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DaysInMonth

Function DaysInMonth(const AValue: TDateTime): Word

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := DaysInMonth(EncodeDate(2024, 2, 15)); // 29
end;

Usage​

DaysInMonth returns the number of days in the calendar month containing a TDateTime value.

Parameters

NameTypeDescription
AValueTDateTime, constDate/time whose year and month select the month-length table. Time is ignored.

Returns

28, 29, 30 or 31 for a valid encoded date.

Behaviour

February reflects the decoded year's leap status. Every valid date in the same month produces the same result.

Errors

Valid dates do not raise. Invalid raw date values can propagate a failure from decoding/table access; Velox does not catch or normalise it.

Additional Technical Info​

DaysInMonth decodes the year and month containing a TDateTime value and returns that month's Gregorian length.

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

Implementation

The DateUtils import binds to Delphi 37.0 System.DateUtils.DaysInMonth. It calls DecodeDate into local year/month/day variables, discards the day, then calls DaysInAMonth(Year, Month).

Edge cases and quirks

  • The delegated DaysInAMonth performs no month validation. If an unsupported raw value makes DecodeDate return month 0, that zero becomes an invalid table index and can cause a range/access failure depending on RTL build checks.
  • A valid 0.0 date decodes as December 1899 and returns 31; zero is not an error sentinel.
  • Timezone conversion is not performed before decoding.
  • Free Pascal documents the same valid-date result but not the exact Delphi invalid-index boundary.

Side effects

None.

Performance and concurrency

Constant-time decode and lookup with no allocation or shared state.

Related entries

  • DaysInAMonth accepts explicit year and month fields.
  • DaysInYear returns the containing year length.
  • DayOfTheMonth returns the current day number rather than the month maximum.

External references

Created 2026-07-15