public function getLastCommitDate()
    {
        $process = new Process('git log -n1 --pretty=%ci HEAD', $this->directory);

        if ($process->run() != 0) {
            throw new \RuntimeException('Can\'t run git log. You must ensure that git binary is available.');
        }

        $date = new \DateTime(trim($process->getOutput()));
        $date->setTimezone(new \DateTimeZone('UTC'));

        return $date->format('Y-m-d H:i:s');
    }