src/Order.php
public function expirationPeriod($expirationPeriod = null)
    {
        if ($expirationPeriod !== null) {
            $this->expirationPeriod = new \DateInterval($expirationPeriod);
        }

        return $this->expirationPeriod;
    }
public function expirationPeriod($expirationPeriod = null)
    {
        if ($expirationPeriod !== null) {
            $this->expirationPeriod = new \DateInterval($expirationPeriod);
        }

        return $this->expirationPeriod;
    }
Time interval (ISO 8601 / RFC 3339)
@param string $expirationPeriod
@return \DateInterval|null