DateTimePicker是怎样得到焦点时，会自动选择4位年份？？？(100分)

program DateTimePicker;

uses
  Messages,
  Windows,
  CommCtrl,
  SysUtils,
  XPMan;

var
  WinClass: TWndClassA;
  hIns: HINST;
  hMainWnd: HWND;
  Msgs: TMsg;
  MainWndFont: HFONT;
  LogFont: TLogFont;
  dtpDateTime, btnClose, edtDate: HWND;



function MainWndProc(hMainWnd: HWnd; Msg, wParam, lParam: Integer): Integer;
  stdcall;
var
  SysTime: TSYSTEMTIME;
  SelDate: String;
begin
  Result := 0;

  case Msg of
    WM_COMMAND: begin
      if HWND(lParam) = btnClose then
        SendMessage(hMainWnd, WM_DESTROY, 0, 0);

      Exit;
    end;


    WM_NOTIFY: begin
      if TNMDATETIMECHANGE(Pointer(lParam)^).nmhdr.hwndFrom =
        dtpDateTime then begin
        MonthCal_GetCurSel(DateTime_GetMonthCal(dtpDateTime), SysTime);
        DateTime_GetSystemtime(dtpDateTime, SysTime);
        SelDate := IntToStr(SysTime.wYear) + '年' + IntToStr(SysTime.wMonth) +
          '月' + IntToStr(SysTime.wDay) + '日';
        SetWindowText(edtDate, PChar(SelDate));
      end;
    end;


    WM_DESTROY: begin
      DeleteObject(MainWndFont);
      PostQuitMessage(0);
      
      Exit;
    end;
    
  else
    Result := DefWindowProc(hMainWnd, Msg, wParam, lParam);
  end;
end;


begin
  hIns := hInstance;
  with WinClass do begin
    style         := CS_CLASSDC or CS_PARENTDC;
    lpfnWndProc   := @MainWndProc;
    hInstance     := hIns;
    hbrBackground := COLOR_BTNFACE + 1;
    lpszClassname := 'DateTimePicker';
    hIcon         := 0;
    hCursor       := LoadCursor(0, IDC_ARROW);
  end;
  RegisterClass(WinClass);

  hMainWnd := CreateWindowEx(WS_EX_WINDOWEDGE, 'DateTimePicker',
              PChar('DateTimePicker'),
              WS_MINIMIZEBOX or WS_SYSMENU,
              Round((GetSystemMetrics(SM_CXSCREEN) - 300) / 2),
              Round((GetSystemMetrics(SM_CYSCREEN) - 300) / 2),
              300, 300, 0, 0, hIns, nil);

  dtpDateTime := CreateWindow('SysDateTimePick32', '',
                 WS_VISIBLE or WS_CHILD or WS_TABSTOP,
                 8, 8, 140, 22, hMainWnd, 0, hIns, nil);

  edtDate := CreateWindowEx(WS_EX_CLIENTEDGE, 'Edit','',
             WS_CHILD or WS_VISIBLE or WS_TABSTOP or ES_AUTOHSCROLL,
             8, 36, 96, 20, hMainWnd, 0, hIns, nil);

  btnClose := CreateWindow('Button', '&Close',
              WS_VISIBLE or WS_CHILD or WS_TABSTOP or WS_GROUP or BS_PUSHBUTTON,
              220, 240, 65, 21, hMainWnd, 0, hIns, nil);

  if SystemParametersInfo(SPI_GETICONTITLELOGFONT, SizeOf(LogFont), @LogFont,
    0) then
    MainWndFont := CreateFontIndirect(LogFont)
  else
    MainWndFont := CreateFont(-12, 0, 0, 0, FW_NORMAL, 0, 0, 0, DEFAULT_CHARSET,
      0, 0, 0, 0, '宋体');
  if MainWndFont <> 0 then begin
    SendMessage(dtpDateTime, WM_SETFONT, MainWndFont, 0);
    SendMessage(edtDate, WM_SETFONT, MainWndFont, 0);
    SendMessage(btnClose, WM_SETFONT, MainWndFont, 0);
  end;

  ShowWindow(hMainWnd, SW_SHOW);
  UpdateWindow(hMainWnd);

  while GetMessage(Msgs, 0, 0, 0) do begin
    if not IsDialogMessage(hMainWnd, Msgs) then begin
      TranslateMessage(Msgs);
      DispatchMessage(Msgs);
    end;
  end;

end.