var returnValue="";
function getDate(field,strBil,formname){
  var url ="js_lib/CalendarCn2.htm";
   var calWin = window.showModalDialog (url,window,"dialogHeight: 250px; dialogWidth: 406px;  center: Yes; help: No; resizable: No; status: No");                                                                                                  
   var e=new Date(returnValue);
   if(!isNaN(returnValue)){
     return false;
  }
  else{field.value=returnValue}
}