function WeekOfYear(ADate : TDateTime) : word;
var
day : word;
month : word;
year : word;
FirstOfYear : TDateTime;
begin
DecodeDate(ADate, year, month, day);
FirstOfYear := EncodeDate(year, 1, 1);
Result := Trunc(ADate - FirstOfYear) div 7 + 1;
end;
// Kullanımı:
procedure TForm1.Button1Click(Sender: TObject);
begin
ShowMessage(IntToStr(WeekOfYear(Date)));
end;