A Summary of the International Standard Date and Time Notation
by Markus Kuhn
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.
The international standard date notation is
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
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:
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.
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
For situations where information about the century is really not required, a 2-digit year representation is available:
If only the month or even only the year is of interest:
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
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
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
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:
[Unfortunately, the current version of the C programming language
standard provides in the
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
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
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
which represents the time one second before midnight.
As with the date notation, the separating colons can also be omitted as in
and the precision can be reduced by omitting the seconds or both the seconds and minutes as in
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
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:
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.:
A remark for readers from German speaking countries:
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
[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.]
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
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:
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 email@example.com. 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 unfortunately not available online and interested people will have to order a paper copy from
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.
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.
You might also be interested in the International Standard Paper Sizes Web page.