Time zone and daylight saving time data

Time zone and daylight saving time data
Time zone and daylight saving time data
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
area.
The
tz
database
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 time zone boundaries and daylight saving rules.
This database (known as
tz
tzdb
, or
zoneinfo
is used by several implementations,
including
the
GNU
C Library
(used in
GNU
/Linux
),
Android
Free
BSD
Net
BSD
Open
BSD
ChromiumOS
Cygwin
MariaDB
musl libc
MySQL
webOS
AIX
iOS
macOS
Microsoft Windows
Open
VMS
Oracle Database
Oracle Solaris
and
QNX
Each main entry in the database represents a
timezone
for a set of civil-time clocks that have all agreed since 1970.
Timezones are typically identified by continent or ocean and then by the
name of the largest city within the region containing the clocks.
For example,
America/New_York
represents most of the
US
eastern time zone;
America/Phoenix
represents most of Arizona, which
uses mountain time without daylight saving time (
DST
);
America/Detroit
represents most of Michigan, which uses
eastern time but with different
DST
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 a
POSIX
.1-2024
implementation set the
TZ
environment variable to the location’s full name,
e.g.,
TZ
="America/New_York"
Associated with each timezone is a history of offsets from
Universal
Time
UT
), which is
Greenwich Mean
Time
GMT
) with days beginning at midnight;
for timestamps after 1960 this is more precisely
Coordinated
Universal Time
UTC
).
The database also records when daylight saving time was in use,
along with some time zone abbreviations such as
EST
for Eastern Standard Time in the
US
Downloading the
tz
database
The following
shell
commands download
the latest release’s two
tarballs
to a
GNU
/Linux or similar host.
mkdir tzdb
cd tzdb
wget
wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
gzip
-dc tzcode-latest.tar.gz |
tar
-xf -
gzip -dc tzdata-latest.tar.gz | tar -xf -
Alternatively, the following shell commands download the same
release in a single-tarball format containing extra data
useful for regression testing:
wget
lzip
-dc tzdb-latest.tar.lz | tar -xf -
These commands use convenience links to the latest release
of the
tz
database hosted by the
Time Zone Database website
of the
Internet Assigned Numbers
Authority (IANA)
Older releases are in files named
tzcode
.tar.gz
tzdata
.tar.gz
, and
tzdb-
.tar.lz
where
is the version.
Since 1996, each version has been a four-digit year followed by
lower-case letter (
through
then
za
through
zz
, then
zza
through
zzz
, and so on).
Since version 2022a, each release has been distributed in
POSIX
ustar interchange format
, compressed as described above;
older releases use a nearly compatible format.
Since version 2016h, each release has contained a text file named
version
” whose first (and currently only) line is the version.
Older archived releases are
available
via
HTTPS
rsync
, and
FTP
Alternatively, a development repository of code and data can be
retrieved from
GitHub
via the shell
command:
git
clone
Since version 2012e, each release has been tagged in development repositories.
Untagged commits are less well tested and probably contain
more errors.
After obtaining the code and data files, see the
README
file for what to do next.
The code lets you compile the
tz
source files into
machine-readable binary files, one for each location. The binary files
are in a special format specified by
The
Time Zone Information Format (
TZif
(Internet
RFC
9636).
The code also lets
you read a
TZif
file and interpret timestamps for that
location.
Changes to the
tz
database
The
tz
code and data
are by no means authoritative. If you find errors, please
email changes to
[email protected]
the time zone mailing list.
The mailing list and its archives are public,
so please do not send confidential information.
See
the mailing
list’s main page
to subscribe or to browse its archive of old messages.
