International Standard
ISO 8601 specifies numeric representations of date and time. This
standard notation helps to avoid confusion in international
communication caused by the many different national notations and
increases the portability of computer user interfaces. In addition,
these formats have several important advantages for computer usage
compared to other traditional date and time notations. The time
notation described here is already the de-facto standard in almost all
countries and the date notation is becoming increasingly popular.
Especially authors of Web pages and software engineers who
design user interfaces, file formats, and communication protocols
should be familiar with ISO 8601.
Contents: Date, Time of Day,
Time Zone.
The international standard date notation is
YYYY-MM-DD
where YYYY is the year in the usual Gregorian calendar, MM is the
month of the year between 01 (January) and 12 (December), and DD is
the day of the month between 01 and 31.
For example, the fourth day of February in the year 1995 is written
in the standard notation as
1995-02-04
Other commonly used notations are e.g. 2/4/95, 4/2/95, 95/2/4,
4.2.1995, 04-FEB-1995, 4-February-1995, and many more. Especially the
first two examples are dangerous, because as both are used quite often
in the U.S. and in Great Britain and both can not be distinguished, it
is unclear whether 2/4/95 means 1995-04-02 or 1995-02-04. The date
notation 2/4/5 has at least six reasonable interpretations (assuming
that only the twentieth and twenty-first century are reasonable
candidates in our life time).
Advantages of the ISO 8601 standard date notation compared to other
commonly used variants:
- easily readable and writeable by software (no 'JAN', 'FEB', ...
table necessary)
- easily comparable and sortable with a trivial string comparison
- language independent
- can not be confused with other popular date notations
- consistency with the common 24h time notation system, where
the larger units (hours) are also written in front of the smaller
ones (minutes and seconds)
- strings containing a date followed by a time are also
easily comparable and sortable (e.g. write "1995-02-04 22:45:00")
- the notation is short and has constant length, which makes both
keyboard data entry and table layout easier
- identical to the Chinese date notation, so the largest cultural
group (>25%) on this planet is already familiar with it :-)
- date notations with the order "year, month, day" are in addition
already widely used e.g. in Japan, Korea, Hungary, Sweden, Finland,
Denmark, and a few other countries and people in the U.S. are already
used to at least the "month, day" order
- a 4-digit year representation avoids
overflow
problems after 1999-12-31
As dates will look a little bit strange anyway starting with
2000-01-01 (e.g. like 1/1/0), it has been suggested that the year 2000
is an excellent opportunity to change to the standard date notation.
ISO 8601 is only specifying numeric notations and does not cover
dates and times where words are used in the representation. It is not
intended as a replacement for language-dependent worded date notations
such as "24. Dezember 2001" (German) or "February 4, 1995" (US
English). ISO 8601 should however be used to replace notations such as
"2/4/95" and "9.30 p.m.".
Apart from the recommended primary standard notation
YYYY-MM-DD, ISO 8601 also specifies a number of
alternative formats for use in applications with special requirements.
All of these alternatives can easily and automatically be
distinguished from each other:
The hyphens can be omitted if compactness of the representation is
more important than human readability, for example as in
19950204
For situations where information about the century is really not
required, a 2-digit year representation is available:
95-02-04 or
950204
If only the month or even only the year is of interest:
1995-02 or
1995
In commercial and industrial applications (delivery times,
production plans, etc.), especially in Europe, it is often required to
refer to a week of a year. Week 01 of a year is per definition the
first week that has the Thursday in this year, which is equivalent to
the week that contains the fourth day of January. In other words, the
first week of a new year is the week that has the majority of its
days in the new year. Week 01 might also contain days from the
previous year and the week before week 01 of a year is the last week
(52 or 53) of the previous year even if it contains days from the new
year. A week starts with Monday (day 1) and ends with Sunday (day 7).
For example, the first week of the year 1997 lasts from 1996-12-30 to
1997-01-05 and can be written in standard notation as
1997-W01 or
1997W01
The week notation can also be extended by a number indicating the
day of the week. For example, the day 1996-12-31, which is the Tuesday
(day 2) of the first week of 1997, can also be written as
1997-W01-2 or
1997W012
for applications like industrial planning where many things like
shift rotations are organized per week and knowing the week number and
the day of the week is more handy than knowing the day of the month.
