daqing@MBP14 ~ % curl -vvv https://gems.ruby-china.com
* Host gems.ruby-china.com:443 was resolved.
* IPv6: (none)
* IPv4: 113.105.142.52, 113.125.220.190, 183.60.163.147, 140.249.101.68, 58.42.59.104
*   Trying 113.105.142.52:443...
* Connected to gems.ruby-china.com (113.105.142.52) port 443
* ALPN: curl offers h2,http/1.1
* (304) (OUT), TLS handshake, Client hello (1):
*  CAfile: /etc/ssl/cert.pem
*  CApath: none
* (304) (IN), TLS handshake, Server hello (2):
* (304) (IN), TLS handshake, Unknown (8):
* (304) (IN), TLS handshake, Certificate (11):
* SSL certificate problem: certificate has expired
* Closing connection
curl: (60) SSL certificate problem: certificate has expired
More details here: https://curl.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.