Metadata for mailing list
discussions
and corresponding data changes can be
generated
automatically
Changes to the
tz
code and data are often
propagated to clients via operating system updates, so
client
tz
data can often be corrected by
applying these updates. With GNU/Linux and similar systems, if your
maintenance provider has not yet adopted the
latest
tz
data, you can often short-circuit
the process by tailoring the generic instructions in
the
tz
README
file and installing the latest
data yourself. System-specific instructions for installing the
latest
tz
data have also been published
for
AIX
Android
ICU
IBM
JDK
Joda-Time
MySQL
Noda Time
, and
OpenJDK/Oracle JDK
Since version 2013a,
sources for the
tz
database have been
UTF-8
text files
with lines terminated by
LF
which can be modified by common text editors such
as
GNU Emacs
gedit
, and
vim
Specialized source-file editing can be done via the
Sublime
zoneinfo
package for
Sublime Text
and the
VSCode
zoneinfo
extension for
Visual
Studio Code
For further information about updates, please see
Procedures for
Maintaining the Time Zone Database
(Internet
RFC
6557).
More detail can be
found in
Theory and pragmatics of the
tz
code and data
A0 TimeZone Migration
displays changes between recent
tzdb
versions.
Coordinating with governments and distributors
As discussed in
How
Time Zones Are Coordinated
”, the time zone database relies on
collaboration among governments, the time zone database volunteer
community, and data distributors downstream.
If your government plans to change its time zone boundaries or
daylight saving rules, please send email as described in
Changes to the
tz
database
”.
Do this well in advance,
as this will lessen confusion and will coordinate updates to many cell phones,
computers, and other devices around the world.
In your email, please cite the legislation or regulation that specifies
the change, so that it can be checked for details such as the exact times
when clock transitions occur.
It is OK if a rule change is planned to affect clocks
far into the future, as a long-planned change can easily be reverted
or otherwise altered with a year’s notice before the change would have
affected clocks.
There is no fixed schedule for
tzdb
releases.
However, typically a release occurs every few months.
Many downstream timezone data distributors wait for
tzdb
release before they produce an update
to time zone behavior in consumer devices and software products.
After a release, various parties must integrate, test,
and roll out an update before
end users
see changes.
These updates can be expensive, for both the
quality
assurance
process and the overall cost of shipping and installing
updates to each device’s copy of
tzdb
Updates may be batched with other updates and may take substantial
time to reach end users after a release.
Older devices may no longer be supported and thus may never be updated,
which means they will continue to use out-of-date rules.
For these reasons any rule change should be promulgated at least a
year before it affects how clocks operate; otherwise, there is a good
chance that many clocks will be wrong due to delays in propagating updates,
and that residents will be confused or even actively resist the change.
The shorter the notice, the more likely clock problems will arise; see “
On
the Timing of Time Zone Changes
” for examples.
Commentary on the
tz
database
The article
tz database
is
an encyclopedic summary.
How to Read the
tz Database Source Files
explains the
tz
database format.
literary appreciation of the Olson/Zoneinfo/tz database
comments on the
database’s style.
What time is it:
managing time in the internet
analyzes the database longitudinally.
Web sites using recent versions of the
tz
database
These are listed roughly in ascending order of complexity and fanciness.
Time.is
shows locations’
time and zones.
TimeJones.com
Time Zone Converter
and
The World Clock
are time zone converters.
TimeZoneDB Database
publishes
tzdb
-derived data in
CSV
and
in
SQL
form.
Date and Time Gateway
lets you see the
TZ
values directly.
Current
Time in 1000 Places
uses descriptions of the values.
The World Clock –
Worldwide
lets you sort zone names and convert times.
24TimeZones
has a world
time map and a time converter.
Time Difference
calculates the current time difference between locations.
Weather Now
and
The Time Now
list the weather too.
Network protocols for
tz
data
Time Zone
Data Distribution Service
(TZDIST, Internet
RFC
7808)
is associated with
CalDAV
(Internet
RFC
7809), a calendar access protocol for
transferring time zone data by reference.
TZDIST
implementations
are available.
The
iCalendar format
(Internet
RFC
5445)
covers time zone
data; see its VTIMEZONE calendar component.
