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<title>Sources for time zone and daylight saving time data</title>
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<h1>Sources for time zone and daylight saving time data</h1>
Time zone and daylight saving rules are controlled by individual
governments. They are sometimes changed with little notice, and their
histories and planned futures are often recorded only fitfully. Here
is a summary of attempts to organize and record relevant data in this
<h2>The <code><abbr title="time zone">tz</abbr></code> database</h2>
The <a href="">public-domain</a>
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 <a href="">time zone</a>
boundaries and
<a href="">daylight-saving</a>
This database (often called <code>zoneinfo</code> or
is used by several implementations,
<a href="">the
<abbr title="GNU's Not Unix">GNU</abbr>
C Library</a> (used in
<a href=""><abbr>GNU</abbr>/Linux</a>),
<a href="">Android</a>,
<a href="">B2G
<abbr title="Operating System">OS</abbr></a>,
<a href="">Free<abbr
title="Berkeley Software Distribution">BSD</abbr></a>,
<a href="">Net<abbr>BSD</abbr></a>,
<a href="">Open<abbr>BSD</abbr></a>,
<a href="">Chromium OS</a>,
<a href="">Cygwin</a>,
<a href=""><abbr
title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
<a href="">MINIX</a>,
<a href=""><abbr
title="Web Operating System">webOS</abbr></a>,
<a href=""><abbr
title="Advanced Interactive eXecutive">AIX</abbr></a>,
<a href="">BlackBerry 10</a>,
<a href=""><abbr
title="iPhone OS">iOS</abbr></a>,
<a href="">Microsoft Windows</a>,
<a href="">Open<abbr
title="Virtual Memory System">VMS</abbr></a>,
<a href="">Oracle Database</a>,
<a href="">Oracle Solaris</a>, and
<a href=""><abbr title="Operating System Ten">OS
Each location in the database represents a 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 <abbr title="United States">US</abbr> eastern time zone;
<code>America/Phoenix</code> represents most of Arizona, which
uses mountain time without daylight saving time (<abbr
title="daylight saving time">DST</abbr>);
<code>America/Detroit</code> represents most of Michigan, which uses
eastern time but with different <abbr>DST</abbr> rules in 1975;
and other entries represent smaller regions like Starke County,
Indiana, which switched from central to eastern time in 1991
and switched back in 2006.
To use the database on an extended <a
title="Portable Operating System Interface">POSIX</abbr></a>
implementation set the <code><abbr>TZ</abbr></code>
environment variable to the location's full name,
e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
Associated with each region is a history of offsets from
<a href="">Universal
Time</a> (<abbr>UT</abbr>), which is <a
href="">Greenwich Mean
Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
for time stamps after 1960 this is more precisely <a
Universal Time</a> (<abbr>UTC</abbr>).
The database also records when daylight saving time was in use,
along with alphabetic time zone abbreviations such as <abbr>EST</abbr>
for Eastern Standard Time in the <abbr>US</abbr>.</p>
In the <code><abbr>tz</abbr></code> database's
<a href=""><abbr
title="File Transfer Protocol">FTP</abbr> releases</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 entries are in <code>tzdata<var>D</var>.tar.gz</code>,
where <code><var>D</var></code> is the data's version.
Since 1996, each version has been a four-digit year followed by
lower-case letter (<samp>a</samp> through <samp>z</samp>,
then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
through <samp>zzz</samp>, and so on).
Convenience links to
the <a href="">latest
code</a> and
<a href="">latest data</a> revisions
are also available.
The following <a
href="">shell</a> commands download
these files to a <abbr>GNU</abbr>/Linux or similar host;
see the downloaded
<code>README</code> file for what to do next.</p>
<pre style="margin-left: 2em"><code>mkdir tz
cd tz
<a href="">wget</a> --retr-symlinks '*-latest.tar.gz'
<a href="">gzip</a> -dc tzcode-latest.tar.gz | <a href="">tar</a> -xf -
gzip -dc tzdata-latest.tar.gz | tar -xf -
The code and data files can also be obtained from the
<a href="">Time Zone Database website</a>
of the <a href="">Internet Assigned Numbers
Authority (IANA)</a>.
