a href="http://www.astro.com/cgi-bin/atlw3/aq.cgi?lang=e">Atlas Query
- Astrodienst</a> is Astrodienst's Web version of Shanks's
excellent time zone history atlases published in both <a
href="http://astrocom.com/software/pcatlas.php">computer</a> and <a
href="http://astrocom.com/books/xrefa.php#SHANKS">book</a> form by <a
href="http://astrocom.com/">Astro Communications Services</a>.</li>
<li><a href="http://worldtime.com/">WORLDTIME: interactive atlas,
time info, public holidays</a>
contains information on local time, sunrise and sunset,
and public holidays in several hundred cities around the world.</li>
<li><a href="http://www.worldtimeserver.com/">World Time Server</a>
is another time zone database.</li>
<li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a>
contains data from the Time Service Department of the US Naval Observatory
(USNO), used as the source
for the <code>usno*</code> files in the <code>tz</code> distribution.</li>
<li><a href="http://www.airportcitycodes.com/aaa/">Airlines, Airplanes
and Airports</a> lists current standard times for thousands of
airports around the world. This seems to be derived from
the <a href="http://www.iata.org/sked/publications/">Standard
Schedules Information Manual (SSIM)</a> of the
the <a href="http://www.iata.org/">International Air Transport
Association</a>,
which gives current time zone rules for
all the airports served by commercial aviation.</li>
</ul>
<h2>Maps</h2>
<ul>
<li>The <a href="http://www.odci.gov/">United States Central
Intelligence Agency (CIA)</a> publishes a <a
href="http://www.odci.gov/cia/publications/factbook/reference_maps/pdf/time_zones.pdf">time
zone map</a>; the
<a
href="http://www.lib.utexas.edu/maps/world.html">Perry-Castañeda
Library Map Collection</a>
of the University of Texas at Austin has copies of
recent editions.
The pictorial quality is good,
but the maps do not indicate summer time,
and parts of the data are a few years out of date.</li>
<li><a href="http://worldtimezone.com/">World timezones map with
current time</a>
has several fancy time zone maps; it covers Russia particularly well.
The maps' pictorial quality is not quite as good as the CIA's
but the maps are more up to date.</li>
</ul>
<h2>Time zone boundaries</h2>
<ul>
<li><a href="http://home-4.tiscali.nl/~t876506/Multizones.html">Time
zone boundaries for multizone countries</a> summarizes legal
boundaries between time zones within countries.</li>
<li>Manifold.net's <a
href="http://www.manifold.net/download/freemaps.html">Free Maps and
GIS Data</a> includes a Manifold-format map of world time zone
boundaries distributed under the GPL. The GeoCommunity's <a
href="http://software.geocomm.com/data/intl_timezones.html">International
Time Zones</a> publishes the same data in other formats.</li>
<li>The US Geological Survey's National Atlas of the United States
publishes the <a href="http://www.nationalatlas.gov/timeznm.html">Time
Zones of the United States</a> in the public domain.</li>
<li>The GeoCommunity lists several commercial sources for <a
href="http://spatialnews.geocomm.com/features/timezones/">International
Time Zones and Time Zone Data</a>.</li>
</ul>
<h2>Civil time concepts and history</h2>
<ul>
<li><a href="http://physics.nist.gov/time">A Walk through Time</a>
surveys the evolution of timekeeping.</li>
<li><a href="http://webexhibits.org/daylightsaving/">About Daylight
Saving Time - History, rationale, laws and dates</a>
is an overall history of DST.</li>
<li><a href="http://toi.iriti.cnr.it/">The
Time of Internet</a>
describes time zones and daylight saving time,
with diagrams.