The iCalendar format requires specialized parsers and generators; a
variant
xCal
(Internet
RFC
6321) uses
XML
format, and a variant
jCal
(Internet
RFC
7265)
uses
JSON
format.
Other
tz
compilers
Although some of these do not fully support
tz
data, in recent
tzdb
distributions you can generally work around compatibility problems by
running the command
make rearguard_tarballs
and compiling
from the resulting tarballs instead.
Vzic
is a
program that compiles
tz
source into iCalendar-compatible VTIMEZONE files.
Vzic is freely
available under the
GNU
General Public License (
GPL
DateTime::TimeZone
contains a script
parse_olson
that compiles
tz
source into
Perl
modules. It is part of the Perl
DateTime Project
which is freely
available under both the
GPL
and the Perl Artistic
License. DateTime::TimeZone also contains a script
tests_from_zdump
that generates test cases for each clock
transition in the
tz
database.
The
Time Zone
Database Parser
is a
C++
parser and
runtime library with a
std::chrono
API
that is a standard part of C++.
It is freely available under the
MIT
license.
International Components for
Unicode (
ICU
contains C/C++ and
Java
libraries for internationalization that
has a compiler from
tz
source
and from
CLDR
data
(mentioned
below
into an
ICU
-specific format.
ICU
is freely available under a
BSD
-style license.
The
Tzdata
package for
the
Elixir
language downloads
and compiles
tz
source and exposes
API
s for use. It is
freely available under the
MIT
license.
Java-based compilers and libraries include:
The
TZUpdater
tool
compiles
tz
source into the format used by
OpenJDK
and
Oracle JDK
Although its source code is proprietary, its executable is available under the
Java SE
Timezone Updater License Agreement
The
Java
SE 8 Date and Time
API
can be supplemented by
ThreeTen-Extra
which is freely available under a
BSD
-style license.
Joda-Time – Java date
and time
API
contains a class
org.joda.time.tz.ZoneInfoCompiler
that compiles
tz
source into a binary format. It inspired
Java 8
java.time
, which its users should migrate to once
they can assume Java 8 or later. It is available under the
Apache License
IANA Updater
and
ZIUpdater
are alternatives to TZUpdater. IANA Updater’s license is unclear;
ZIUpdater is licensed under the
GPL
Time4A: Advanced date and
time library for Android
and
Time4J: Advanced date,
time and interval library for Java
compile
tz
source into a binary format.
Time4A is available under the Apache License and Time4J is
available under the
GNU
Lesser
General Public License (
LGPL
ICU
(mentioned
above
) contains compilers and
Java-based libraries.
Noda Time – Date and
time
API
for .NET
is like Joda-Time and Time4J, but for the .NET framework instead of Java.
It is freely available under the Apache License.
Many modern
JavaScript
runtimes support
tz
natively via the
timeZone
option of
Intl.DateTimeFormat
This can be used as-is or with most of the following libraries,
many of which also support runtimes lacking the
timeZone
option.
The
Intl.DateTimeFormat
timezone polyfill
is freely available under a
BSD
-style license.
The
date-fns
library manipulates timezone-aware timestamps in browsers and
in
Node.js
It is freely available under the
MIT
license.
Day.js
is a
minimalist replacement for the date and time API of
the
now-legacy Moment.js
date
manipulation library.
It is freely available under the
MIT
license.
Luxon
improves
timezone support for the
Intl
API.
It is freely available under the
MIT
license.
Moment Timezone
is a
Moment.js plugin.
It is freely available under the
MIT
license.
Timezone
is a
JavaScript library that supports date arithmetic that is time zone
aware. It is freely available under the
MIT
license.
@tubular/time
supports live
tzdb
updates,
astronomical and atomic time, a command-line interface,
and full
TypeScript
Its companion
@tubular/time-tzdb
can generate
TZif
and other files, and a companion website
Timezone Database Explorer
lets you
convert timestamps, view transition histories, and download code and data.