An <a href="">unofficial development
repository</a> of the code and data is available
in <a href="">Git</a> form
from <a href="">GitHub</a>; be careful, as this
repository is less well tested and probably contains more errors.
The code lets you compile the <code><abbr>tz</abbr></code> source files into
machine-readable binary files, one for each location. It also lets
you read a <code><abbr>tz</abbr></code> binary file and interpret time stamps for that
<h2>Changes to the <code><abbr>tz</abbr></code> database</h2>
The <code><abbr>tz</abbr></code> code and data
are by no means authoritative. If you find errors, please
send changes to the <a href="">time zone
mailing list</a>. You can also <a
href="">browse recent
messages</a> sent to the mailing list, <a
href="">subscribe</a> to it,
and browse the <a
href="">archive of old
If your government plans to change its time zone boundaries or
daylight saving rules, let the mailing list know well in advance. With
less than a year's notice there is a good chance that some
computer-based clocks will operate incorrectly after the change, due
to delays in propagating updates to software and data. The shorter
the notice, the more likely clock problems will arise.
<p>Sources for the <code><abbr>tz</abbr></code> database are
<a href=""><abbr
title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
<a href="">text files</a>
with lines terminated by <a href=""><abbr
which can be modified by common text editors such
as <a href="">GNU Emacs</a>,
<a href="">gedit</a>, and
<a href="">vim</a>. One
editor has a package to simplify editing further:</p>
<li><a href="">Sublime
zoneinfo</a> is a <a href="">Sublime
Text</a> package for syntax highlighting <code><abbr>tz</abbr></code>
source files.</li>
For further information about updates, please see
<a href="">Procedures for
Maintaining the Time Zone Database</a> (Internet <abbr
title="Request For Comments">RFC</abbr> 6557).</p>
<h2>Commentary on the <code><abbr>tz</abbr></code> database</h2>
<li>The article
<a href="">tz database</a> is
an encyclopedic summary.</li>
<li><a href="tz-how-to.html">How to Read the
tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
database format.</li>
the Timing of Time Zone Changes</a> gives examples of problems caused
by inadequate notice by governments of time zone and daylight saving
rule changes.</li>
literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
database's style.</li>
<h2>Web sites using recent versions of the <code><abbr>tz</abbr></code> database</h2>
These are listed roughly in ascending order of complexity and fanciness.
<li><a href=""></a> shows locations'
time and zones in a simple format.</li>
<li><a href=""></a> is a simple
time zone converter.</li>
href="">Date and Time Gateway</a>
lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
Time in 1000 Places</a> uses descriptions of the values.</li>
<li><a href="">Time Zone
uses a pulldown menu.</li>
<li><a href="">Complete
timezone information for all countries</a> displays tables of DST rules.
<li><a href="">The World Clock &ndash;
Worldwide</a> lets you sort zone names and convert times.</li>
<li><a href="">Time Difference</a>
calculates the current time difference between locations.</li>
<li><a href="">Weather Now</a> lists the weather too.</li>
<li><a href="">The Time Now</a> also lists weather.</li>
<li><a href=""></a>
also contains data about time zone boundaries; it supports queries via place
names and shows location maps.</li>
<h2>Network protocols for <code><abbr>tz</abbr></code> data</h2>
<li>The <a href="">Internet Engineering Task Force</a>'s
<a href="">Time Zone Data
Distribution Service (tzdist) working group</a> defined <a
(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
along with a <a href="">calendar access
protocol for transferring time zone data by reference</a>
(Internet <abbr>RFC</abbr> 7809). This work was based
on the iCalendar and CalConnect efforts described below.</li>
<li>The <a href="">
Internet Calendaring and Scheduling Core Object Specification
(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
covers time zone
data; see its VTIMEZONE calendar component.
