function addDays(date, dayAdd) {
	millisec = (dayAdd) * 86400000;
	newTime = date.getTime() + millisec;
	return new Date(newTime);
}

function setLastDay() {
  var date_ini_str = document.getElementById('arr_date').value;
  var date_end_str = document.getElementById('dep_date').value;
  var date_ini=date_ini_str.split("-");
  var date_end=date_end_str.split("-");
	var dataStart = new Date(date_ini[0],date_ini[1]-1,date_ini[2],12);

  dataEnd =  new Date();
  dataEnd = addDays (dataStart, Number(document.getElementById('nights').value));
  var d  = dataEnd.getDate();
  var day = (d < 10) ? '0' + d : d;
  var m = dataEnd.getMonth() + 1;
  var month = (m < 10) ? '0' + m : m;
  document.getElementById('dep_date').value = dataEnd.getFullYear()+"-"+month+"-"+day;

	return true;
}