calculo de data com datepicker em JS

const milis_day = 1000 * 60 * 60 * 24;

$(function() {
    $('input[name="daterange"]').daterangepicker();
});
$('input[name="daterange"]').on('apply.daterangepicker', function(ev, picker) {
  var pick1 = new Date(picker.startDate.format('YYYY-MM-DD')); // primeira data
  var pick2 = new Date(picker.endDate.format('YYYY-MM-DD')); // segunda data
  var utc1 = Date.UTC(pick1.getFullYear(), pick1.getMonth(), pick1.getDate());
  var utc2 = Date.UTC(pick2.getFullYear(), pick2.getMonth(), pick2.getDate());
  
  var diff = Math.floor((utc2 - utc1) / milis_day)
  $('#qtdDias').val(diff);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script type="text/javascript" src="//cdn.jsdelivr.net/momentjs/latest/moment.min.js"></script>
<link rel="stylesheet" type="text/css" href="//cdn.jsdelivr.net/bootstrap/3/css/bootstrap.css" />
 
<!-- Include Date Range Picker -->
<script type="text/javascript" src="//cdn.jsdelivr.net/bootstrap.daterangepicker/2/daterangepicker.js"></script>
<link rel="stylesheet" type="text/css" href="//cdn.jsdelivr.net/bootstrap.daterangepicker/2/daterangepicker.css" />



<input type="text" name="daterange" value="" />
<br><br>Dias de diferença: <br>
<input id="qtdDias" readonly disabled>