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.
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 2001 September 11. 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 should be avoided, and instead the ISO 8601 format should be used.
Dates are of the form YYYY-MM-DD.
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.
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.
Most of the world has adopted the standard, a list of countries and equivalent standards numbers can be found.
A discussion group for ISO 8601 issues has been operating since 1999.