Use the following command to check the expiry date of a SSL/TLS certificate. This command also includes SNI (server name indication)
echo | openssl s_client -connect techoverflow.net:443 2>&1 | sed --quiet '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' | openssl x509 -noout -text | grep --after 2 Validity
Remember to replace both instances of techoverflow.net by the domain name to test!
Example output:
Validity
Not Before: Aug 18 00:00:00 2021 GMT
Not After : Aug 17 23:59:59 2022 GMT