Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

function LastDayOfCurrentMonth: TDate;
 var
   y, m, d: Word;
 begin
   DecodeDate(now, y, m, d);
   m := m + 1;
   if m  12 then
   begin
     y := y + 1;
     m := 1;
   end;
   Result := EncodeDate(y, m, 1) - 1;
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   ShowMessage(DateToStr(LastDayOfCurrentMonth));
 end;