"Week date" redirects here. Not to be confused with Week, Week-day names, or Week number.
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Current week expressed according to ISO 8601 [refresh]
Date
2024-05-15
Week
2024-W20
Week with weekday
2024-W20-3
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 Wednesday, 15 May 2024 corresponds to day number 3 in the week number 20 of 2024, and is written as 2024-W20-3 (in extended form) or 2024W203 (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.
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