public function listGists(string $username = null, string $since = '1970-01-01'): array
    {
        $url = '/gists';
        if (null !== $username) {
            $url = '/users/:username/gists';
        }

        return $this->getApi()->request($this->getApi()->sprintf(':url?:arg', $url, $username,
            http_build_query(['since' => (new DateTime($since))->format(DateTime::ATOM)])));
    }