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ISOWEEKNUM

Supported, behaves as documented

Category: Date and time · Last tested 2026-08-06

Real, executed compatibility results for the ISOWEEKNUM function across Microsoft Excel, Google Sheets, and LibreOffice Calc — verified by actually running it. Syntax and links to each vendor’s official documentation are below.

Support matrix

EngineDocumentedLive-testedVerdict
Excel Yes Not yet n/a
Google Sheets Yes Not yet n/a
LibreOffice Calc Yes Yes (25.8.7.3, 2026-08-06) Supported, behaves as documented

LibreOffice version history

We executed the same test cases under each LibreOffice release to show exactly when ISOWEEKNUM’s support changed — not documentation claims, real results.

LibreOffice versionVerdictTested
24.2.0.3 Supported, behaves as documented 2026-07-19
24.8.7.2 Supported, behaves as documented 2026-07-19
25.2.0.3 Supported, behaves as documented 2026-07-24
25.8.7.3 Supported, behaves as documented 2026-08-06

Executed test cases

LibreOffice Calc 25.8.7.3 (tested 2026-08-06)

FormulaDescriptionResultExpectedVerdict
=ISOWEEKNUM(DATE(2012,3,9)) Microsoft's own documented example date/result. Verified via https://support.microsoft.com/en-us/office/isoweeknum-function-1c2d0afe-d25b-4ab1-8894-8d0520e90e0e 10 10 Matched
=ISOWEEKNUM(DATE(2023,1,1)) Genuine algorithmic divergence from plain WEEKNUM: ISO 8601 week 1 is defined as the week containing the year's first Thursday (Monday-start weeks), NOT the week containing Jan 1. Jan 1, 2023 is a Sunday, so it belongs to the week Dec 26, 2022 - Jan 1, 2023, whose Thursday (Dec 29, 2022) is in 2022 -- so this date is ISO week 52 of 2022, even though plain WEEKNUM would call it week 1 of 2023 52 52
Cross-checked against Python's datetime.date(2023,1,1).isocalendar() -> (2022, 52, 7), which implements ISO 8601		 Matched
=ISOWEEKNUM(DATE(2024,1,1)) Contrast case: Jan 1, 2024 is a Monday, so (unlike the 2023 case above) it DOES start ISO week 1 of 2024 -- showing the year-boundary quirk depends entirely on which weekday Jan 1 falls on 1 1
Cross-checked against Python's datetime.date(2024,1,1).isocalendar() -> (2024, 1, 1)		 Matched
=ISOWEEKNUM(DATE(2024,12,31)) The same phenomenon at the OTHER end of the year: Dec 31, 2024 is a Tuesday, and the Monday-start week containing it (Dec 30, 2024 - Jan 5, 2025) has its Thursday (Jan 2, 2025) in the following calendar year, so this December date is actually ISO week 1 of 2025 1 1
Cross-checked against Python's datetime.date(2024,12,31).isocalendar() -> (2025, 1, 2)		 Matched

Docs & syntax