The iCalendar format requires specialized parsers and generators; a
variant <a href="">xCal</a>
(Internet <abbr>RFC</abbr> 6321) uses
<a href=""><abbr
title="Extensible Markup Language">XML</abbr></a> format, and a variant
<a href="">jCal</a>
(Internet <abbr>RFC</abbr> 7265)
uses <a href=""><abbr
title="JavaScript Object Notation">JSON</abbr></a> format.
<a href="">CalConnect, The Calendaring and Scheduling
Consortium</a> is promoting further work in this area. <a
TIMEZONE Problems and Recommendations</a> offers guidelines and
recommendations for the use of VTIMEZONE and <code><abbr>tz</abbr></code>.</li>
<li>The <a
Registry and Service Recommendations</a> of CalConnect's
<a href="">TIMEZONE
Technical Committee</a> discusses a
strategy for defining and deploying a time zone
registration process that would establish unique names for each
version of each <code><abbr>tz</abbr></code> zone, along with a polygonal
representation of the geographical area corresponding to the
<li>The <a
list discusses <a
title="Resource Description Framework">RDF</abbr></a>-based calendar
and group scheduling systems, and has a <a
href="">workspace on time zone
data</a> converted from <code><abbr>tz</abbr></code>.</li>
<h2>Other <code><abbr>tz</abbr></code> compilers</h2>
<li><a href="">Vzic</a> is a <a
program that compiles
<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
Vzic is freely
available under the <a
General Public License (<abbr
title="General Public License">GPL</abbr>)</a>.</li>
<li><a href="">tziCal &ndash; tz
database conversion utility</a> is like Vzic, except for the <a
href="">.NET framework</a>
and with a <abbr>BSD</abbr>-style license.</li>
contains a script <code>parse_olson</code> that compiles
<code><abbr>tz</abbr></code> source into <a href="">Perl</a>
modules. It is part of the Perl <a
href="">DateTime Project</a>, which is freely
available under both the <abbr>GPL</abbr> and the Perl Artistic
License. DateTime::TimeZone also contains a script
<code>tests_from_zdump</code> that generates test cases for each clock
transition in the <code><abbr>tz</abbr></code> database.</li>
<li>The <a href="">Time Zone
Database Parser</a> is a
<a href="">C++</a> parser and
runtime library. It is freely available under the
<a href="">Creative Commons
Attribution 4.0 International Public License</a>.</li>
<li><a href="">International Components for
Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
libraries for internationalization that
has a compiler from <code><abbr>tz</abbr></code> source
and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
(mentioned below)
into an <abbr>ICU</abbr>-specific format.
<abbr>ICU</abbr> is freely available under a
<abbr>BSD</abbr>-style license.</li>
<li>The <a href="">Tzdata</a> package for
the <a href="">Elixir</a> language downloads
and compiles tz source and exposes <abbr
title="Application Program Interface">API</abbr>s for use. It is
freely available under the <abbr
title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
<li>The <a
tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
Oracle Java.</li>
<li><a href="">Joda-Time &ndash; Java date
and time <abbr>API</abbr></a> contains a class
<code></code> that compiles
<code><abbr>tz</abbr></code> source into a Joda-specific binary format. Joda Time
is freely available under a <abbr>BSD</abbr>-style license.</li>
<li><a href="">Noda Time &ndash; Date and
time <abbr>API</abbr> for .NET</a>
and <a href="">TZ4Net</a>
are similar to Joda Time, but for the .NET framework instead of
Java. They are freely available under the
<a href="">Apache License</a>
and a <abbr>BSD</abbr>-style license, respectively.</li>
<li><a href="">JavaScript</a>-based
compilers and libraries include:
<li><a href="">Moment Timezone</a> is a
plugin for the <a href="">Moment.js</a> date
manipulation library. It is freely available under the <abbr>MIT</abbr>
<li><a href="">TimezoneJS.Date</a>'s
<abbr>API</abbr> is upward compatible with standard JavaScript
Dates. It is freely available under the Apache License.</li>
<li><a href="">Walltime-js</a>
translates <abbr>UT</abbr> to local time. It is freely available under
the <abbr>MIT</abbr> license.</li>
<li><a href="">pytz &ndash; World Timezone
Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
<a href="">Python</a>.
