The International Standards Organisation (ISO) issued Standard ISO 8601, "Representation of Dates" in 1988, superseding the earlier ISO 2015. The bulk of the standard consists of standards for representing dates in the Gregorian calendar including the highly recommended "YYYY-MM-DD" form which is unambiguous, free of cultural bias, can be sorted into order without rearrangement, and is Y9K compliant. In addition, ISO 8601 formally defines the "calendar week" often encountered in commercial transactions in Europe. The first calendar week of a year: week 1, is that week which contains the first Thursday of the year (or, equivalently, the week which includes January 4th of the year; the first day of that week is the previous Monday). The last week: week 52 or 53 depending on the date of Monday in the first week, is that which contains December 31 of the year. The first ISO calendar week of a given year starts with a Monday which can be as early as December 29th of the previous year or as late as January 4th of the present; the last calendar week can end as late as Sunday, January 3rd of the subsequent year. ISO 8601 dates in year, week, and day form are written with a "W" preceding the week number, which bears a leading zero if less than 10, for example February 29th, 2000 is written as 2000-02-29 in year, month, day format and 2000-W09-2 in year, week, day form; since the day number can never exceed 7, only a single digit is required. The hyphens may be elided for brevity and the day number omitted if not required. You will frequently see date of manufacture codes such as "00W09" stamped on products; this is an abbreviation of 2000-W09, the ninth week of year 2000.

In solar calendars such as the Gregorian, only days and years have physical significance: days are defined by the rotation of the Earth, and years by its orbit about the Sun. Months, decoupled from the phases of the Moon, are but a memory of forgotten lunar calendars, while weeks of seven days are entirely a social construct--while most calendars in use today adopt a cycle of seven day names or numbers, calendars with name cycles ranging from four to sixty days have been used by other cultures in history.

ISO 8601 permits us to jettison the historical and cultural baggage of weeks and months and express a date simply by the year and day number within that year, ranging from 001 for January 1st through 365 (366 in a leap year) for December 31st. This format makes it easy to do arithmetic with dates within a year, and only slightly more complicated for periods which span year boundaries. You'll see this representation used in project planning and for specifying delivery dates. ISO dates in this form are written as "YYYY-DDD", for example 2000-060 for February 29th, 2000; leading zeroes are always written in the day number, but the hyphen may be omitted for brevity.

All ISO 8601 date formats have the advantages of being fixed length (at least until the Y10K crisis rolls around) and, when stored in a computer, of being sorted in date order by an alphanumeric sort of their textual representations. The ISO week and day and day of year calendars are derivative of the Gregorian calendar and share its accuracy.

You can download the ISO 8601 standard from the ISO Web site; to read this PDF document you'll need Adobe Acrobat Reader, which is available as a free download from Adobe's site.

Adapted from Formilab's Calendar Converter.