Depending to-date with jQuery datepicker

date_popup.js

(function($) {
  Drupal.behaviors.date_popup_custom = {};

  Drupal.behaviors.date_popup_custom.attach = function(context, settings) {
    // Add departure date same as arrival date
    $('.form-item-d-departure-date').find('input').click(function() {
      var arrival_date = $('.form-item-d-arrival-date').find('input').val();
      $('.form-item-d-departure-date').find('input').datepicker( "setDate" , arrival_date );
    });
  };
})(jQuery);