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StartOfAWeek

Function StartOfAWeek( const AYear, AWeekOfYear : Word; const ADayOfWeek : Word) : TDateTime

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := StartOfAWeek(2026, 29, 4); // Thursday 16 July 2026
end;

Usage​

StartOfAWeek constructs midnight for a supplied ISO week-year, week number and ISO weekday.

Parameters

NameTypeDescription
AYearWord, constISO week-year from 1 through 9999, subject to encodable boundary dates.
AWeekOfYearWord, constISO week number from 1 through 52 or 53 where that week-year contains week 53.
ADayOfWeekWord, constISO weekday: 1 Monday through 7 Sunday.

Returns

The encoded calendar date for the requested ISO week date at 00:00:00.000.

Behaviour

  • Week one follows ISO 8601 and can begin in the preceding calendar year.
  • Week 52 or 53 can extend into the following calendar year.
  • The supplied weekday is returned, not necessarily Monday.
  • The result is timezone-neutral and contains no current-clock state.

Errors

Invalid year/week/weekday combinations raise DateUtils EConvertError through the encode-week path.

Usage notes

Use StartOfTheWeek to derive Monday midnight from an existing date/time value.

Additional Technical Info​

StartOfAWeek constructs midnight for a specific day in an ISO 8601 week date. The parameters identify an ISO week-year, week number and Monday-one weekday.

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

Implementation

The DateUtils import binds to System.DateUtils.StartOfAWeek, which immediately calls EncodeDateWeek(AYear, AWeekOfYear, ADayOfWeek). That routine validates the ISO fields, resolves ISO week-year ownership and encodes the resulting Gregorian date.

Edge cases and quirks

  • Despite the name, the result is only the start of the whole week when ADayOfWeek = 1. Passing 2 through 7 returns midnight on the selected later weekday.
  • Delphi declares a default weekday of Monday, but Velox's exact script declaration requires all three arguments; scripts cannot rely on the omitted default.
  • Week 53 is accepted only for ISO week-years that actually contain it.
  • Free Pascal exposes a similar overload/default model; the explicit Velox declaration above controls script syntax.

Side effects

None.

Performance and concurrency

Constant-time ISO calendar arithmetic and encoding with no I/O or shared state.

Related entries

External references

Created 2026-07-15