The time zone map is out of date, however.</li>
<li><a href="http://www.phys.uu.nl/~vgent/idl/idl.htm">A History of
the International Date Line</a> tells the story of the most important
time zone boundary.</li>
<li><a href="http://www.statoids.com/tconcept.html">Basic Time
Zone Concepts</a> discusses terminological issues behind time zones.</li>
</ul>
<h2>National histories of legal time</h2>
<dl>
<dt>Australia</dt>
<dd>The Community Relations Division of the New South Wales (NSW)
Attorney General's Department maintains a <a
href="http://www.lawlink.nsw.gov.au/crd.nsf/pages/time2">history of
daylight saving in NSW</a>.</dd>
<dt>Austria</dt>
<dd>The Federal Office of Metrology and Surveying publishes a
table of <a href="http://www.metrologie.at/pdf/sommerzeit.pdf"
hreflang="de">daylight saving time in Austria (in German)</a>.</dd>
<dt>Belgium</dt>
<dd>The Royal Observatory of Belgium maintains a table of <a
href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
<dt>Brazil</dt>
<dd>The Time Service Department of the National Observatory
records <a href="http://pcdsh01.on.br/DecHV.html"
hreflang="pt-BR">Brazil's daylight saving time decrees (in
Portuguese)</a>.</dd>
<dt>Canada</dt>
<dd>The Institute for National Measurement Standards publishes current
and some older information about <a
href="http://inms-ienm.nrc-cnrc.gc.ca/time_services/daylight_savings_e.html">Time
Zones and Daylight Saving Time</a>.</dd>
<dt>Chile</dt>
<dd>WebExhibits publishes a <a
href="http://webexhibits.org/daylightsaving/chile.html"
hreflang="es">history of official time (in Spanish)</a> originally
written by the Chilean Hydrographic and Oceanographic Service.</dd>
<dt>Germany</dt>
<dd>The National Institute for Science and Technology maintains the <a
href="http://www.ptb.de/en/org/4/44/441/dars_e.htm">Realisation of
Legal Time in Germany</a>.</dd>
<dt>Israel</dt>
<dd>The Interior Ministry periodically issues <a
href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements/"
hreflang="he">announcements (in Hebrew)</a>.</dd>
<dt>Mexico</dt>
<dd>The Investigation and Analysis Service of the Mexican Library of
Congress has published a <a
href="http://www.cddhcu.gob.mx/bibliot/publica/inveyana/polisoc/horver/"
hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
<dt>Malaysia</dt>
<dd>See Singapore below.</dd>
<dt>Netherlands</dt>
<dd><a href="http://www.phys.uu.nl/~vgent/wettijd/wettijd.htm"
hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
covers the history of local time in the Netherlands from ancient times.</dd>
<dt>New Zealand</dt>
<dd>The Department of Internal Affairs maintains a brief history <a
href="http://www.dia.govt.nz/diawebsite.nsf/wpg_URL/Resource-material-Information-We-Provide-About-Daylight-Saving">about
daylight saving</a>. The privately-maintained <a
href="http://www.astrologyhouse.co.nz/timechanges.htm">Time Changes in
New Zealand</a> has more details.</dd>
<dt>Singapore</dt>
<dd><a
href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
is Singapore in the "Wrong" Time Zone?</a> details the
history of legal time in Singapore and Malaysia.</dd>
<dt>United Kingdom</dt>
<dd><a
href="http://www.srcf.ucam.org/~jsm28/british-time/">History of
legal time in Britain</a> discusses in detail the country
with perhaps the best-documented history of clock adjustments.
