finde sidste dag i måned

function DaysInMonth: Integer;
var
  Year, Month, Day: Word;
begin
  DecodeDate(Now, Year, Month, Day);
  Result := MonthDays[IsLeapYear(Year), Month];
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(IntToStr(DaysInMonth));
end;