procedure TForm1.DateTimePicker1DropDown(Sender: TObject);
var
  dwstyle: dword;
  mch: THandle;
  rct: TRect;
begin
  mch := SendMessage((Sender as TDateTimePicker).Handle, DTM_GETMONTHCAL, 0,0);
  dwStyle := GetWindowLong(mch, GWL_STYLE);

  // add this line if you want to see week numbers in the dropdown calendar
  dwStyle := dwStyle or MCS_WEEKNUMBERS;
  // add this line if you do not want to see the circle before today's date in the dropdown calendar
  dwStyle := dwStyle or MCS_NOTODAYCIRCLE;
  // add this line if you do not want to see today's date in the dropdown calendar
  dwStyle := dwStyle or MCS_NOTODAY;

  SetWindowLong(mch, GWL_STYLE, dwStyle);
  // adapt the size of the dropdown calendar as the default size is a bit too small
  SendMessage(mch, MCM_GETMINREQRECT, 0, integer(@rct));
  MoveWindow(mch, 0, 0, rct.Right + 2, rct.Bottom + 2, True);
end;