The certificate for mc.sdf.org expired on 12/11/2023

Since its not yet 12/11/2023 (presuming MM/DD/YYYY) your system clock may be set incorrectly, though I'd expect you to break all SSL sites if that were the case. Also, examining the certificate right now shows that it doesn't expire in December (or November) at all...

> openssl s_client -connect mc.sdf.org:443 -servername mc.sdf.org 2> /dev/null | openssl x509 -noout -dates
notBefore=Oct 13 14:47:57 2023 GMT
notAfter=Jan 11 14:47:56 2024 GMT