protected function getModifiedTimestamp(array $jsonld)
    {
        $modified = $this->getFirstPredicate(
            $jsonld,
            $this->modifiedDatePredicate
        );

        if (empty($modified)) {
            throw new \RuntimeException(
                "Could not parse {$this->modifiedDatePredicate} from " . json_encode($jsonld),
                500
            );
        }

        $date = \DateTime::createFromFormat(
            \DateTime::W3C,
            $modified
        );

        return $date->getTimestamp();
    }