function LastDayCurrMon: 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(LastDayCurrMon));
end;