Why does Jan 1 sometimes belong to the previous year?

ISO weeks must contain at least one day of the week's owning year. If the year starts on a Friday/Saturday/Sunday, Jan 1 sits inside the last week of the previous ISO year — that's why the ISO year and the calendar year can differ for early-January and late-December dates.

When does a year have 53 ISO weeks?

When the calendar year starts on a Thursday, or is a leap year that starts on a Wednesday. Otherwise it has 52 weeks.

ISO 8601 Week Calculator

Time

ISO 8601 weeks start on Monday and the year that owns each week is the one that owns its Thursday — so the first week of the year is the one containing 4 January. This tool implements that exactly. Useful for fiscal calendars, project scheduling, or any pipeline that reports by ISO week (most European business systems, GitHub insights, etc.). The reverse direction lets you pick a year + week to see exactly which seven dates it spans.

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Date

ISO week24
ISO year2026
Week start2026-06-08 (Mon)
Week end2026-06-14 (Sun)
ISO weekday3 (Wed)
Day of year161
Weeks in ISO year53

Weeks start on Monday. Week 1 is the week that contains January 4th.

How to use

Date → Week: pick a date to see its ISO year, week, weekday, and the full week range.
Week → Date: enter an ISO year + week number to see the seven dates that fall in that week.

Frequently asked questions

Why does Jan 1 sometimes belong to the previous year?: ISO weeks must contain at least one day of the week's owning year. If the year starts on a Friday/Saturday/Sunday, Jan 1 sits inside the last week of the previous ISO year — that's why the ISO year and the calendar year can differ for early-January and late-December dates.
When does a year have 53 ISO weeks?: When the calendar year starts on a Thursday, or is a leap year that starts on a Wednesday. Otherwise it has 52 weeks.