This should do the trick
var start = $('#start_date').val();
var end = $('#end_date').val();
// end - start returns difference in milliseconds
var diff = new Date(end - start);
// get days
var days = diff/1000/60/60/24;
Example
var start = new Date("2010-04-01"),
end = new Date(),
diff = new Date(end - start),
days = diff/1000/60/60/24;
days; //=> 8.525845775462964
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