An abbreviated version of the year and week number like
95W05
is sometimes useful as a compact code printed on a product that
indicates when it has been manufactured.
The ISO standard avoids explicitly stating the possible range of
week numbers, but this can easily be deduced from the definition:
Theorem: Possible ISO week numbers are in the
range 01 to 53. A year always has a week 52. (There is one historic
exception: the year in which the Gregorian calendar was introduced had
less than 365 days and less than 52 weeks.)
Proof: Per definition, the first week of a year is
W01 and consequently days before week W01 belong to the previous year
and so there is no week with lower numbers. Considering the highest
possible week number, the worst case is a leap year like 1976 that
starts with a Thursday, because this keeps the highest possible number
of days of W01 in the previous year, i.e. 3 days. In this case, the
Sunday of W52 of the worst case year is day number 4+51*7=361 and
361-366=5 days of W53 belong still to this year, which guarantees that
in the worst case year day 4 (Thursday) of W53 is not yet in the next
year, so a week number 53 is possible. For example, the 53 weeks of
the worst case year 1976 started with 1975-12-29 = 1976-W01-1 and
ended with 1977-01-02 = 1976-W53-7. On the other hand, considering the
lowest number of the last week of a year, the worst case is a non-leap
year like 1999 that starts with a Friday, which ensures that the first
three days of the year belong to the last week of the previous year.
In this case, the Sunday of week 52 would be day number 3+52*7=367,
i.e. only the last 367-365=2 days of the W52 reach into the next year
and consequently, even a worst case year like 1999 has a week W52
including the days 1999-12-27 to 2000-01-02. q.e.d.
[Unfortunately, the current version of the C programming language
standard provides in the strftime() function no means to
generate the ISO 8601 week notation. A required extension would be
four new formatting codes: for the year of the week to which the
specified day belongs (both 2-digit and 4-digit), for the number of
the week between 01 and 53, and for the day of the week between 1
(Monday) and 7 (Sunday). Another trivial mistake in the description of
strftime() in the C standard is that the range of seconds
goes from 00 to 61, because at one time only one single leap second 60
can be inserted into UTC and consequently there will never be a leap
second 61. Contribution N764 to the
ISO C committee
suggests to fix some of this in the next revision of the ISO C
standard. The author of this text has also developed a proposal for a
modernised clock and calendar API for C, which
provides full proper treatment of leap seconds and timezones and fixes
numerous other problems in the current C timing library functions. It
also serves as an excellent model for those who want to design clock
library functions for other programming languages.]
Both day and year are useful units of structuring time, because the
position of the sun on the sky, which influences our lives, is
described by them. However the 12 months of a year are of some obscure
mystic origin and have no real purpose today except that people are
used to having them (they do not even describe the current position of
the moon). In some applications, a date notation is preferred that
uses only the year and the day of the year between 001 and 365 (366 in
leap years). The standard notation for this variant representing
the day 1995-02-04 (that is day 035 of the year 1995) is
1995-035 or
1995035
Leap years are years with an additional day YYYY-02-29, where the
year number is a multiple of four with the following exception: If a
year is a multiple of 100, then it is only a leap year if it is also a
multiple of 400. For example, 1900 was not a leap year, but 2000 is one.
The international standard notation for the time of day is
hh:mm:ss
where hh is the number of complete hours that have passed since
midnight (00-24), mm is the number of complete minutes that have
passed since the start of the hour (00-59), and ss is the number of
complete seconds since the start of the minute (00-59). If the hour
value is 24, then the minute and second values must be zero. [Although
ISO 8601 does not mention this, the value 60 for ss might sometimes be
needed during an inserted leap second in an
atomic time scale like Coordinated Universal Time (UTC). A single leap
second 23:59:60 is inserted into the UTC time scale every few years as
announced by the International Earth
Rotation Service in Paris to keep UTC from wandering away more
than 0.9 s from the less constant astronomical time scale UT1
that is defined by the actual rotation of the earth.]
An example time is
23:59:59
which represents the time one second before midnight.
