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DaysInAMonth

Function DaysInAMonth(const AYear, AMonth: Word): Word

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := DaysInAMonth(2024, 2); // 29
end;

Usage​

DaysInAMonth returns the number of days in a specified year and month, including leap-year February.

Parameters

NameTypeDescription
AYearWord, constYear used for the February leap-year decision. Use the Velox encodeable range 1..9999 for calendar dates.
AMonthWord, constMonth number. The supported range is 1..12.

Returns

28, 29, 30 or 31 for a valid year/month pair.

Behaviour

Leap years are divisible by 4, except century years not divisible by 400. Thus February 2000 has 29 days while February 1900 has 28.

Errors

No exception for a valid pair. Invalid AMonth behaviour is deliberately not normalised by Velox; any runtime range/access failure propagates.

Usage notes

Use an upstream validation rule or IsValidDate before processing untrusted numeric calendar fields.

Additional Technical Info​

DaysInAMonth returns the Gregorian length of a supplied month in a supplied year. February is selected from the common-year or leap-year table according to Delphi's divisibility rules.

The example is fictional, deterministic and source-reviewed. It was not executed by the documentation workflow.

Implementation

The DateUtils import binds directly to Delphi 37.0 System.DateUtils.DaysInAMonth. The implementation indexes the RTL MonthDays table with two dimensions: (AMonth = 2) and IsLeapYear(AYear) selects the common/leap row, and AMonth selects the month column.

Edge cases and quirks

  • The installed function performs no local range validation before using AMonth as an array index. Values outside 1..12 are unsupported and can raise a range error or read an invalid table element depending on the RTL build's range-checking configuration. Validate the month before calling.
  • AYear is a Word, so the signature accepts 0 and values above 9999. IsLeapYear will still apply arithmetic to them even though EncodeDate cannot create those years. Keep the year within 1..9999 for an actual Delphi calendar date.
  • There is no locale or timezone dependency.
  • Free Pascal documents the same valid month/year purpose, but it does not define the error behaviour of this installed Delphi table access.

Side effects

None.

Performance and concurrency

Constant-time leap calculation and table lookup with no allocation or mutable state.

Related entries

  • DaysInMonth derives year and month from an existing TDateTime.
  • DaysInAYear returns the complete year length.

External references

Created 2026-07-15