private function getModifiedSinceHeaderValue(CacheItemInterface $cacheItem)
    {
        $data = $cacheItem->get();
        // The isset() is to be removed in 2.0.
        if (!isset($data['createdAt'])) {
            return;
        }

        $modified = new \DateTime('@'.$data['createdAt']);
        $modified->setTimezone(new \DateTimeZone('GMT'));

        return sprintf('%s GMT', $modified->format('l, d-M-y H:i:s'));
    }