sometimes you need to quickly check an endpoint or site and grab the dates/ when a certificate expires. Of course you could just log onto the server in question and inspect the certificate or try and coax your browser into letting you view the certificate properties. That said this is often quicker and could be handy as part of a larger script, check or automation piece.
This quick one liner will show us when the certificate on centos.org is valid for, just change the site name and port to that of the endpoint you want to check.
[root@tst-sh01~]# echo | openssl s_client -connect centos.org:443 2>/dev/null | openssl x509 -noout -dates
notBefore=Jul 29 00:00:00 2014 GMT
notAfter=Aug 2 12:00:00 2017 GMT