js比较年月日时分秒时间大小

var beginTime = "2015-09-21 00:00:33"; 

var endTime    = "2015-09-21 00:00:33";  

    var beginTimes =beginTime.replace(/-/g,"/");
    var endTimes   =endTime.replace(/-/g,"/");
      
    alert(beginTimes);  //2015/09/21 00:00:33
    alert(endTimes);  //015/09/21 00:00:33
    var a =(Date.parse(endTimes)-Date.parse(beginTimes))/3600/1000;
   
    alert(a);  
    if(a<0){  
        alert("endTime小!");  
    }else if (a>0){  
        alert("endTime大!");  
    }else if (a==0){  
        alert("相等!");  
    }