public function getDiffMessage($count, $invert, $unit)
    {
        if (0 === $count) {
            throw new \InvalidArgumentException('The count must not be null.');
        }

        $unit = strtolower($unit);

        if (!in_array($unit, array('year', 'month', 'day', 'hour', 'minute', 'second'))) {
            throw new \InvalidArgumentException(sprintf('The unit \'%s\' is not supported.', $unit));
        }

        return $this->doGetDiffMessage($count, $invert, $unit);
    }