DAYSINMONTH Function (OpenOffice Calc)

• Introduced: First release not established; documented by OpenOffice.org 3.3.
• Supported in: Apache OpenOffice 4.1.16; this is a verified release, not a final supported version.
• Scope: Desktop Calc, English function names and semicolon-separated formulas.
• Checked: October 14, 2026. Eight ordinary reference formulas were recalculated from generated ODF in 4.1.16 on Linux x86-64, Build 9816. Source and independent calendar arithmetic were checked.

DAYSINMONTH returns the number of calendar days in the month containing Date: 28, 29, 30 or 31. It belongs to the date add-in family. The argument is a date serial, not a month number, year number or number of elapsed days. Release implementation.

DAYSINMONTH(Date)

• Date: one required date value, preferably a date cell or DATE(year;month;day).
• The implementation receives an integer date serial and uses the document’s configured null date. Supply date-only values deliberately; fractional conversion was not part of this native fixture.

The native column contains saved 4.1.16 results; independent references use exact date arithmetic or rational fractions. Numerical values are shown to twelve decimal places where needed. Enter the DATE expressions directly rather than substituting locale-dependent text.

The generated ODF uses ORG.OPENOFFICE.DAYSINMONTH, mapped to the English worksheet name DAYSINMONTH by the release function map. The saved operation was verified after recalculation. This establishes the ODF-format path rather than manual English-name entry.

If A2 contains =DATE(2024;2;15), =DAYSINMONTH(A2) gives 29. Replacing the year with 2023 gives 28. For a monthly allocation whose rule is an equal amount per calendar day, a monthly amount of 290 divided by 29 gives 10 per day. Whether that allocation rule is appropriate is a separate model decision.

The day within the month does not change its length. April 1 and April 30, 2024 both belong to a 30-day month. The example table uses explicit DATE formulas so the intended month is visible.

The calendar rule is Gregorian: years divisible by 4 are leap years except century years not divisible by 400. Consequently February 2000 has 29 days, while February 1900 and February 2100 each have 28. Do not use a simple “divisible by four” rule for all years.

With a valid date in A2, a transparent month-end construction is =DATE(YEAR(A2);MONTH(A2);DAYSINMONTH(A2)). For February 15, 2024 it represents February 29. Another useful identity is DAYSINMONTH(A2)=DAY(EOMONTH(A2;0)); the date and serial meanings should agree. These explanatory identities were checked independently, beyond the eight listed native calls.

Entering DAYSINMONTH(2) asks about date serial 2 in the active date system; it does not mean February. Likewise, 2024 is a serial when passed directly. Construct the intended date explicitly.

The tested ODF stored this add-in under its registered ORG.OPENOFFICE name. Its English worksheet identity was checked in the release registry. No manual-entry, other-locale or invalid-input execution is claimed. Preserve date values when importing text or copying between workbooks with different date systems.

DATE, EOMONTH, DAYSINYEAR.

• Function-specific release implementation

The generated ODF produced the informational load warning “Not all attributes could be read.” The sheet was hard-recalculated and saved separately. All eight ordinary function identities and results were reconciled; a stale SUM cache changed from 999999 to 3 and an uncached SUM returned 7. This establishes the stated examples, not every workbook style or invalid input. Other explanatory identities are independent calculations. The authoring-tool XLSX was not executed in OpenOffice, and its preview values are not the native results reported here.