code: private function formatExpires()
      {
          $format = CookieExpiresFormat::getInstance();
          $offset = Util::cleanTimeZoneOffset($this->timeZoneOffset);
          $date   = $format->format($this->expires, $offset);
          return "expires={$date}";
      }
docs: expires 属性を書式化します.
  
  @return string "expires=Wdy, DD-Mon-YY HH:MM:SS GMT" 形式の文字列