DecodeDateDay

Procedure DecodeDateDay(const AValue: TDateTime; var AYear, ADayOfYear: Word)

Example

procedure ScriptEvent(var Value: variant);
var
  YearValue, DayValue: Word;
begin
  DecodeDateDay(EncodeDate(2024, 12, 31), YearValue, DayValue);
  Value := YearValue * 1000 + DayValue; // 2024366
end;

Usage

DecodeDateDay writes the calendar year and one-based day-of-year components of a TDateTime value.

Parameters

Name	Type	Description
AValue	TDateTime, const	Value whose calendar date is decoded; time is ignored.
AYear	Word, var	Overwritten with the calendar year.
ADayOfYear	Word, var	Overwritten with 1..365, or 1..366 for a leap year.

Behaviour

January 1 returns day 1. Leap day and all following days are shifted by one in a leap year. Both output variables are overwritten.

Errors

Valid dates do not raise. An invalid decoded year can propagate an encoding EConvertError; unsupported numeric values can propagate conversion errors.

Additional Technical Info

DecodeDateDay decomposes a date into its calendar year and one-based ordinal day within that year.

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.DecodeDateDay. The routine sets AYear := YearOf(AValue), then sets ADayOfYear := DayOfTheYear(AValue). The latter subtracts the encoded start of that year from the truncated input date and adds one.

Edge cases and quirks

• The result is based on calendar ordinals, not elapsed 24-hour periods from the exact input time.
• An unsupported date that decodes year 0 is not safely represented: the subsequent start-of-year calculation attempts to encode year 0 and can raise EConvertError instead of returning zero outputs.
• The script exposes Delphi out semantics as var; incoming output-variable contents are ignored.
• Free Pascal documents the same outputs. Velox uses the installed Delphi helper chain and its exact invalid-year behaviour.

Side effects

Only AYear and ADayOfYear are changed.

Performance and concurrency

Constant-time decode, encode and arithmetic with no shared state.

Related entries

• DayOfTheYear returns only the ordinal.
• DecodeDate returns calendar month and day instead.
• DecodeDateWeek uses ISO week-numbering fields.

External references

• Embarcadero System.DateUtils.DecodeDateDay - documents the exact Delphi outputs.
• Free Pascal DecodeDateDay - documents the compatible Free Pascal operation.

Created 2026-07-15