DayOfTheWeek

Function DayOfTheWeek(const AValue: TDateTime): Word

Example

procedure ScriptEvent(var Value: variant);
begin
  Value := DayOfTheWeek(EncodeDate(2026, 7, 20)); // 1: Monday
end;

Usage

DayOfTheWeek returns the ISO weekday number for a TDateTime value, with Monday 1 and Sunday 7.

Parameters

Name	Type	Description
AValue	TDateTime, const	Date/time whose encoded calendar date supplies the weekday.

Returns

1 Monday, 2 Tuesday, 3 Wednesday, 4 Thursday, 5 Friday, 6 Saturday or 7 Sunday.

Behaviour

Weekday numbering is fixed and independent of the process locale's preferred first day of week. It matches the numbering returned by DecodeDateWeek, DecodeDateMonthWeek and DecodeDayOfWeekInMonth.

Errors

Valid dates do not raise. Timestamp conversion failures from invalid numeric values propagate.

Additional Technical Info

DayOfTheWeek returns an ISO-8601-style weekday number: Monday is 1 and Sunday is 7. This numbering is used by Delphi's ISO week and month-week functions.

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.DayOfTheWeek. It converts the value to a Delphi timestamp, then calculates (TimeStamp.Date - 1) mod 7 + 1. The time-of-day field is not used.

Edge cases and quirks

• Do not confuse this with DayOfWeek, where Sunday is 1. The two functions return different numbers for every weekday; never infer the numbering convention from the similar names alone.
• The date is interpreted exactly as encoded; no timezone conversion is performed before finding the weekday.
• Unsupported raw values outside Delphi's timestamp domain are not validated and modulo behaviour for them is not a supported contract.
• Free Pascal's DateUtils function follows the same Monday-first concept.

Side effects

None.

Performance and concurrency

Constant-time timestamp conversion and modulo arithmetic with no shared state.

Related entries

• DayOfWeek uses Sunday-first Delphi/SysUtils numbering.
• DecodeDateWeek returns ISO week-year, week and this weekday convention together.
• DecodeDayOfWeekInMonth returns the weekday occurrence within a calendar month.

External references

• Embarcadero System.DateUtils.DayOfTheWeek - documents the exact ISO numbering used by Delphi.
• Free Pascal DayOfTheWeek - documents the compatible Free Pascal routine.

Created 2026-07-15