Why does the ISO year sometimes differ from the calendar year?

Because a week's ISO year is defined by whichever calendar year contains that week's Thursday, which can pull the first days of January into the previous ISO year, or the last days of December into the next one.

Week Number Calculator

Find the ISO 8601 week number for a date. ISO weeks start Monday, and week 1 is the week containing January 4.

Want to know if a whole year has 52 or 53 weeks? See Weeks-in-Year Calculator.

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ISO Week Numbers vs. Plain Calendar Weeks

Worked example

January 1, 2021 falls in ISO week 53 of 2020, not week 1 of 2021. Under ISO 8601, a week belongs entirely to whichever year contains its Thursday, and late-December or early-January dates often cross this boundary in a way that surprises people expecting week numbers to reset cleanly on January 1. December 31, 2029 falls in ISO week 1 of 2030 for the same reason, since that week's Thursday lands in January 2030.

ISO week versus locale week

Some calendar systems, and some countries, start the week on Sunday and number weeks differently. This engine always reports the ISO 8601 standard, never a locale-specific display, and states the result's own ISO week-year clearly when it differs from the calendar year.

Common mistakes

Assuming the ISO week-year always matches the calendar year of the date entered. The first few days of January and the last few days of December are the dates most likely to belong to a different ISO week-year than their calendar year suggests.
Comparing an ISO week number to a locale week number from a spreadsheet or calendar app without checking which convention it uses. A Sunday-start week display and an ISO Monday-start week can number the same date differently.

Limitations

This reports the ISO week number only. It does not reflect a country-specific numbering system that differs from ISO 8601, so a locale display that starts weeks on Sunday will not always match this result.

Frequently asked questions

Why does the ISO year sometimes differ from the calendar year?: Because a week's ISO year is defined by whichever calendar year contains that week's Thursday, which can pull the first days of January into the previous ISO year, or the last days of December into the next one.