protected function _convertUnit($unit)
    {
        if ($unit === 'week') {
            return self::ONE_WEEK;
        }

        if ($unit === 'day') {
            return self::ONE_DAY;
        }

        if ($unit === 'hour') {
            return self::ONE_HOUR;
        }

        if ($unit === 'minute') {
            return self::ONE_MINUTE;
        }

        if ($unit === 'second') {
            return self::ONE_SECOND;
        }

        if ($unit === 'millisecond') {
            return self::ONE_MILLISECOND;
        }

        throw new BuildException("Illegal unit '$unit'");
    }