ISOWEEKNUM

Supported, behaves as documented

Category: Date and time · Last tested 2026-09-01

Real compatibility results for the ISOWEEKNUM function: executed in Excel for the web, Google Sheets and LibreOffice Calc, with desktop Excel behavior from Microsoft’s official documentation (we do not run desktop Excel — Excel for the web is a different application and is executed separately). Syntax and links to that documentation are below.

Support matrix

Engine	Documented	Live-tested	Verdict
Excel (desktop)	Yes	No — documented only	n/a
Excel for the web	—	Yes (recalc, 2026-09-01)	Supported, behaves as documented
Google Sheets	Yes	Yes (Drive import, 2026-08-29)	Supported, behaves as documented
LibreOffice Calc	Yes	Yes (25.8.7.3, 2026-07-29)	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 version	Verdict	Tested
24.2.0.3	Supported, behaves as documented	2026-08-29
24.8.7.2	Supported, behaves as documented	2026-08-29
25.2.0.3	Supported, behaves as documented	2026-08-29
25.8.7.3	Supported, behaves as documented	2026-07-29

Executed test cases

Excel for the web (executed 2026-09-01 via OneDrive recalculation)

These values come from Excel for the web, not from desktop Excel. They are two different implementations of the calculation engine, and this run measured only the web one: the corpus was uploaded to OneDrive as .xlsx, recalculated by Excel for the web on open, and downloaded again for readback. Excel for the web is a rolling service with no pinnable version, so the run is identified by its date. Where a value here disagrees with the Expected column — which is Microsoft’s documentation of the desktop product — we cannot tell you whether the web engine diverges from the desktop one or the documentation is wrong about both, because we do not run desktop Excel.

Formula	Description	Result	Expected	Verdict
=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 Provenance 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 Provenance 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 Provenance Cross-checked against Python's datetime.date(2024,12,31).isocalendar() -> (2025, 1, 2)	Matched

Google Sheets (executed 2026-08-29 via Drive import)

Google Sheets is a rolling service with no pinnable version, so this run is identified by its date. The corpus was imported to Drive as .xlsx, recalculated by Sheets, and exported back for readback.

Formula	Description	Result	Expected	Verdict
=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 Provenance 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 Provenance 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 Provenance Cross-checked against Python's datetime.date(2024,12,31).isocalendar() -> (2025, 1, 2)	Matched

LibreOffice Calc 25.8.7.3 (tested 2026-07-29)

Formula	Description	Result	Expected	Verdict
=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 Provenance 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 Provenance 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 Provenance Cross-checked against Python's datetime.date(2024,12,31).isocalendar() -> (2025, 1, 2)	Matched

Docs & syntax

Excel (desktop): official documentation
Google Sheets: official documentation
LibreOffice Calc: official documentation