It is freely available under a <abbr>BSD</abbr>-style license.</li>
<li><a href="">TZInfo &ndash;
Ruby Timezone Library</a>
compiles <code><abbr>tz</abbr></code> source into
<a href="">Ruby</a>.
It is freely available under the <abbr>MIT</abbr> license.</li>
<li>The <a href="">Chronos Date/Time
Library</a> is
a <a href="">Smalltalk</a> class
library that compiles <code><abbr>tz</abbr></code> source into a time
zone repository whose format
is either proprietary or an <abbr>XML</abbr>-encoded
<li><a href="">Tcl</a>
contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
source into text files, along with a runtime that can read those
files. Tcl is freely available under a <abbr>BSD</abbr>-style
<h2>Other <code><abbr>tz</abbr></code> binary file readers</h2>
<li>The <a
href=""><abbr>GNU</abbr> C
has an independent, thread-safe implementation of
a <code><abbr>tz</abbr></code> binary file reader.
This library is freely available under the
<a href="">
<abbr>GNU</abbr> Lesser General Public License
(<abbr title="Lesser General Public License">LGPL</abbr>)</a>,
and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
<li><a href="">GNOME</a>'s Glib has
a <code><abbr>tz</abbr></code> binary file reader written in C that
creates a <code>GTimeZone</code> object representing sets
of <abbr>UT</abbr> offsets.
It is freely available under the <abbr>LGPL</abbr>.</li>
<a href="">BDE Standard Library</a>'s
<code>baltzo::TimeZoneUtil</code> component contains a C++
implementation of a binary file reader. It is freely available under
the Apache License.</li>
<li><a href="">CCTZ</a> is a simple C++
library that translates between UTC and civil time and can read binary
files. It is freely available under the Apache License.</li>
<li><a href=""></a>
is a <code><abbr>tz</abbr></code> binary file reader written in Java.
It is freely available under the <abbr>LGPL</abbr>.</li>
<li><a href="">Timezone</a> is a
JavaScript library that supports date arithmetic that is time zone
aware. It is freely available under the <abbr>MIT</abbr> license.</li>
<li>Tcl, mentioned above, also contains a
<code><abbr>tz</abbr></code> binary file reader.</li>
<li><a href="">
is a <code><abbr>tz</abbr></code> binary file reader written in Perl.
It is freely available under the same terms as Perl
(dual <abbr>GPL</abbr> and Artistic license).</li>
public-domain <a href="">tz.js</a>
library contains a Python tool that
converts <code><abbr>tz</abbr></code> binary data into
<abbr>JSON</abbr>-format data suitable for use
in its JavaScript library for time zone conversion. Dates before 1970
are not supported.</li>
<li>The <a
package contains <a href="">Haskell</a> code that
parses and uses <code><abbr>tz</abbr></code> binary data. It is freely
available under a <abbr>BSD</abbr>-style license.</li>
<h2>Other <code><abbr>tz</abbr></code>-based time zone software</h2>
<li><a href="">FoxClocks</a>
is an extension for <a href="">Google
Chrome</a> and for <a
Toolkit</a> applications like <a
href="">Firefox</a> and <a
It displays multiple clocks in the application window, and has a mapping
interface to <a href="">Google Earth</a>.
