About this data
The dates on Week Numbers USA should be dates you can rely on. This page explains where they come from, how we calculate them, how often we check them and what to do if you spot a mistake.
Where does the data come from?
Week numbers follow the international standard ISO 8601. No data source is needed: every week number follows directly from the calendar.
How do we calculate the dates?
We show two week numbers. Under ISO 8601 a week starts on Monday, and week 1 is the week containing the first Thursday of the year, so a year has 52 or 53 weeks. In the common US system a week starts on Sunday and week 1 is the week containing January 1, as in Excel's WEEKNUM function and Outlook's default. Both calculations are covered by automated tests, including around the turn of the year.
How often do we check the data?
The ISO standard does not change, so the week numbers themselves never need rechecking. The pages are refreshed daily, so today's week number is always right.
Spotted a mistake?
Is a date wrong, or is something missing? Let us know via the contact form, ideally with a link to the source. We review every report and correct the data where needed.
Report a mistake via the contact form