jQuery UI Datepicker - Modify another datepicker based on the difference of the last value and newly selected value

gistfile1.js

$('#form_starts_at').datepicker('option', 'onSelect', function(dateText, inst) {
  var then = $.datepicker.parseDate(inst.settings.dateFormat, inst.lastVal);
  var now = $.datepicker.parseDate(inst.settings.dateFormat, dateText);
  var distance = now.getTime() - then.getTime();
  var ends_at = $("#form_ends_at").datepicker('getDate')
  ends_at.setTime(ends_at.getTime() + distance);
  $("#form_ends_at").datepicker('setDate', ends_at);
});