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Week number
Find the week number of any date, or the dates in a week.
ISO 8601 week dates · weeks start Monday · week 1 holds the year’s first Thursday
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- 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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