Calculating the number of days between two dates on JSP

<script>
   function changeTimelyFlag(){
     var date1 = GetXFormFieldValueById('fd_arrival_date');    /*到达日期id*/
     var date2 = GetXFormFieldValueById('fd_current_date');    /*当前日期id*/
var dateStartMillons = new Date(date2).getTime();
var dateEndMillons = new Date(date1).getTime();
var difDays = (dateEndMillons - dateStartMillons) / (24 * 60 * 60 * 1000);
//Calculate the difference between the two dates. 
     IF (difDays> = . 7 ) { 
       SetXFormFieldValueById ( ' fd_timely_yes_no ' , " is " ); 
     } 
     the else 
       SetXFormFieldValueById ( ' fd_timely_yes_no ' , " NO " ); 
   } 
/ * Arrival Date change event * / 
AttachXFormValueChangeEventById ( ' fd_arrival_date ' , changeTimelyFlag); 
 / * arrival date * / 
AttachXFormValueChangeEventById ( ' fd_delivery_date ', changeTimelyFlag ); 
</script>