JSP上計算兩個日期之間的天數

<script>
   function changeTimelyFlag(){
     var date1 = GetXFormFieldValueById('fd_arrival_date');    /*到達日期id*/
     var date2 = GetXFormFieldValueById('fd_current_date');    /*當前日期id*/
var dateStartMillons = new Date(date2).getTime();
var dateEndMillons = new Date(date1).getTime();
var difDays = (dateEndMillons - dateStartMillons) / (24 * 60 * 60 * 1000);
//計算出兩個日期的差值。
     if(difDays>=7){
       SetXFormFieldValueById('fd_timely_yes_no', "是");
     }
     else
       SetXFormFieldValueById('fd_timely_yes_no', "否");
   }
/*到達日期變化事件*/
AttachXFormValueChangeEventById('fd_arrival_date', changeTimelyFlag ); 
/*到貨日期*/
AttachXFormValueChangeEventById('fd_delivery_date', changeTimelyFlag ); 
</script>