It is freely available under the
MIT
license.
The proposed
Temporal
objects
let programs access an abstract view of
tzdb
data, and are designed to replace
JavaScript’s
problematic
Date
objects
when working with dates and times.
JuliaTime
contains a
compiler from
tz
source into
Julia
. It is freely available
under the
MIT
license.
TZDB
IANA
Time Zone Database for Delphi/
FPC
compiles from
tz
source into
Object Pascal
as compiled by
Delphi
and
FPC
It is freely available under a
BSD
-style license.
pytz – World Timezone
Definitions for Python
compiles
tz
source into
Python
It is freely available under a
BSD
-style license.
In code that can assume Python 3.6 or later it is largely superseded; see
pytz:
The Fastest Footgun in the West
TZInfo –
Ruby Timezone Library
compiles
tz
source into
Ruby
It is freely available under the
MIT
license.
The
Chronos Date/Time
Library
is
Smalltalk
class
library that compiles
tz
source into a time
zone repository whose format
is either proprietary or an
XML
-encoded
representation.
Tcl
contains a developer-oriented parser that compiles
tz
source into text files, along with a runtime that can read those
files. Tcl is freely available under a
BSD
-style
license.
Other
TZif
readers
The
GNU
Library
has an independent, thread-safe implementation of
TZif
file reader.
This library is freely available under the LGPL
and is widely used in
GNU
/Linux systems.
GNOME
’s
GLib
has
TZif
file reader written in C that
creates a
GTimeZone
object representing sets
of
UT
offsets.
It is freely available under the
LGPL
The
BDE Standard Library
’s
baltzo::TimeZoneUtil
component contains a C++
implementation of a
TZif
file reader. It is freely available under
the Apache License.
CCTZ
is a simple C++
library that translates between
UT
and civil time and
can read
TZif
files. It is freely available under the Apache
License.
The
Go programming language
has a
TZif
file reader
LoadLocationFromTZData
The
posix_tz_db
package
contains Python code
to generate
CSV
and
JSON
tables that map
tz
settings to proleptic TZ approximations.
For example, it maps
"Africa/Cairo"
to
"EET-2EEST,M4.5.5/0,M10.5.4/24"
an approximation valid for Cairo timestamps from 2023 on.
This can help porting to platforms that support only proleptic TZ.
The package is freely available under the MIT license.
Timelib
is a C
library that reads
TZif
files and converts
timestamps from one time zone or format to another.
It is used by
PHP
HHVM
and
MongoDB
It is freely available under the
MIT
license.
Tcl, mentioned
above
, also contains a
TZif
file reader.
DateTime::TimeZone::Tzfile
is a
TZif
file reader written in Perl.
It is freely available under the same terms as Perl
(dual
GPL
and Artistic license).
Python has a
zoneinfo.ZoneInfo
class
that reads
TZif
data and creates objects
that represent
tzdb
timezones.
Python is freely available under the
Python Software Foundation
License
A companion
PyPI
module
tzdata
supplies TZif data if the underlying system data cannot be found;
it is freely available under the Apache License.
The
public-domain
tz.js
library contains a Python tool that
converts
TZif
data into
JSON
-format data suitable for use
in its JavaScript library for time zone conversion. Dates before 1970
are not supported.
The
timezone-olson
package contains
Haskell
code that
parses and uses
TZif
data. It is freely
available under a
BSD
-style license.
Other
tz
-based time zone software
FoxClocks
is an extension for
Google
Chrome
Firefox
and
Thunderbird
It displays multiple clocks in the application window, and has a mapping
interface to
Google Earth
It is freely available under the
GPL
Microsoft Windows 8.1
and later has
tz
data and
CLDR
data (mentioned
below
) used by the
Windows Runtime
Universal Windows Platform
classes
DateTimeFormatter
and
Calendar
Exploring
Windows Time Zones with
System.TimeZoneInfo
describes
the older, proprietary method of Microsoft Windows 2000 and later,
which stores time zone data in the
Windows Registry
. The
Zone
→ Tzid table
or
XML
file
of the
CLDR
data maps proprietary zone IDs
to
tz
names.
