public function createUpdateXML() {
    $doc = $this->createDocument();

    $root = $doc->appendChild($doc->createElement($this->getNodeName()));

    foreach ($this->getUpdatableAttributes() as $attr) {
      $val = $this->$attr;

      if ($val instanceof DateTime) {
        $val = $val->format('c');
      }

      $root->appendChild($doc->createElement($attr, $val));
    }

    return $this->renderXML($doc);
  }