public function toNIS()
    {
        if ($this->getAttribute("timeStamp")) {
            // from NIS timestamp
            $ts = $this->getAttribute("timeStamp");
            if (! empty($ts))
                return $ts;

            return $this->diff("now", static::$nemesis);
        }
        else {
            // from UTC timestamp
            $utc = $this->getAttribute("utc");
            return $this->diff($utc, static::$nemesis);
        }
    }