ISO week converter
Convert a calendar date to an ISO 8601 week and back. A missing week 53 does not invent a date.
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Tool guide
ISO week converter: how it works
Practical information to understand the result and use this tool correctly.
About ISO week converter
ISO weeks start on Monday. Week 1 is the week that contains the first Thursday of the calendar year.
A date near New Year can belong to the previous or next ISO week-year. 2021-01-01 is 2020-W53-5.
Week 53 is accepted only when that week exists. The tool does not invent a date for 2019-W53.
How to use it
- 1Enter 2021-01-01 or use the date example.
- 2Or enter 2024-W01-1 to get the Monday of that week.
- 3Copy the result. Reset clears both fields.
Frequently asked questions
- What ISO week is 2021-01-01?
- 2020-W53-5. Friday 1 January 2021 belongs to ISO week 53 of 2020.
- What date is 2024-W01-1?
- 2024-01-01. That week starts on Monday 1 January 2024.
- Why is 2019-W53-1 rejected?
- 2019 has 52 ISO weeks. The tool does not invent a date for a week the year does not have.