It is freely available under the <abbr>GPL</abbr>.</li>
<li><a href="">Go programming language</a>
implementations contain a copy of a 32-bit subset of a recent
<code><abbr>tz</abbr></code> database in a
Go-specific format.</li>
clock (intclock)</a> is a clock that displays multiple time zones on
<abbr>GNU</abbr>/Linux and similar systems. It is freely available
under the <abbr>GPL</abbr>.</li>
<li>Microsoft Windows 8.1
and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
data (mentioned below) used by
<a href="">Windows Runtime</a>
classes such as <a
Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
the older, proprietary method of Microsoft Windows 2000 and later,
which stores time zone data in the
<a href="">Windows Registry</a>. The
href="">Zone &rarr;
Tzid table</a> or <a
file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
to <code><abbr>tz</abbr></code> names.
Java</a> contains a copy of a subset of a recent
<code><abbr>tz</abbr></code> database in a
Java-specific format.</li>
<li><a href="">Time Zone</a> is
a <a href="">WordPress</a> plugin. It is freely
available under a <abbr>BSD</abbr>-style license.</li>
<li><a href="">Time Zone
Master</a> is a Microsoft Windows clock program that can automatically
download, compile and use the <code>tzdata<var>D</var>.tar.gz</code>
files as they are released. The Basic version is free.</li>
href="">VelaTerra</a> is
an <abbr>OS X</abbr> program. Its developers
<a href="">offer free
licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
<h2>Other time zone databases</h2>
<li><a href="">Time-zone Atlas</a>
is Astrodienst's Web version of Shanks and Pottenger's
time zone history atlases published in both <a
and book form (<a
href="">one volume
for the <abbr>US</abbr></a>, and <a
href="">one for
other locations</a>) by <a
href="">Astro Computing Services</a>.
These atlases are extensive but unreliable, as Shanks appears to have
guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
sources and do not indicate which entries are guesswork.</li>
<li><a href="">HP-UX</a> has a database in
its own <code>tztab</code>(4) format.</li>
<li>Microsoft Windows has proprietary data mentioned above.</li>
<li><a href="">World Time Server</a>
is another time zone database.</li>
<li><a href="">World Time Zones</a>
contains data from the Time Service Department of the
<abbr>US</abbr> Naval Observatory.</li>
<li>The <a href="">Standard
Schedules Information Manual</a> of the
International Air Transport Association
gives current time zone rules for airports served by commercial aviation.</li>
<li>The <a href="">United States Central
Intelligence Agency (<abbr
title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
zone map</a>; the
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="">Current time around the world
and standard time zones map of the world</a>
has several fancy time zone maps; it covers Russia particularly well.
The maps' pictorial quality is not quite as good as the
but the maps are more up to date.</li>
much is time wrong around the world?</a> maps the difference between
mean solar and standard time, highlighting areas such as western China
where the two differ greatly. It's a bit out of date, unfortunately.</li>
<h2>Time zone boundaries</h2>
<li><a href=""><abbr>TZ</abbr> timezones
maps</a> contains <a
href="">shapefiles</a> of
sets of <code><abbr>tz</abbr></code> regions.</li>
<li>The <a href="">latlong package</a>
maps geographical coordinates to a <code><abbr>tz</abbr></code> region.
