protected function _readTimestamp()
    {
        $data = array();
        if (empty($data) && file_exists($this->path . static::BUILD_TIME_FILE)) {
            $data = file_get_contents($this->path . static::BUILD_TIME_FILE);
            if ($data) {
                $data = unserialize($data);
            }
        }
        return $data;
    }