If you are getting the date from DatePicker, then try the below:
var fromDate = $('#date1').datepicker('getDate');
var toDate = $('#date2').datepicker('getDate');
var diff = 0;
if (fromDate && toDate) {
diff = Math.floor((toDate.getTime() - fromDate.getTime()) / 86400000);
}
console.log(diff);