ISO week date

Current week expressed according to ISO 8601 [refresh]
Date	2024-06-08
Week	2024-W23
Week with weekday	2024-W23-6

The ISO week date system is effectively a leap week calendar system that is part of the ISO 8601 date and time standard issued by the International Organization for Standardization (ISO) since 1988 (last revised in 2019) and, before that, it was defined in ISO (R) 2015 since 1971. It is used (mainly) in government and business for fiscal years, as well as in timekeeping. This was previously known as "Industrial date coding". The system specifies a week year atop the Gregorian calendar by defining a notation for ordinal weeks of the year.

The Gregorian leap cycle, which has 97 leap days spread across 400 years, contains a whole number of weeks (20871). In every cycle there are 71 years with an additional 53rd week (corresponding to the Gregorian years that contain 53 Thursdays). An average year is exactly 52.1775 weeks long; months (1⁄12 year) average at exactly 4.348125 weeks/month.

An ISO week-numbering year (also called ISO year informally) has 52 or 53 full weeks. That is 364 or 371 days instead of the usual 365 or 366 days. These 53 week years occur on all years that have Thursday as the 1st of January and on leap years that start on Wednesday the 1st. The extra week is sometimes referred to as a leap week, although ISO 8601 does not use this term.

Weeks start with Monday and end on Sunday. Each week's year is the Gregorian year in which the Thursday falls. The first week of the year, hence, always contains 4 January. ISO week year numbering therefore usually deviates by 1 from the Gregorian for some days close to 1 January.

Examples of contemporary dates around New Year's Day
English short		ISO
Sat 1 Jan 1977	1977-01-01	1976-W53-6
Sun 2 Jan 1977	1977-01-02	1976-W53-7

Sat 31 Dec 1977	1977-12-31	1977-W52-6
Sun 1 Jan 1978	1978-01-01	1977-W52-7
Mon 2 Jan 1978	1978-01-02	1978-W01-1

Sun 31 Dec 1978	1978-12-31	1978-W52-7
Mon 1 Jan 1979	1979-01-01	1979-W01-1

Sun 30 Dec 1979	1979-12-30	1979-W52-7
Mon 31 Dec 1979	1979-12-31	1980-W01-1
Tue 1 Jan 1980	1980-01-01	1980-W01-2

Sun 28 Dec 1980	1980-12-28	1980-W52-7
Mon 29 Dec 1980	1980-12-29	1981-W01-1
Tue 30 Dec 1980	1980-12-30	1981-W01-2
Wed 31 Dec 1980	1980-12-31	1981-W01-3
Thu 1 Jan 1981	1981-01-01	1981-W01-4

Thu 31 Dec 1981	1981-12-31	1981-W53-4
Fri 1 Jan 1982	1982-01-01	1981-W53-5
Sat 2 Jan 1982	1982-01-02	1981-W53-6
Sun 3 Jan 1982	1982-01-03	1981-W53-7
Notes: Both years 1979 start with the same day. 1980 is a leap year. 1980W is 2 days shorter: 1 day longer at the start, 3 days shorter at the end. 1981W begins three days before the end of 1980. 1981W has 53 weeks and ends three days into 1982.

A precise date is specified by the ISO week-numbering year in the format YYYY, a week number in the format ww prefixed by the letter 'W', and the weekday number, a digit d from 1 through 7, beginning with Monday and ending with Sunday. For example, the Gregorian date Saturday, 8 June 2024 corresponds to day number 6 in the week number 23 of 2024, and is written as 2024-W23-6 (in extended form) or 2024W236 (in compact form). The ISO year is slightly offset to the Gregorian year; for example, Monday 30 December 2019 in the Gregorian calendar is the first day of week 1 of 2020 in the ISO calendar, and is written as 2020-W01-1 or 2020W011.