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.

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 September 11, 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.

Times are of the form HH:MM:SS.

Numbered weeks of the year are defined. Weeks run from Monday (day 1) through to Sunday (day 7). Weeks are numbered within a year from 1 to 52 or 53, week 1 is the first week that has a Thursday in the year (note that this week has the majority of its days in the year with which it is associated. This week scheme is used in some European business contexts.

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

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

Markus Kuhn has an an excellent summary.

The full standard can be ordered from the International Organization for Standardization.