关于定时刷新窗口的问题

我做了一个简单的有背景图片的时钟显示的小程序，定时的刷新STATIC窗口
代码如下void CClockShow::OnTimer(UINT nIDEvent) 
{
// TODO: Add your message handler code here and/or call default
    //背景重绘
    CDC *dc;
    dc=GetDC();
     CDC dcMem; 
     dcMem.CreateCompatibleDC(dc); 
     CBitmap bmpBackground;
     bmpBackground.LoadBitmap(IDB_CLOCK);//载入背景图片
     dcMem.SelectObject(bmpBackground);
     dc->BitBlt(0,0,119,88,&dcMem,0,0,SRCCOPY);//???
    
     //时间重绘    
     CString time;
      SYSTEMTIME sysTime;
  GetLocalTime(&sysTime);
      dc->SetBkMode(TRANSPARENT);
  time.Format("%02d:%02d:%02d",sysTime.wHour, 
  sysTime.wMinute, sysTime.wSecond);
      CFont font;
  font.CreatePointFont (140,_TEXT("宋体"));
  dc->SelectObject (&font);
      dc->TextOut(25,60,time);
   time.Format("星期");
   switch(sysTime.wDayOfWeek)
   {
   case 0:
          time+="日";
  break;
  case 1:
          time+="一";
  break;
  case 2:
          time+="二";
  break;
  case 3:
          time+="三";
  break;
  case 4:
          time+="四";
  break;
  case 5:
          time+="五";
  break;
  case 6:
          time+="六";
  break;
   }
   dc->TextOut(30,35,time);
   time.Format("%d-%d-%d",sysTime.wYear,sysTime.wMonth,sysTime.wDay);
            dc->TextOut(20,10,time);
   CStatic::OnTimer(nIDEvent);
}这个程序运行20分钟以后就会弹出错误对话框！ 请教各位高人可能是什么原因？？