var TODAY = new Date().getTime();

function checkDate(aSource) {
  var form = document.getElementById('reservation-form');

  var add = new Date(form.arrivalMonth.value + '/' + form.arrivalDay.value + '/' + form.arrivalYear.value);
  var ad = add.getTime();

  var n = form.numberOfNights.value * 86400000;
  var dd = ad + n;

  var arrivalDate = new Date(ad);
  form.arrivalDay.value = arrivalDate.getDate();
  form.arrivalMonth.value = arrivalDate.getMonth() + 1;
  form.arrivalYear.value = arrivalDate.getFullYear();

  var departureDate = new Date(dd);
  form.departureDayDisplay.value = departureDate.getDate();
  form.departureMonthDisplay.value = departureDate.getMonth() + 1;
  form.departureYearDisplay.value = departureDate.getFullYear();

  form.departureDay.value = departureDate.getDate();
  form.departureMonth.value = departureDate.getMonth() + 1;
  form.departureYear.value = departureDate.getFullYear();
}