function createDuration() {
    if( !isset( $this->duration['value']['week'] ) &&
        !isset( $this->duration['value']['day'] )  &&
        !isset( $this->duration['value']['hour'] ) &&
        !isset( $this->duration['value']['min'] )  &&
        !isset( $this->duration['value']['sec'] ))
      return;
    $attributes = $this->_createParams( $this->duration['params'] );
    return $this->_createElement( 'DURATION', $attributes, $this->_format_duration( $this->duration['value'] ));
  }