ISO and US week numbers are different
Under ISO 8601 — used across Europe and in most business software — weeks start on Monday and week 1 is the one containing the first Thursday of the year. In the common US convention, weeks start on Sunday and week 1 is the one containing 1 January.
They frequently disagree by one, especially in the first days of January. A report "for week 3" can mean two different weeks depending on who produced it, which is a reliable source of confusion in international teams.
Why some years have 53 weeks
A year of 52 weeks is 364 days, one short. The extra days accumulate, and a year gets a 53rd week when it starts on a Thursday, or on a Wednesday in a leap year. Payroll and rota systems built on 52 weeks break in those years — which is a known and recurring problem, not an unusual one.
It also means early January can belong to the previous year’s final week, and late December to week 1 of the next.
Frequently asked questions
Why does my week number differ from a colleague’s?
Almost certainly ISO versus the US convention. ISO weeks start Monday and week 1 holds the first Thursday; the US convention starts Sunday and week 1 holds 1 January.
Can a year have 53 weeks?
Yes — when it starts on a Thursday, or on a Wednesday in a leap year. Systems that assume 52 break in those years.
Which week does 1 January fall in?
Under ISO it can belong to the last week of the previous year. Late December can likewise belong to week 1 of the next.
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