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ISO Week Number Calculator: Date to Week and Back

Date to ISO week date and back: week 1 holds the first Thursday, weeks start Monday.

Direction

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Rules this tool follows
  • ISO 8601-1 §7.3.6: weeks run Monday-Sunday; week 1 is the week containing the first Thursday of the year.
  • An ISO year has exactly 52 or 53 weeks. Week 53 is refused for years that do not have one.
  • Calendar-year ≠ ISO year at boundaries: 1 January can sit in the previous ISO year, 31 December in the next.
  • All math is exact UTC-calendar arithmetic: no formatter heuristics.
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