ClarityHow to use this tool
- Enter calendar date.
- Select Calculate to view the result.
- Check the method and assumptions below before using the result.
Input reference and sample values
Use the units and choices shown for each field. Sample values are provided for practice, not as recommendations.
| Input | Format and units | Sample |
|---|---|---|
| Calendar date | Use a calendar date in year-month-day order. | 2026-01-01 |
Result for these sample inputs
2026-W01
- Week starts Monday
- 2025-12-29
- Week ends Sunday
- 2026-01-04
- ISO week-year
- 2026
Select Try the example in the form to load these values and calculate them again. This replaces the current form inputs. Compare the result with the method and worked explanation below.
The method, explained
ISO weeks start Monday. Week 1 is the week containing January 4, equivalently the year’s first Thursday. The ISO week-year can differ from the calendar year.
January 1, 2021 belongs to 2020-W53. Its week begins December 28, 2020 and ends January 3, 2021.
What to keep in mind
Uses ISO week-date conventions, not Sunday-start calendars or company fiscal week numbers. Dates near the supported year limits may be rejected when their full week falls outside 0100–9999.
Common questions
Can January dates belong to the previous week-year?
Yes. Days before the first ISO week may belong to the final ISO week of the preceding year.
Does every year have 52 ISO weeks?
No. Some ISO week-years have 53 weeks.
Methodology maintained by ClarityKit. How these tools are built and checked.