WeekOfTheYear的问题

WeekOfTheYear当月的第一天是星期天的时候,返回的周居然是上一周的??
procedure TForm1.Button1Click(Sender: TObject);
var
Year, Month, Day: Word;
begin
  DecodeDate(now, Year, Month, Day);
  showMessage(IntToStr(WeekOfTheYear(EncodeDate(Year, Month, 1))));
end;