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WEEKNUM Function in Apache OpenOffice Calc: Syntax, Examples and Compatibility

Date & Time Beginner Apache OpenOffice Calc

Use WEEKNUM in Apache OpenOffice Calc to calculate an ISO-8601 week number in Apache OpenOffice Calc without accidentally applying the Excel non-ISO WEEKNUM convention. Two original examples, expected results, cautions and official software documentation.

Verification status: Official documentation referenced; example outcomes independently reasoned or arithmetically checked where stated. Not executed in Apache OpenOffice Calc. How formula examples are checked

Quick answer: calculate an ISO-8601 week number in Apache OpenOffice Calc without accidentally applying the Excel non-ISO WEEKNUM convention in Apache OpenOffice Calc. The reproducible examples below use explicit values so the formula’s meaning is testable without a large workbook.

Evidence: The function name and syntax are documented in the linked vendor function reference. Expected outputs were independently reasoned or checked outside the application. These are not native Apache OpenOffice Calc test results. A guide being published here does not certify every software release or file-import scenario.

Syntax and arguments

=WEEKNUM(date; mode)

date is the date; mode=2 starts the week on Monday. Apache OpenOffice WEEKNUM follows ISO week numbering (the week containing January 4 is week 1), unlike the default Excel WEEKNUM.

Best use case: Calculate an ISO-8601 week number in Apache OpenOffice Calc without accidentally applying the Excel non-ISO WEEKNUM convention.

Set up the example sheet

Prepare two fixed dates with DATE in separate cells: Friday 1 January 2021 and Monday 4 January 2021. Use ISO week numbering with Monday-first mode 2.

Enter the formula in an unused cell. For a formula returning several values, leave an unobstructed area where those values can expand. These examples use semicolons between formula arguments in the documented Calc-style syntax; adjust separators if your application or regional settings differ.

Worked example 1: WEEKNUM with known input

=WEEKNUM(DATE(2021;1;1);2)

Expected output: 53.

1 January 2021 falls in ISO week 53 of the 2020 ISO week-year, not week 1 of a calendar-year counting scheme.

Worked example 2: a second WEEKNUM case

=WEEKNUM(DATE(2021;1;4);2)

Expected output: 1.

Monday 4 January 2021 begins ISO week 1 of the 2021 ISO week-year.

Compare both results before replacing literal values or short sample ranges with your own workbook data. For a spilling formula, compare every output cell, not only the top-left result.

Practical advice and troubleshooting

When this formula helps: For a non-ISO week number matching traditional Excel WEEKNUM semantics, consult OpenOffice WEEKNUM_ADD rather than assuming identical function names imply identical rules.

Important edge case: A date near New Year can belong to the previous or next ISO week-year. Combine ISO week year and week number in a calendar-based report.

If the result differs, first verify the text-versus-number types of the input cells, the argument separators for your locale, and the range size. For dynamic arrays, inspect occupied cells in the output area. An error that looks like a missing function may reflect a software version rather than incorrect data.

Compatibility and version considerations

The Apache OpenOffice Calc official guide documents this name. OpenOffice often uses semicolon-separated function arguments, and older versions can require Ctrl+Shift+Enter array entry. Critical compatibility difference: Apache OpenOffice WEEKNUM follows ISO weeks. Excel WEEKNUM(date,2) treats the week containing January 1 as week 1. OpenOffice WEEKNUM_ADD is the separate function used for Excel-style week numbering. The Excel/Google comparison is background only, not a claim of identical behavior in Apache OpenOffice Calc.

Check your installed release for availability and dynamic-array support.

Function availability and parameter behavior must be judged for the specific application and release; a function documented in one spreadsheet does not automatically work in another.

Application-specific source note: The Apache OpenOffice Calc official guide documents this name. OpenOffice often uses semicolon-separated function arguments, and older versions can require Ctrl+Shift+Enter array entry.

Formula separator: These examples use semicolons for the function-argument separators. Locale settings can change entry conventions.

Editorial verification: Function name checked in official vendor catalogue on 2026-10-12. The two example outputs are independently reasoned; no native application run, screenshot, or version-specific output is claimed. Review the editorial testing standards if you require stronger proof before importing a workbook.