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ISO 8601

ISO 8601 is an international standard for calendar date and time representations. Its full English title is Data elements and interchange formats -- Information interchange -- Representation of dates and times. The current version is the second edition, ISO 8601:2000. This replaces the ISO 8601:1988 First Edition.

Date and time format

The standard describes many formats, but the format favoured by most proponents of the standard uses the YYYY-MM-DD form for dates. For example, 2001-09-11 is simply September 11 in the year 2001. Note that the ISO 8601 form is unambiguous; traditionally this date might be written 09/11/01 in the US, but 11.09.01 in Europe. On a global and international medium such as the Internet, local conventions such as these can cause confusion, and unambiguous forms such as the ISO 8601 format can be seen to be preferable.

Dates prior to 999 are written as, for example, 0487-09-11 for September 11 in the year 487. BC dates are offset by one in order to obtain zero-based dates, so -0487-09-11 is September 11 in the year 488 BC rather than 487 BC, as might be supposed. 0000-09-11 is September 11 in the year 1 BC.

The Long Now Foundation suggests that years should be written with five digits (ie 02003 for the year 2003) in order to avoid the Year 10,000 problem.

Times are of the form HH:MM:SS.

Week - day format

As an alternative to the year, calendar date format described above, the ISO 8601 standard specifies a year, week, day format in the form YYYY-WW-D, where YYYY is the Gregorian year, WW is a two digit week number and D is the day of the week.

ISO 8601 standard weeks run from Monday (day 1) through Sunday (day 7). ISO 8601 assumes that Monday is the first day of the week, a practice that is commonly followed in Europe. The USA commonly assumes that Sunday is the first day of the week (following a tradition derived from ancient Jews and ancient Egyptians)

For the week numbering it is relevant to note that in this system Thursday is the day in the middle of the week, even though the Wednesday is in the middle of a working week from Monday through Friday.

Weeks are numbered within a year from 1 to 52 or 53. A complete week that spans two year numbers will adopt the year number and week number from the portion of the week that has the greater number of days within either of the two years. Equivalently, the Thursdays of a year determine the week numbering: week 1 is defined as the week that contains the first Thursday of the year, etc.

In some European business contexts dates are not specified by month and day of the month but by this week number and the day of the week.


On the Internet, ISO 8601 is used by the W3C, defining a profile of the standard that restricts the supported formats to reduce the scope for error and the complexity of software to UTC. See also WRC link in External links, below.


The standard also defines Ordinal Day of Year to mean that the days are numbered from 001 for January 01 through to 365 for December 31 (numbered as 366 in a Leap year).

There are also extensions for sub-second timestamps, and for indicating time zones.

Various abbreviations are allowed, but all maintain an unambiguous interpretation.

External links