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DaysInYear

Function DaysInYear(const AValue: TDateTime): Word

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := DaysInYear(EncodeDate(1900, 6, 1)); // 365
end;

Usage​

DaysInYear returns the number of days in the calendar year containing a TDateTime value.

Parameters

NameTypeDescription
AValueTDateTime, constDate/time whose decoded year is evaluated. Time is ignored.

Returns

365 for a common year or 366 for a leap year.

Behaviour

All valid values within the same calendar year return the same number. No locale or timezone logic is involved.

Errors

No ordinary exception for valid dates. Unsupported non-finite values have no validated contract.

Additional Technical Info​

DaysInYear decodes the calendar year containing a TDateTime and returns 365 or 366 according to Gregorian leap-year rules.

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.DaysInYear. It calls YearOf(AValue), which uses DecodeDate, then delegates to DaysInAYear and its IsLeapYear table selection.

Edge cases and quirks

  • An unsupported date before Delphi year 1 can decode its year as 0. The downstream arithmetic treats year 0 as divisible by 400 and can return 366 rather than signalling that the date was invalid.
  • A valid TDateTime at the 1899 epoch follows ordinary Gregorian rules.
  • This is a proleptic Gregorian calculation and does not model jurisdiction-specific calendar history.
  • Free Pascal supplies the same common/leap result; Delphi source remains authoritative for Velox.

Side effects

None.

Performance and concurrency

Constant-time decode and leap-year arithmetic with no shared state.

Related entries

  • DaysInAYear accepts the year explicitly and exposes the year-0/out-of-range quirk directly.
  • DayOfTheYear returns the current ordinal.
  • DaysInMonth returns the containing month length.

External references

Created 2026-07-15