// JavaScript Document

function calcDays(){
  var date1 = document.getElementById('res[from_date]').value;
  var date2 = document.getElementById('res[to_date]').value;
  date1 = date1.split("-");
  date2 = date2.split("-");
  var sDate = new Date(date1[1]+"/"+date1[0]+"/"+date1[2]);
  var eDate = new Date(date2[1]+"/"+date2[0]+"/"+date2[2]);
  var daysApart = Math.abs(Math.round((sDate-eDate)/86400000));
  document.getElementById('res[nights]').value = daysApart;
}

onload=calcDays;