GTM - Days between 2 dates

function() {
  var arrival = new Date({{dlv - ArrivalDt}}).toISOString();
  var booked = new Date({{dlv - BookDt}}).toISOString();
  var daysbetween = Math.floor((Date.parse(arrival) - Date.parse(booked)) / 86400000);
  return daysbetween;
}