code: public function setLastModified(Request $request, \DateTime $lastModified)
      {
          $this->cache->save($this->getCacheKeyRequest(sha1($request->getUri()), 'lastmodified'), $lastModified->format('r'));
  
          foreach ($this->itemIds as $itemId => $bool) {
              $key                             = $this->getCacheKeyItem($itemId, 'uri');
              $urisForItem                     = Option::fromValue($this->cache->fetch($key), false)->getOrElse(array());
              $urisForItem[$request->getUri()] = $bool;
              $this->cache->save($key, $urisForItem);
              Option::fromValue($this->logger)->map(function (LoggerInterface $logger) use ($request, $itemId) {
                  $logger->debug(sprintf('[ContentfulBundle:RequestLastModifiedCache] "%s" is used on "%s".', $itemId, $request->getUri()));
              });
          }
      }
docs: Stores the last modified date for a request.
  
  @param Request   $request
  @param \DateTime $lastModified