Date arithmetic is tested on every build against hand-worked cases, including month ends, leap days and two-digit years. How it is checked
Enter a date, or a week number and a year, to see the week in both conventions in common use: ISO 8601 and the US one. The two disagree for a good part of the year. The ISO week-year is shown too, because a week number without one is ambiguous every January.
The two conventions, and why they disagree
Under ISO 8601, which most of Europe and most business software outside the United States uses, weeks start on Monday and week 1 is the week containing the first Thursday of the year. Put more simply, week 1 is the week containing 4 January. Under the common United States convention, weeks start on Sunday and week 1 is whichever week contains 1 January, so the first week is usually a stub of one to six days.
The two agree for much of the year and then drift apart, sometimes by one and sometimes by two. If a colleague says “week 34” and your calendar says 33, you may both be right under different definitions, so say which one you are using.
Why a week number needs a year attached to it
Under ISO, early January can belong to the last week of the previous year, and late December can belong to week 1 of the next one. 1 January 2027 falls on a Friday, so it sits in week 53 of week-year 2026. That is the rule working as intended, and it is why the ISO date format writes the week-year out in full, as 2026-W53.
So do not file or sort by a bare week number across a year boundary. The readout above gives the week-year alongside the week, and the week-year is sometimes different from the calendar year of the date you entered.
Fifty-two weeks, or fifty-three
The years with 53 ISO weeks are the ones that begin on a Thursday, plus the leap years that begin on a Wednesday. That comes to seventy-one years in every four hundred; the rest have 52. A payroll or rota system that assumes 52 weeks comes up a week short in those years, and that is a common source of a missing pay period.
Questions
What week number is it right now?
Both answers are in the strip above, because there are two conventions in common use. The ISO figure is the one most business software outside the United States means; the US figure is the one most American consumer calendars show. When you quote a week number to somebody, name the convention with it: that one word clears up most of the confusion these numbers cause.
Why does my calendar show a different week number than my work system?
Almost certainly because one uses ISO 8601 and the other uses the US convention. ISO weeks start on Monday and week 1 is the one containing the first Thursday; the US convention starts weeks on Sunday and always puts 1 January in week 1. They agree for stretches and then differ by one or two. The page opens on today's date, so both numbers for this week are already in the strip above.
Why is 1 January sometimes in week 52 or 53 of the previous year?
Because ISO keeps every week whole, and a week belongs to whichever year holds its Thursday. 1 January 2027 is a Friday, so its week's Thursday falls on 31 December 2026 and the whole week belongs to week-year 2026. That is why an ISO week number is properly written with its week-year, as in 2026-W53, and why sorting records by a bare week number across a year boundary puts them in the wrong order.
Do all years have 52 weeks?
No. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday, which happens seventy-one times in every four hundred years. Every other year has 52. Systems that hardcode 52, payroll schedules and rota planners especially, lose a week in those years, and the failure usually surfaces as a pay period that does not exist.
What is the difference between a week-year and a calendar year?
The calendar year is the year in the date. The week-year is the year that owns the ISO week that date falls in, and for a few days at either end of the year the two can differ. A date of 1 January 2027 has calendar year 2027 and week-year 2026. Any system that reports a week number should report the week-year with it, because the pair is unambiguous and either one alone is not.
Dates and Days