public function getEtag($visitor_identifier, $use_cache = true)
    {
        $timestamp_field = $this->getTimestampField();

        if ($timestamp_field && ($this->tag === false || !$use_cache)) {
            $this->tag = $this->prepareTagFromBits($this->getAdditionalIdentifier(), $visitor_identifier, $this->getTimestampHash($timestamp_field));
        }

        return $this->tag;
    }