As with the date notation, the separating colons can also be
omitted as in
235959
and the precision can be reduced by omitting the seconds or both
the seconds and minutes as in
23:59, 2359, or
23
It is also possible to add fractions of a second after a decimal
dot or comma, for instance the time 5.8 ms before midnight can be
written as
23:59:59.9942 or
235959.9942
As every day both starts and ends with midnight, the two notations
00:00 and 24:00 are available to
distinguish the two midnights that can be associated with one date.
This means that the following two notations refer to exactly the same
point in time:
1995-02-04 24:00 =
1995-02-05 00:00
In case an unambiguous representation of time is required, 00:00 is
usually the preferred notation for midnight and not 24:00. Digital
clocks display 00:00 and not 24:00.
ISO 8601 does not specify, whether its notations specify a point in
time or a time period. This means for example that ISO 8601 does not
define whether 09:00 refers to the exact end of the ninth hour of the
day or the period from 09:00 to 09:01 or anything else. The users of
the standard must somehow agree on the exact interpretation of the
time notation if this should be of any concern.
If a date and a time are displayed on the same line, then always
write the date in front of the time. If a date and a time value are
stored together in a single data field, then ISO 8601 suggests that
they should be separated by a latin capital letter T, as in
19951231T235959.
A remark for readers from the U.S.:
The 24h time notation specified here has already been
the de-facto standard all over the world in written language for
decades. The only exception are some English speaking countries, where
still notations with hours between 1 and 12 and additions like "a.m."
and "p.m." are in wide use. The common 24h international standard
notation starts to get widely used now even in England. Most other
languages don't even have abbreviations like "a.m." and "p.m." and the
12h notation is certainly hardly ever used on Continental Europe to
write or display a time. Even in the U.S., the military and computer
programmers have been using the 24h notation for a long time.
The old English 12h notation has many disadvantages like:
- It is longer than the normal 24h notation.
- It takes somewhat more time for humans to compare two times
in 12h notation.
- It is not clear, how 00:00, 12:00 and 24:00 are represented.
Even encyclopedias and style manuals contain contradicting
descriptions and a common quick fix seems to be to avoid
"12:00 a.m./p.m." altogether and write "noon", "midnight", or
"12:01 a.m./p.m." instead, although the word "midnight" still
does not distinguish between 00:00 and 24:00.
- It makes people often believe that the next day starts at the
overflow from "12:59 a.m." to "1:00 a.m.", which is a common
problem not only when people try to program the timer of VCRs
shortly after midnight.
- It is not easily comparable with a string compare operation.
- It is not immediately clear for the unaware, whether the
time between "12:00 a.m./p.m." and "1:00 a.m./p.m." starts
at 00:00 or at 12:00, i.e. the English 12h notation is more
difficult to understand.
Please consider the 12h time to be a relic from the dark ages when
Roman numerals were used, the number zero had not yet been invented
and analog clocks where the only known form of displaying a
time. Please avoid using it today, especially in technical
applications! Even in the U.S., the widely respected Chicago
Manual of Style now recommends using the international
standard time notation in publications.
A remark for readers from German speaking countries:
In May 1996, the German standard DIN 5008, which
specifies typographical rules for German texts written on typewriters,
has been updated. The old German numeric date notations DD.MM.YYYY and
DD.MM.YY have been replaced by the ISO date notations YYYY-MM-DD and
YY-MM-DD. Similarly, the old German time notations hh.mm and hh.mm.ss
have been replaced by the ISO notations hh:mm and hh:mm:ss. Those new
notations are now also mentioned in the latest edition of the
Duden. The German alphanumeric date notation continues to
be for example "3. August 1994" or "3. Aug. 1994". The corresponding
Austrian standard has already used the ISO 8601 date and time
notations before.
ISO 8601 has been adopted as European Standard EN 28601 and is
therefore now a valid standard in all EU countries and all conflicting
national standards have been changed accordingly.
