Comaprison of two dates that are selected from date picker

    Here I selected two dates using date picker and want to check the start date must be less than end date.

        var startDate = $("#startDate").val();
        var endDate = $("#endDate").val();

        startDate = $.datepicker.parseDate('mm/dd/yy', startDate);
        endDate = $.datepicker.parseDate('mm/dd/yy', endDate);

        var difference = (endDate - startDate) / (86400000);
        alert(difference)
        if (difference < 0 && flag == 0) {
            flag = 1;
            alert("The start date must come before the end date.");        
        }