js中截取时间段，精确到秒

function changeDiffDate(){
var s=document.forms[0].elements["record.START_DATE"].value;
var e=document.forms[0].elements["record.END_DATE"].value;
var beginTimes=s.substring(0,10).split('-'); 
var endTimes=e.substring(0,10).split('-');
var begin=beginTimes[1]+'-'+beginTimes[2]+'-'+beginTimes[0]+' '+s.substring(10,19); 
var end=endTimes[1]+'-'+endTimes[2]+'-'+endTimes[0]+' '+e.substring(10,19); 
var a =(Date.parse(end)-Date.parse(begin))/3600/1000;
if(a>12){
 alert("时间超过12小时，请确认时间是否正确!");
}
document.forms[0].elements["record.DIFFDATE"].value=a.toFixed(4);
}