Time & Date Tools
Week Number Lookup
Look up an ISO week number, week-year, day of year, quarter and week of month for any date.
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About this tool
Look up an ISO week number, week-year, day of year, quarter and week of month for any date.
Use the tool above first; keep original inputs or files when the result matters, and verify the output in the destination system when there is a strict requirement.
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Worked example and limits
Calendar acceptance checks: 2026-01-31 plus one month is 2026-02-28; 2024-02-29 plus one year is 2025-02-28. The date difference from 2026-09-25 to 2026-09-28 is three days despite daylight saving. January 5–9, 2026 inclusive has five weekdays, or four when January 5 is a custom holiday. Leap-day age conventions depend on the receiving service; confirm ISO week rules when using week numbers.