var a=new Date('2007/12/1');
   var b=new Date("2008/2/1");
   
   function monthDiff(startdate,stopdate){
        if(typeof (startdate)!='object' || typeof (stopdate)!='object')return; 
        if(startdate>stopdate)return;
                //自已判断输入值合法性
        var y=stopdate.getYear()-startdate.getYear();
        var m=stopdate.getMonth()-startdate.getMonth();
        
        if(y==0 && m<0) return;

        if(m>0){
            return 12*y+m;
        }
        if(m<0){
            return (12+m)+(y-1)*12
        }
   }
   
   alert(monthDiff(a,b)) //