dates-are-crazy.js

(function ($) {
  Drupal.behaviors.datePicker = {
    attach: function (context, settings) {
      var $fieldDate = $('form-item-attributes-field-date', context);
      var $fieldDateSelect = $fieldDate.find('select', context);
      var $fieldDateOptions = $fieldDate.find('option', context);

      // Insert datepicker div
      $('<div id="datepicker" />').insertBefore('.form-item-attributes-field-date');

      // Get array of valid dates
      var validDates = [];
      $fieldDateOptions.each(function () {
        validDates.push($(this).text());
      });

      $("#datepicker").datepicker({
        dateFormat: "dd/mm/yy",
        defaultDate: $fieldDate.find("option:selected").text(),
        beforeShowDay: function (date) {
          var string = jQuery.datepicker.formatDate('dd/mm/yy', date);
          return [validDates.indexOf(string) != -1]
        },
        onSelect: function (dateText, inst) {
          var selected = $fieldDateOptions.filter(function () {
            return $(this).html() == dateText;
          }).val();
          $fieldDateSelect.val(selected).trigger("change");
        }
      });
    }
  };
})(jQuery);