CalculationTime

Calendar Week Converter

Time Duration Calculator
CalculationTime

Calendar Week Converter

A printable converter for ISO week dates, Sunday-start calendar weeks, week boundaries and date labels beside the CalculationTime clockface.

Inputs, result, proof and time.

Use the instrument above, then check the calculation record, assumptions and related tools below.

Week Converter

Convert dates and week labels

ISO week date
Sunday-start week

Selected date

10 Sept

Thu, 10 Sept 2026 converts to 2026-W37-4
ISO week
2026-W37
ISO range
07 Sept-13 Sept
Sunday week
Week 37
Sunday range
06 Sept-12 Sept
2026-W37-4ISO week-date for selected date
2026-W37-4Reverse ISO lookup lands on Thu, 10 Sept 2026
2026 Sunday week 37, day 5Sunday-start label for selected date
2026 Sunday week 37, day 5Reverse Sunday-start lookup lands on Thu, 10 Sept 2026
  1. Monday2026-W37-107 Sept
  2. Tuesday2026-W37-208 Sept
  3. Wednesday2026-W37-309 Sept
  4. Thursday2026-W37-410 Sept
  5. Friday2026-W37-511 Sept
  6. Saturday2026-W37-612 Sept
  7. Sunday2026-W37-713 Sept

Conversion check

Thu, 10 Sept 2026 is ISO weekday 4, inside ISO week 37 of 2026. The same date is day 5 of Sunday-start week 37 in 2026.

Week boundaries

ISO week range: Mon, 07 Sept 2026 to Sun, 13 Sept 2026. Sunday-start range: Sun, 06 Sept 2026 to Sat, 12 Sept 2026.

Input limits

ISO year 2026 has 53 ISO weeks. Sunday-start year 2026 has 53labelled weeks under this page's January 1 rule.

Notes

Use this report for release planning, editorial calendars, roster labels and any handoff where ISO week numbers need a civil-date check.

Method

Week labels depend on the week system you choose

ISO 8601 weeks start on Monday and week 1 is the week containing the first Thursday of the ISO week-year. The Sunday-start comparison labels week 1 as the week containing January 1, with each week running Sunday to Saturday.

ISO conversion

The date is moved to the Thursday of its week, then compared with the first Thursday week of that ISO year to find the week number.

Reverse lookup

ISO week-year, week and weekday inputs are converted back to the Monday-start week range and selected civil date.

Limits

The tool uses local Gregorian calendar dates at noon. It does not claim payroll, school, fiscal or regional compliance rules.

Source Notes

ISO week dates use the published ISO 8601 convention

ISO 8601 defines week dates around Monday-start weeks and week-years. Browser date objects provide the local Gregorian date stepping used for the on-page calculations.

ReferencesISO 8601 date and time formatECMAScript Date objects