public function extendExpiration()
    {
        if (null === $this->ttl) {
            throw new DomainException('There is no TTL set for this Lock.');
        }

        if (!$this->expiresAt) {
            $this->expiresAt = new \DateTime();
            $this->expiresAt->setTimestamp(time());
        }

        $this->expiresAt->add($this->ttl);
    }