public function formatDifference(Verta $v = null)
    {
        $difference = $this->diffSeconds($v);
        $absolute = $difference < 0 ? self::POST : self::PRE;
        $difference = abs($difference);

        for ($j = 0; $difference >= static::$unitNumber[$j] and $j < count(static::$unitNumber) - 1; $j++) {
            $difference /= static::$unitNumber[$j];
        }
        $difference = intval(round($difference));

        if($difference === 0) {
            return self::NOW;
        }

        return sprintf('%s %s %s', $difference, static::$unitName[$j], $absolute);
    }