protected function getDurationFormat(array $config)
    {
        if (isset($config['durationFormat'])) {
            if (!is_string($config['durationFormat'])) {
                throw new \DomainException(
                    'durationFormat must reference a string',
                    self::ERR_INVALID_DURATIONFORMAT
                );
            }
            return $config['durationFormat'];
        }
        return '%im%ss';
    }