An ISO week date is a date format defined by ISO 8601 that identifies a date by its week-based year, week number, and weekday number. The format is YYYY-Www-D, for example 2026-W22-4 for Thursday, 28 May 2026.
Format components
The ISO week date has 3 parts:
- YYYY — the week-based year, which differs from the calendar year for dates near 1 January.
- Www — the week number, ranging from
W01toW52orW53. - D — the weekday, numbered 1 (Monday) through 7 (Sunday).
The literal W separates the year and week number. Week numbers are always 2 digits and zero-padded.
Distinct from the calendar date
The ISO week date differs from the calendar date format YYYY-MM-DD in 2 ways. The week-based year may not match the calendar year. The middle component is a week number, not a month. Both formats refer to the same underlying day, but applications must convert between them explicitly.