These mappings can be performed programmatically via the
TimeZoneConverter
.NET library,
or the ICU Java and C++ libraries mentioned
above
Oracle
Java
contains a copy of a subset of a recent
tz
database in a
Java-specific format.
Other time zone databases
Time-zone Atlas
is Astrodienst’s Web version of Shanks and Pottenger’s out-of-print
time zone history atlases
for the US
and
for the world
Although these extensive atlases
were
sources for much of the older
tz
data
they are unreliable as Shanks appears to have
guessed many
UT
offsets and transitions. The atlases cite no
sources and do not indicate which entries are guesswork.
HP-UX
has a database in
its own
tztab
(4) format.
Microsoft Windows has proprietary data mentioned
above
World Time Server
is another time zone database.
The
Standard
Schedules Information Manual
of the
International Air Transport Association
gives current time zone rules for airports served by commercial aviation.
Maps
World Time Zone Map
with current time
has several fancy time zone maps; it covers Russia particularly well.
How
much is time wrong around the world?
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.
The
Perry–Castañeda
Library Map Collection
of the University of Texas at Austin has
copies of old maps taken from
The
World Factbook
, formerly published by the
US Central Intelligence Agency
Although the maps’ pictorial quality is good,
the maps do not indicate daylight saving time.
Time zone boundaries
Geographical boundaries between timezones are available
from several
Internet
geolocation
services and other sources.
Timezone
Boundary Builder
extracts
Open Street Map
data to build
boundaries of
tzdb
timezones.
Its code is freely available under the
MIT
license, and
its data entries are freely available under the
Open Data Commons
Open Database License
. The borders appear to be quite accurate.
Its main web page lists more than twenty libraries
for looking up a timezone name from a GPS coordinate.
Free access via a network API, if you register a key, is provided by
the
GeoNames
Timezone web service
, the
Google
Maps Time Zone API
, and
the
TimeZoneDB API
Commercial network API access is provided
by
AskGeo
and
GeoGarage
How
to get a time zone from a location using latitude and longitude
coordinates?
” discusses other geolocation possibilities.
Administrative
Divisions of Countries (“Statoids”)
lists
political subdivision data related to time zones.
Manifold Software
– GIS and Database Tools
includes a Manifold-format map of
world time zone boundaries circa 2007, distributed under the
GPL
A ship within the
territorial
waters
of any nation uses that nation’s time. In international
waters, time zone boundaries are meridians 15° apart, except that
UT
−12 and
UT
+12 are each 7.5°
wide and are separated by
the 180° 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
common.
Civil time concepts and history
Walk through Time
surveys the evolution of timekeeping.
The history of daylight saving time is surveyed in
About Daylight
Saving Time – History, rationale, laws & dates
and summarized in
A Brief
History of Daylight Saving Time
Time
Lords
discusses how authoritarians manipulate civil time.
Working with Time
and Time Zones
contains guidelines and best practices for software applications that
deal with civil time.
A History of
the International Date Line
tells the story of the most important
time zone boundary.
Basic Time
Zone Concepts
discusses terminological issues behind time zones.
National histories of legal time
Australia
The Bureau of Meteorology publishes a list of
Implementation
Dates of Daylight Savings Time within Australia
Belgium
The Royal Observatory of Belgium maintains a table of time in
Belgium (in
Dutch
and
French
).
Brazil
The Time Service Department of the National Observatory
records
Brazil’s daylight saving time decrees (in
Portuguese)
Canada
National Research Council Canada publishes current
and some older information about
time
zones and daylight saving time
Chile
The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
history of
Chile’s official time (in Spanish)
China
The Hong Kong Observatory maintains a
history of
summer time in Hong Kong
and Macau’s Meteorological and Geophysical Bureau maintains a
similar
history for Macau
Unfortunately the latter is incomplete and has errors.
