Calculate week number

Convert a Gregorian date to its ISO week-year or expand an ISO week into Monday-through-Sunday dates.

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ToolWeek Number Calculator
Input
Output
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How it works

The week’s Thursday determines its ISO year, while reverse lookup anchors week 1 to the week containing 4 January. Checking 28 December rejects week 53 in an ISO year that contains only 52 weeks.

  • ISO weeks always begin Monday and can cross calendar-year boundaries.
  • No current date is assumed, keeping every lookup reproducible.

Worked example

A date at the year boundary
Late December that already belongs to next year's week 1
Input
											2018-12-31
Output
												ISO year: 2019
ISO week: 1
Week start: 2018-12-31
Week end: 2019-01-06

When to use this

Business reports group by ISO week, manufacturing plans expand week labels, and boundary tests cover late-December week years.

Edge cases

  • 2018-12-31 is ISO week 1 of 2019 despite its Gregorian year.
  • Week 53 is rejected when 28 December proves that ISO year ends at week 52.
  • Sunday is the final day of its Monday-starting ISO week.

References