Why an ISO Week Can Belong to a Different Year
Understand Monday-first ISO weeks and the January dates that fall in the previous week-year.
The ISO week has its own year
ISO weeks begin on Monday. Week 1 is the week containing January 4, which is also the week containing the year’s first Thursday. The week-year can therefore differ from the Gregorian year printed in a date. This matters when naming reports, files and production schedules.
A boundary example
January 1, 2021 was a Friday in ISO week 53 of 2020. January 4, 2021 began ISO week 1 of 2021. Calling both dates week 1 because they occur in January would merge two different ISO reporting periods. Keep the ISO week-year together with the week number.
Some years have 53 weeks
Most ISO week-years have 52 weeks, but some have 53 because of the way weekdays align with the calendar. A week 53 result is not an error. Never assume the final reporting week is always 52, and do not combine a Gregorian year with a week number taken from another system.
Check the system used by your team
Other calendars can start weeks on Sunday or count the partial week containing January 1 as week 1. This calculator uses ISO rules throughout. If an employer or spreadsheet reports a different number, compare the week-start and first-week settings before changing the date.
Put it into practice
Day of the Week Calculator
Find which day of the week any Gregorian date falls on, plus its day number in the year and whether it is a leap year.
Date & calendarISO Week Number Calculator
Find the ISO 8601 week number and week-year for a date, together with the Monday and Sunday of that week.
Date & calendarPrintable Monthly Calendar
View and print a clean Gregorian monthly calendar, with Monday or Sunday as the first day of the week.
Date & calendar