Czech Republic
When daylight saving time starts and ends (in Czech)
summarizes and cites historical
DST
regulations.
Germany
The National Institute for Science and Technology maintains the
Realisation
of Legal Time in Germany
Israel
Israel Timezone Files
lists official time-change announcements and laws since 1940,
almost all in Hebrew.
Malaysia
See Singapore
below
Mexico
The Investigation and Analysis Service of the Mexican Library of
Congress has published a
history of Mexican local time (in Spanish)
Netherlands
Legal time in the Netherlands (in Dutch)
covers the history of local time in the Netherlands from ancient times.
New Zealand
The Department of Internal Affairs maintains a brief
History
of Daylight Saving in NZ
Palestine
The Ministry of Telecom and Digital Economy publishes a
history of clock changes (in Arabic)
Portugal
The Lisbon Astronomical Observatory publishes a
history of
legal time (in Portuguese)
Singapore
Why
is Singapore in the “Wrong” Time Zone?
details the
history of legal time in Singapore and Malaysia.
United Kingdom
History of
legal time in Britain
discusses in detail the country
with perhaps the best-documented history of clock adjustments.
United States
The Department of Transportation’s
Recent
Time Zone Proceedings
lists changes to
official written time zone boundaries, and its
Time
Zones dataset
maps current boundaries.
These boundaries are only for standard time, so the current map puts
all of Arizona in one time zone even though part of Arizona
observes
DST
and part does not.
Uruguay
The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
Navy (SOHMA) publishes an annual
almanac
(in Spanish)
Costs and benefits of time shifts
Various sources argue for and against daylight saving time and time
zone shifts, and many scientific studies have been conducted. This
section summarizes reviews and position statements based on
scientific literature in the area.
Carey RN, Sarma KM.
Impact of
daylight saving time on road traffic collision risk: a systematic
review
BMJ Open.
2017;7(6):e014319. doi:
10.1136/bmjopen-2016-014319
This reviews research literature and concludes that the evidence
neither supports nor refutes road safety benefits from
shifts in time zones.
Havranek T, Herman D, Irsova D.
Does daylight saving save electricity? A meta-analysis.
Energy J.
2018;39(2):35–61.
doi:
10.5547/01956574.39.2.thav
This analyzes research literature and concludes, “Electricity savings
are larger for countries farther away from the equator, while
subtropical regions consume more electricity because of
DST
.”
Neumann P, von Blanckenburg K.
What
time will it be? A comprehensive literature review on daylight saving time
Time Soc
. 2025;34(4):684–745.
doi:
10.1177/0961463X241310562
This reviews
DST
’s effects on electricity, health, crime, road
safety, and the economy, focusing on research since 2010, and concludes that
year-round standard time is preferable overall.
Romigi A, Franco V, Scoditti E
et al
The effects of daylight saving time and clock time transitions on sleep and
sleepiness: a systematic review.
Sleep Med Rev.
2025;84:102161. doi:
10.1016/j.smrv.2025.102161
This reviews
DST
and
DST
transitions,
and concludes that they both harm sleep, health and behavior.
The following medical societies have taken positions on the
advisability of clock shifts:
In 2022 the American Medical Association issued a
statement
supporting permanent standard time
on health grounds.
Crawford MR, Winnebeck EC, von Schantz M
et al
The
British Sleep Society position statement on Daylight Saving Time in the UK
J Sleep Res.
2025;34(3):e14352.
doi:
10.1111/jsr.14352
This recommends that the UK abolish
DST
for health reasons.
Malow BA.
It is time
to abolish the clock change and adopt permanent
standard time in the United States:
a Sleep Research Society position statement
Sleep.
2022;45(12):zsac236.
doi:
10.1093/sleep/zsac236
The Sleep Research Society
advocates permanent standard time due to its health benefits.
