code: public function getLastModified(Request $request)
      {
          $minModified          = $this->getLastMinModifiedDate();
          $optionalLastModified = Option::fromValue($this->cache->fetch($this->getCacheKeyRequest(sha1($request->getUri()), 'lastmodified')), false);
          if ($optionalLastModified->isEmpty()) {
              return $minModified;
          }
          return max($minModified, new \DateTime($optionalLastModified->get()));
      }
docs: Returns the last modified date for a request.
  
  @param Request $request
  
  @return \DateTime