On 15 June 1988, the International Organization for Standardization (ISO) issued the Representation of Dates and Times standard (referred to as ISO 8601:1988) for regularizing the notation of dates and times in administrative, scientific, commercial and industrial documents. This standard superseded a number of earlier ISO documents (2014:1976, 2015:1976, 2711:1973, 3307:1975 and 4031:1978) that provided previous standards on the same topic. A revision of this standard was issued on 21 December 2000 and is referred to as ISO 8601:2000 or the “second edition”. The latest revision of this standard was issued on 3 December 2004 and is referred to as ISO 8601:2004 or the “third edition”.
Although the bulk of the standard deals with the proposal to introduce a uniform and unambiguous notation for representing calendar dates and the times of the day, it also provides standards for defining the day number in a week (assumed to start on Monday) and the week number in a year.
The purpose of this website is to point out some of the mathematical properties of the week numbering scheme of the ISO 8601 calendar and how to determine the ISO week number of an arbitrary calendar date.
