How to use this reference
ISO weeks start on Monday, and week 1 is the week containing January 4—the first Thursday of the year. This convention is useful for work schedules and international reporting, but it is not the same as simply dividing the day of the year by seven.
The ISO week-year can differ from the calendar year near New Year. January 1, 2021 belongs to ISO week 53 of 2020, while December 30, 2024 belongs to week 1 of 2025. Always include the ISO week-year with a week number to avoid attaching a report to the wrong year.
Calendar-year time remaining is reported separately from ISO week numbering. Use the Day Counter for exact date gaps, the Weeks Until/Since Calculator for seven-day planning intervals, and the Date Calculator for general date arithmetic.
Frequently asked questions
Can January 1 be in the previous ISO year?
Yes. Dates before the first Monday of ISO week 1 can belong to the final ISO week of the previous year.
Do all ISO years contain 52 weeks?
No. ISO years have 52 or 53 weeks, depending on the placement of weekdays and leap days.
Are weeks remaining the same as the final ISO week minus today's week?
No. ISO week-years overlap calendar-year boundaries. Remaining calendar days must be calculated separately.