Event.observe(window, 'load', function() {
  showRecurringEndDate( $('recurring').checked );
  Event.observe($('recurring'), 'change', function() {
    showRecurringEndDate(this.checked);
  });
});

function showRecurringEndDate(checked) {
  if ( checked ) {
    $('recurringEndDate').show();
  } else {
    $('recurringEndDate').hide();
  }
}