ISO 8601 is the international standard for representing dates and times, published by the International Organization for Standardization (ISO). It is the system most widely used in software, international business, and European countries.
Week numbering rules
Under ISO 8601:
- Weeks start on Monday and end on Sunday.
- Week 1 of any given year is the week containing the first Thursday of that year — equivalently, the week containing 4 January.
- Years can have either 52 or 53 weeks depending on how the calendar falls.
When does it matter?
ISO 8601 week numbers are useful any time consistency across regions matters — for example, when coordinating release schedules across teams in different countries, or aligning fiscal reports against international peers.
Compare ISO 8601 with the US Standard and Australian Financial Year numbering on the home page.