Without any further additions, a date and time as written above is
assumed to be in some local time zone. In order to indicate that a
time is measured in Universal Time (UTC), you can append a capital
letter Z to a time as in
23:59:59Z or 2359Z
[The Z stands for the "zero meridian", which goes through Greenwich
in London, and it is also commonly used in radio communication where
it is pronounced "Zulu" (the word for Z in the international radio
alphabet). Universal
Time (sometimes also called "Zulu Time") was called Greenwich Mean
Time (GMT) before 1972, however this term should no longer be
used. Since the introduction of an international atomic time scale,
almost all existing civil time zones are now related to UTC, which is
slightly different from the old and now unused GMT.]
The strings
+hh:mm, +hhmm, or
+hh
can be added to the time to indicate that the used local time zone
is hh hours and mm minutes ahead of UTC. For time zones west of the
zero meridian, which are behind UTC, the notation
-hh:mm, -hhmm, or
-hh
is used instead. For example, Central European Time (CET) is +0100
and U.S./Canadian Eastern Standard Time (EST) is -0500. The following
strings all indicate the same point of time:
12:00Z = 13:00+01:00
= 0700-0500
There exists no international standard that specifies
abbreviations for civil time zones like CET, EST, etc. and sometimes
the same abbreviation is even used for two very different time zones.
In addition, politicians enjoy modifying the rules for civil time
zones, especially for daylight saving times, every few years, so the
only really reliable way of describing a local time zone is to specify
numerically the difference of local time to UTC. Better use directly
UTC as your only time zone where this is possible and then you do not
have to worry about time zones and daylight saving time changes at
all.
Arthur David Olson and
others maintain a database of all current and
many historic time zone changes and daylight saving time
algorithms. It is available via ftp from elsie.nci.nih.gov in the
tzcode* and tzdata* files. Most Unix time
zone handling implementations are based on this package. If you want
to join the tz mailing list, which is dedicated to
discussions about time zones and this software, please send a request
for subscription to tz-request@elsie.nci.nih.gov. You can read previous discussion
there in the tz
archive.
Some other interesting sources of information about date and time
on the Internet are for example the Glossary of
Frequency and Timing Terms and the FAQ
provided by NIST,
the Yahoo
Science:Measurements and Units:Time link collection, the U.S. Naval Observatory Server,
the International Earth Rotation
Service (IERS) (for time gurus only!), the University of Delaware NTP Time
Server, the time and calendar section of the USENET sci.astro
FAQ, and the Calendar FAQ.
This was a brief overview of the ISO 8601 standard, which covers
only the most useful notations and includes some additional related
information. The full standard defines in addition a number of more
exotic notations including some for periods of time. The ISO 8601:1988 document is
fortunately now also available online, or you
can order a paper copy from
International Organization
for Standardization
Case postale 56
1, rue de Varembé
CH-1211 Genève 20
Switzerland
phone: +41 22 749 01 11
fax: +41 22 733 34 30
email: sales@isocs.iso.ch
A more detailed online summary of ISO 8601 than this one is the
text ISO 8601:1988 Date/Time Representations available
from
ftp.informatik.uni-erlangen.de/pub/doc/ISO/ISO8601.ps.Z
(PostScript, 16 kb, 5 pages) written by Gary Houston, now also
available in HTML. Ian Galpin (G1SMD) proposes to use ISO 8601 as a Common Date-Time
Standard for Amateur Radio. Steve Adams has written another web
page about the ISO date format that is partially based on this
text.
ISO TC 154 decided in 1996 to revise ISO 8601. Louis Visser is coordinating
this project. If you want to contribute to this work, you should
contact your national ISO member
organization.
I wish to thank Edward
M. Reingold for developing the fine GNU Emacs calendar functions,
as well as Rich Wales,
Mark Brader, Paul Eggert, and others
in the comp.std.internat, comp.protocols.time.ntp, and
sci.astro USENET discussion groups for
valuable comments about this text. Further comments and hyperlinks to
this page are very welcome.
Some journalists recently got interested in the international date
and time format and reported about it. Examples include:
- An article by Jon G.
Auerbach in the 1999-06-01 issue of the Wall Street Journal, page
A1.
If you are a journalist and need information on this or related
topics, please feel free to contact me.
You might also be interested in the International
Standard Paper Sizes Web page.
Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>
created 1995 -- last modified 2000-01-24 --
http://www.cl.cam.ac.uk/~mgk25/iso-time.html