Rishi MA, Cheng JY, Strang AR
et al
Permanent standard time
is the optimal choice for health and safety:
an American Academy of Sleep Medicine position statement
J Clin Sleep Med.
2024;20(1):121–125.
doi:
10.5664/jcsm.10898
The AASM argues for permanent standard time due to health and safety risks
and economic costs of both
DST
transitions and
permanent
DST
Roenneberg T, Wirz-Justice A, Skene DJ
et al
Why
should we abolish Daylight Saving Time?
J Biol Rhythms.
2019;34(3):227–230.
doi:
10.1177/0748730419854197
The Society for Research on Biological Rhythms
opposes
DST
changes and permanent
DST
and advocates that governments adopt
“permanent Standard Time for the health and safety of their citizens”.
Precision timekeeping
The
Science of Timekeeping
is a thorough introduction
to the theory and practice of precision timekeeping.
The Science of
Time 2016
contains several freely readable papers.
NTP
: The Network
Time Protocol
(Internet
RFC
5905)
discusses how to synchronize clocks of
Internet hosts.
The
Huygens
family of software algorithms can achieve accuracy to a few tens of
nanoseconds in scalable server farms without special hardware.
The
Precision
Time Protocol
IEEE
1588)
can achieve submicrosecond clock accuracy on a local area network
with special-purpose hardware.
Timezone
Options for
DHCP
(Internet
RFC
4833)
specifies a
DHCP
option for a server to configure
a client’s time zone and daylight saving settings automatically.
Time
Scales
describes astronomical time scales like
TDT
TCG
, and
TDB
The
IAU
’s
SOFA
collection contains C and
Fortran
code for converting among time scales like
TAI
TDB
TDT
and
UTC
. It is freely available under the
SOFA license
Mars24 Sunclock
– Time on Mars
describes Airy Mean Time (
AMT
) and the
diverse local time
scales used by each landed mission on Mars.
LeapSecond.com
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.
The rules for leap seconds are specified in Annex 1 (Time scales) of
Standard-frequency
and time-signal emissions
, International Telecommunication Union –
Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).
IERS
Bulletins
contains official publications of the International
Earth Rotation and Reference Systems Service, which decides when leap
seconds occur.
The
tz
code and data support leap seconds
via an optional
"right"
configuration where a computer’s internal
time_t
integer clock counts every
TAI
second,
as opposed to the default
"posix"
configuration
where the internal clock ignores leap seconds.
The two configurations agree for timestamps starting with 1972-01-01 00:00:00
UTC
time_t
63 072 000) and diverge for
timestamps starting with
time_t
78 796 800,
which corresponds to the first leap second
1972-06-30 23:59:60
UTC
in the
"right"
configuration,
and to
1972-07-01 00:00:00
UTC
in the
"posix"
configuration.
In practice the two configurations also agree for timestamps before
1972 even though the historical situation is messy, partly because
neither
UTC
nor
TAI
is well-defined for sufficiently old timestamps.
The
NTP
Leap Second File
covers the text file
leap-seconds.list
, which lists the currently known leap seconds.
The
IERS
maintains this file, and a copy is distributed by
tzdb
for use by
NTP
implementations like
classic
ntpd
and
NTPsec
The
tz
database also distributes leap second
information in a differently-formatted
leapseconds
text file,
as well as in the
"right"
configuration in binary form; for
example,
right/UTC
can be used
by
chrony
another
NTP
implementation.
Leap Smear
discusses how to gradually adjust
POSIX
clocks near a
leap second so that they disagree with
UTC
by at most a
half second, even though every
POSIX
minute has exactly
sixty seconds. This approach works with the default
tz
"posix"
configuration, is
supported
by the abovementioned
NTP
implementations,
supports
conversion between
UTC
and smeared
POSIX
timestamps, and is used by major
cloud service providers. However, according to
§3.7.1 of
Network Time Protocol Best Current Practices
(Internet
RFC
8633), leap smearing is not suitable for
applications requiring accurate
UTC
or civil time,
and is intended for use only in single, well-controlled environments.
