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Date & Time Calculator
Calendar arithmetic, reviewed October 2026 — weekends count unless a page says otherwise, and the sample holiday list is not an official gazette.
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An ISO week calculator returns the week number for any date under ISO 8601: weeks start on Monday and week 1 is the week containing the year's first Thursday — the numbering used in business and manufacturing calendars. 1 January 2026 (a Thursday) is Week 1, while 31 December 2026 lands in Week 53: 2026 is one of the years with 53 ISO weeks.
Enter any date and read the week number. Under ISO rules early January can fall in the previous year's final week and late December in week 1 of the next — that spillover is the standard working as intended, not an error. Use it for sprint planning, payroll weeks, and production calendars that number weeks instead of naming months.
It does not convert to Sunday-start US week numbering, does not export a calendar, and only numbers the date — early-January spillover into the previous year's final week is the standard working as intended.
ISO 8601 numbering only — not US Sunday-start week numbering.
Because ISO ties week 1 to the first Thursday. If the year starts late in the week, those first days belong to the previous year's final week.
Yes. Years that start on a Thursday, and leap years starting on a Wednesday, have 53 ISO weeks.
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