The National Physical Laboratory also maintains an <a
href="http://www.npl.co.uk/time/summer_time_archive.html">archive
of summer time dates</a>.</dd>
</dl>
<h2>Precision timekeeping</h2>
<ul>
<li><a
href="http://literature.agilent.com/litwebbin/purl.cgi?org_id=tmo&pub_id=5965-7984E">The
Science of Timekeeping</a> is a thorough introduction
to the theory and practice of precision timekeeping.</li>
<li><a href="http://www.ntp.org/">NTP: The Network Time Protocol</a>
discusses how to synchronize clocks of
Internet hosts.</li>
<li><a href="http://gauss.gge.unb.ca/GMT.UT.and.the.RGO.txt"
charset="macintosh">A
Few Facts Concerning GMT, UT, and the RGO</a>
answers questions like "What is the difference between GMT and UTC?"</li>
<li><a
href="http://www.gb.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
Times</a> explains more abstruse astronomical time scales like TT, TCG,
and TDB.</li>
<li>The <a href="http://www.iau.org/">IAU</a>'s <a
href="http://www.iau-sofa.rl.ac.uk/">Standards Of Fundamental
Astronomy</a> (SOFA) initiative publishes Fortran code for converting
among time scales like TAI, TDB, TT and UTC.</li>
<li><a href="http://www.jpl.nasa.gov/basics/bsf2-3.htm">Basics of
Space Flight - Reference Systems - Time Conventions</a>
briefly explains interplanetary space flight timekeeping.</li>
<li><a
href="http://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a> briefly
describes Mars Coordinated Time (MTC) and the diverse local time
scales used by each landed mission on Mars.</li>
<li><a
href="http://hpiers.obspm.fr/eop-pc/products/bulletins/bulletins.html">Bulletins
maintained by the IERS EOP (PC)</a> contains official publications of
the Earth Orientation Parameters Product Center of the
International Earth Rotation Service, the committee that decides
when leap seconds occur.</li>
<li>The <a
href="http://www.mail-archive.com/leapsecs@rom.usno.navy.mil/">Leap
Second Discussion List</a> covers McCarthy and Klepczynski's proposal
to discontinue leap seconds, published in <a
href="http://www.gpsworld.com/">GPS World</a> <strong>10</strong>, 11
(1999-11), 50–57 and discussed further in R. A. Nelson et al.,
<a href="http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
leap second: its history and possible future</a>,
<a href="http://www.bipm.fr/metrologia/metrologia.html">Metrologia</a>
<strong>38</strong> (2001), 509–529.
<a href="http://www.ucolick.org/~sla/leapsecs/onlinebib.html">The
Future of Leap Seconds</a> catalogs information about this
contentious issue.</li>
</ul>
<h2>Time notation</h2>
<ul>
<li>
<a href="http://www.cl.cam.ac.uk/~mgk25/iso-time.html">A Summary of
the International Standard Date and Time Notation</a> is a good
summary of ISO
8601:1988 - Data elements and interchange formats - Information interchange
- Representation of dates and times (which has been superseded by
<a href="http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=26780">ISO 8601:2000</a>).</li>
<li>
Section 3.3 of <a
href="ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt">Internet RFC 2822</a>
specifies the time notation used in email and <a
href="ftp://ftp.rfc-editor.org/in-notes/rfc2616.txt">HTTP</a> headers.</li>
<li>
<a href="ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt">Internet RFC
3339</a> specifies an ISO 8601 profile for use in new Internet
protocols.</li>
<li>
<a href="http://www.exit109.com/~ghealton/y2k/yrexamples.html">The
Best of Dates, the Worst of Dates</a> covers many problems encountered
by software developers when handling dates and time stamps.</li>
<li>
Alphabetic time zone abbreviations should not be used as unique
identifiers for UTC offsets as they are ambiguous in practice. For
example, "EST" denotes 5 hours behind UTC in English-speaking North
America, but it denotes 10 or 11 hours ahead of UTC in Australia;
and French-speaking North Americans prefer "HNE" to "EST". For
compatibility with <a href="http://www.pasc.org/#POSIX">POSIX</a> the
<code>tz</code> database contains English abbreviations for all time
stamps but in many cases these are merely inventions of the database
maintainers.</li>
</ul>
<h2>Related indexes</h2>
<ul>
<li><a href="tz-art.htm">Time and the Arts</a></li>
<li><a href="http://dmoz.org/Reference/Time/">Open Directory -
Reference: Time</a></li>
<li><a href="http://directory.google.com/Top/Reference/Time/">Google Directory - Reference > Time</a></li>
<li><a href="http://dir.yahoo.com/Science/Measurements_and_Units/Time/">Yahoo! Science > Measurements and Units > Time</a></li>
</ul>
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