Different week-number systems can assign different numbers to the same date. A Monday start alone does not necessarily make a calculation ISO-compliant.
Choose the intended convention
=ISOWEEKNUM(A2) uses ISO weeks, which begin on Monday and define week 1 using the year’s first Thursday. Dates near New Year can belong to the adjacent ISO week-year.
=WEEKNUM(A2,2) also begins weeks on Monday but uses the system whose first week contains January 1. WEEKNUM with type 21 uses the ISO system. The default type 1 begins weeks on Sunday.
Keep the week-year consistent
Do not combine an ISO week number with the calendar year blindly. Early January can still belong to the previous ISO week-year, while late December can belong to the next one.
For reports, use a reviewed year-and-week key or the actual week’s starting date. A week number by itself repeats every year and can merge unrelated periods.
Test the boundary dates
Check several dates spanning late December and early January against the organization’s reporting calendar. Include a year with an ISO week 53 when relevant.
Confirm the input cells are actual dates and avoid ambiguous text inside function arguments. Document the selected WEEKNUM type in formulas rather than leaving the default unexplained. If another system disagrees, compare conventions before altering source dates. A mismatch can reflect different valid definitions rather than incorrect data, and changing the week start may still leave the first-week rule unresolved. Preserve the convention used by historical reports when comparing periods.
References
Reviewed against the linked official product guidance on November 3, 2026. Features, limits, screen labels, and access can vary by account, platform, region, and organization policy.