The
LEAPSECS List
covers
McCarthy
and Klepczynski’s 1999 proposal to discontinue leap seconds
discussed further in
The
leap second: its history and possible future
UTC
might be redefined
without Leap Seconds
gives pointers on this
contentious issue.
The General Conference on Weights and Measures (CGPM)
decided in 2022
to discontinue the use of leap seconds by 2035, and requested that no
discontinuous adjustments be made to UTC for at least a century.
The World Radiocommunication Conference
resolved
in 2023
to cooperate with this process. A draft
Resolution
C to make continuous UTC effective on 2027-05-20
and thereby discontinue leap seconds,
has been scheduled for the 28th CGPM starting 2026-10-13 in Paris.
One proposal to implement this
would replace leap seconds with seven 13-second leap smears occurring once per
decade until 2100, with leap smears after that gradually increasing in size.
See:
Levine J.
proposal to change the leap-second adjustments to
coordinated universal time
Metrologia.
2024;61(5):055002. doi:
10.1088/1681-7575/ad6266
with followups in doi:
10.1088/1681-7575/ade314
and doi:
10.1088/1681-7575/ade315
However, there is still no consensus on whether this is the best way
to replace leap seconds.
Time notation
The
Unicode Common Locale Data
Repository (
CLDR
) Project
has localizations for time
zone names, abbreviations, identifiers, and formats. For example, it
contains French translations for “Eastern European Summer Time”,
EEST
”, and “Bucharest”. Its
by-type
charts
show these values for many locales. Data values are available in
both
LDML
(an
XML
format) and
JSON
A summary of
the international standard date and time notation
covers
ISO
8601-1:2019 – Date and time – Representations for information
interchange – Part 1: Basic rules
XML
Schema: Datatypes – dateTime
specifies a format inspired by
ISO
8601 that is in common use in
XML
data.
§3.3 of
Internet Message Format
(Internet
RFC
5322)
specifies the time notation used in email and
HTTP
headers.
Date and Time
on the Internet: Timestamps
(Internet
RFC
3339)
specifies an
ISO
8601 profile for use in new Internet protocols.
An extension,
Date
and Time on the Internet: Timestamps with Additional Information
(Internet
RFC
9557) extends this profile
to let you specify the
tzdb
timezone of a timestamp
via suffixes like
[Asia/Tokyo]
Date & Time
Formats on the Web
surveys web- and Internet-oriented date and time
formats.
Alphabetic time zone abbreviations should not be used as unique
identifiers for
UT
offsets as they are ambiguous in
practice. For example, in English-speaking North America
CST
” denotes 6 hours behind
UT
but in China it denotes 8 hours ahead of
UT
and French-speaking North Americans prefer
HNC
” to
CST
”. The
tz
database contains English abbreviations for many timestamps;
unfortunately some of these abbreviations were merely the database maintainers’
inventions, and these have been removed when possible.
Numeric time zone abbreviations typically count hours east of
UT
, e.g., +09 for Japan and
−10 for Hawaii. However,
POSIX
proleptic
TZ
settings use the opposite convention.
For example, one might use
TZ
="
JST
-9"
and
TZ
="
HST
10"
for Japan and Hawaii, respectively. If the
tz
database is available, it is usually better to use
settings like
TZ
="Asia/Tokyo"
and
TZ
="Pacific/Honolulu"
instead, as this should avoid
confusion, handle old timestamps better, and insulate you better from
any future changes to the rules. One should never set
POSIX
TZ
to a value like
"GMT-9"
, though, since this would incorrectly imply that
local time is nine hours ahead of
UT
and the time zone
is called “
GMT
”.
See also
Theory and pragmatics of the
tz
code and data
Time and the Arts