function DateMonthEnd(const DT: TDateTime): TDateTime;
var
  Day, Month, Year: Word;
  LastDay: Byte;
begin
  SysUtils.DecodeDate(DT, Year, Month, Day);
  LastDay := DaysInMonth(DT);
  Result := SysUtils.EncodeDate(Year, Month, LastDay);
end;
//delphi/2160