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

uses
   CommCtrl;

 type
   THackCommonCalendar = class(TCommonCalendar);

   {...}

 procedure TForm1.DateTimePicker1DropDown(Sender: TObject);
 var
   Style: Integer;
   ReqRect: TRect;
   MaxTodayWidth: Integer;
 begin
   with THackCommonCalendar(Sender as TDateTimePicker) do
   begin
     // set style to include week numbers 
    Style := GetWindowLong(CalendarHandle, GWL_STYLE);
     SetWindowLong(CalendarHandle, GWL_STYLE, Style or MCS_WEEKNUMBERS);
     FillChar(ReqRect, SizeOf(TRect), 0);
     // get required rect 
    Win32Check(MonthCal_GetMinReqRect(CalendarHandle, ReqRect));
     // get max today string width 
    MaxTodayWidth := MonthCal_GetMaxTodayWidth(CalendarHandle);
     // adjust rect width to fit today string 
    if MaxTodayWidth > ReqRect.Right then
       ReqRect.Right := MaxTodayWidth;
     // set new height & width 
    SetWindowPos(CalendarHandle, 0, 0, 0, ReqRect.Right, ReqRect.Bottom,
       SWP_NOACTIVATE or SWP_NOMOVE or SWP_NOZORDER);
   end;
 end;