private function mapTimestamp(Document $document, \DateTime $created = null, \DateTime $changed = null)
    {
        $document->addField(
            $this->factory->createField('created', $created ? $created->format('c') : null, 'string')
        );

        $document->addField(
            $this->factory->createField('changed', $changed ? $changed->format('c') : null, 'string')
        );
    }