It is written in Go and is freely available under the Apache License.</li>
<li><a href="">What Time
is It Here?</a> applies <a href="">MongoDB</a>
geospatial query operators to shapefiles' data.</li>
<li><a href="">Administrative
Divisions of Countries ("Statoids")</a> contains lists of
political subdivision data related to time zones.</li>
<li><a href="">Time
zone boundaries for multizone countries</a> summarizes legal
boundaries between time zones within countries.</li>
<li>'s <a
href="">Free Stuff for
Manifold System Users</a> includes a Manifold-format map of
world time zone boundaries distributed under the
<li>The GeoCommunity lists several commercial sources for <a
Time Zones and Time Zone Data</a>.</li>
<li>A ship within the <a
waters</a> of any nation uses that nation's time. In international
waters, time zone boundaries are meridians 15&deg; apart, except that
<abbr>UTC</abbr>&minus;12 and <abbr>UTC</abbr>+12 are each 7.5&deg;
wide and are separated by
the 180&deg; meridian (not by the International Date Line, which is
for land and territorial waters only). A captain can change ship's
clocks any time after entering a new time zone; midnight changes are
<h2>Civil time concepts and history</h2>
<li><a href="">A
Walk through Time</a>
surveys the evolution of timekeeping.</li>
<li><a href="">About Daylight
Saving Time &ndash; History, rationale, laws &amp; dates</a>
is an overall history of <abbr>DST</abbr>.</li>
<li><a href="">Working with Time Zones</a>
contains guidelines and best practices for software applications that
deal with civil time.</li>
<li><a href="">Saving Time,
Saving Energy</a> discusses a primary justification for <abbr>DST</abbr>.</li>
<li><a href="">A Brief
History of Daylight Saving Time</a> summarizes some of the contentious
history of <abbr>DST</abbr>.</li>
<li><a href="">A History of
the International Date Line</a> tells the story of the most important
time zone boundary.</li>
<li><a href="">Basic Time
Zone Concepts</a> discusses terminological issues behind time zones.</li>
<h2>National histories of legal time</h2>
<dd>The Parliamentary Library has commissioned a <a
paper on daylight saving time in Australia</a>.
The Bureau of Meteorology publishes a list of <a
Dates of Daylight Savings Time within Australia</a>.</dd>
<dd>The Royal Observatory of Belgium maintains a table of <a
hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
<dd>The Time Service Department of the National Observatory
records <a href=""
hreflang="pt-BR">Brazil's daylight saving time decrees (in
<dd>National Research Council Canada publishes current
and some older information about <a
zones &amp; daylight saving time</a>.</dd>
<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
<a href="" hreflang="es">history of
Chile's official time (in Spanish)</a>.</dd>
<dd>The National Institute for Science and Technology maintains the <a
of Legal Time in Germany</a>.</dd>
<dd>The Interior Ministry periodically issues <a
hreflang="he">announcements (in Hebrew)</a>.</dd>
<dd>The Investigation and Analysis Service of the Mexican Library of
Congress has published a <a
hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
<dd>See Singapore below.</dd>
<dd><a href=""
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 <a
href="">History of
Daylight Saving</a>. The privately-maintained <a
href="">History of New Zealand
time</a> has more details.</dd>
is Singapore in the "Wrong" Time Zone?</a> details the
history of legal time in Singapore and Malaysia.</dd>
<dt>United Kingdom</dt>
href="">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
of Summer time dates</a>.</dd>
<dt>United States</dt>
<dd>The Department of Transportation's <a
Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
<h2>Precision timekeeping</h2>
Science of Timekeeping</a> is a thorough introduction
to the theory and practice of precision timekeeping.</li>
<li><a href=""><abbr
title="Network Time Protocol">NTP</abbr>: The Network
Time Protocol</a>
discusses how to synchronize clocks of
Internet hosts.</li>
<li>The <a href="">Precision
Time Protocol</a> (<abbr
title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
can achieve submicrosecond clock accuracy on a local area network.</li>
Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
(Internet <abbr>RFC</abbr> 4833)
specifies a <a
option for a server to configure
a client's time zone and daylight saving settings automatically.</li>
<li><a href="">A Few Facts
Concerning <abbr>GMT</abbr>, <abbr>UT</abbr>, and
the <abbr title="Royal Greenwich Observatory">RGO</abbr></a>
answers questions like "What is the
difference between <abbr>GMT</abbr> and <abbr>UTC</abbr>?"</li>
Times</a> explains more abstruse astronomical time scales like
<abbr title="Terrestrial Dynamic Time">TDT</abbr>,
<abbr title="Geocentric Coordinate Time">TCG</abbr>, and
<abbr title="Barycentric Dynamic Time">TDB</abbr>.
