/**
  * @static
  * @param Google\Http\Request $resp
  * @return bool True if the HTTP response is considered to be expired.
  * False if it is considered to be fresh.
  */
 public static function isExpired(Request $resp)
 {
     // HTTP/1.1 clients and caches MUST treat other invalid date formats,
     // especially including the value “0”, as in the past.
     $parsedExpires = false;
     $responseHeaders = $resp->getResponseHeaders();
     if (isset($responseHeaders['expires'])) {
         $rawExpires = $responseHeaders['expires'];
         // Check for a malformed expires header first.
         if (empty($rawExpires) || is_numeric($rawExpires) && $rawExpires <= 0) {
             return true;
         }
         // See if we can parse the expires header.
         $parsedExpires = strtotime($rawExpires);
         if (false == $parsedExpires || $parsedExpires <= 0) {
             return true;
         }
     }
     // Calculate the freshness of an http response.
     $freshnessLifetime = false;
     $cacheControl = $resp->getParsedCacheControl();
     if (isset($cacheControl['max-age'])) {
         $freshnessLifetime = $cacheControl['max-age'];
     }
     $rawDate = $resp->getResponseHeader('date');
     $parsedDate = strtotime($rawDate);
     if (empty($rawDate) || false == $parsedDate) {
         // We can't default this to now, as that means future cache reads
         // will always pass with the logic below, so we will require a
         // date be injected if not supplied.
         throw new Exception("All cacheable requests must have creation dates.");
     }
     if (false == $freshnessLifetime && isset($responseHeaders['expires'])) {
         $freshnessLifetime = $parsedExpires - $parsedDate;
     }
     if (false == $freshnessLifetime) {
         return true;
     }
     // Calculate the age of an http response.
     $age = max(0, time() - $parsedDate);
     if (isset($responseHeaders['age'])) {
         $age = max($age, strtotime($responseHeaders['age']));
     }
     return $freshnessLifetime <= $age;
 }