<a href="">Time
Scales</a> goes into more detail, particularly for historical variants.</li>
<li>The <a href=""><abbr
title="International Astronomical Union">IAU</abbr></a>'s <a
title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
collection contains C and <a
code for converting among time scales like
<abbr title="International Atomic Time">TAI</abbr>,
<abbr>TDB</abbr>, <abbr>TDT</abbr> and
<li><a href="">Basics of
Space Flight &ndash; Reference Systems &ndash; Time Conventions</a>
briefly explains interplanetary space flight timekeeping.</li>
Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a> briefly
describes Mars Coordinated Time (<abbr
title="Mars Coordinated Time">MTC</abbr>) and the
diverse local time
scales used by each landed mission on Mars.</li>
<li><a href=""></a> is
dedicated not only to leap seconds but to precise time and frequency
in general. It covers the state of the art in amateur timekeeping, and
how the art has progressed over the past few decades.</li>
title="International Earth Rotation and Reference Systems Service">IERS</abbr>
Bulletins</a> contains official publications of the International
Earth Rotation and Reference Systems Service, which decides
when leap seconds occur.</li>
<li>The <a
Second Discussion List</a> covers <a
and Klepczynski's proposal to discontinue leap seconds</a>,
discussed further in
<a href="">The
leap second: its history and possible future</a>.
<a href=""><abbr>UTC</abbr>
might be redefined
without Leap Seconds</a> gives pointers on this
contentious issue.</li>
<h2>Time notation</h2>
<li>The <a href="">Unicode Common Locale Data
Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
zone names, abbreviations, identifiers, and formats. For example, it
contains French translations for "Eastern European Summer Time",
"<abbr title="Eastern European Summer Time">EEST</abbr>", and
"Bucharest". Its
<a href="">by-type
charts</a> show these values for many locales. Data values are available in
both <abbr title="Locale Data Markup Language">LDML</abbr>
(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
<a href="">A summary of
the international standard date and time notation</a> is a good
summary of
title="International Organization for Standardization">ISO</abbr>
8601:2004 &ndash; Data elements and interchange formats &ndash; Information
interchange &ndash; Representation of dates and times</a>.</li>
<a href=""><abbr>XML</abbr>
Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
<a href="">Internet
Message Format</a> (Internet <abbr>RFC</abbr> 5322) &sect;3.3
specifies the time notation used in email and <a
<a href="">Date and Time
on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
specifies an <abbr>ISO</abbr> 8601
profile for use in new Internet
<a href="">Date &amp; Time
Formats on the Web</a> surveys web- and Internet-oriented date and time
<a href="">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 <abbr>UTC</abbr> offsets as they are ambiguous in
practice. For example, in English-speaking North America
"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UTC</abbr>,
but in China it denotes 8 hours ahead of <abbr>UTC</abbr>,
and French-speaking North Americans prefer
"<abbr title="Heure Normale du Centre">HNC</abbr>" to
"<abbr>CST</abbr>". For <abbr>POSIX</abbr> the <code><abbr>tz</abbr></code>
database contains English abbreviations for all time stamps but in
many cases these are merely inventions of the database
<li>Numeric time zone abbreviations typically count hours east of
<abbr>UTC</abbr>, e.g., +09 for Japan and
&minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
<code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
For example, one might use <code><abbr>TZ</abbr>="<abbr
title="Japan Standard Time">JST</abbr>-9"</code> and
<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
for Japan and Hawaii, respectively. If the
<code><abbr>tz</abbr></code> database is available, it is usually better to use
settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
confusion, handle old time stamps better, and insulate you better from
any future changes to the rules. One should never set
<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
<code>"GMT-9"</code>, though, since this would falsely claim that
local time is nine hours ahead of <abbr>UTC</abbr> and the time zone
is called "<abbr>GMT</abbr>".</li>
<h2>See also</h2>
<li><a href="tz-art.htm">Time and the Arts</a></li>
This web page is in the public domain, so clarified as of
2009-05-17 by Arthur David Olson.
Please send corrections to this web page to the
<a href="">time zone mailing list</a>.
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