If you need a quick way to determine when a certain SSL certificate expires you can utilize following approaches. In both examples server I am trying to check is called webserver.domain.com.
If you have Nagios plugins installed you could type
# /usr/lib/nagios/plugins/check_http -p 443 -S -C 15 webserver.domain.com
CRITICAL - Certificate expired on 11/01/2009 11:23.
That’s easy. However what if you don’t have Nagios plugins. In that case you can do the same with OpenSSL and s_client. Look for notAfter field.
# echo | openssl s_client -connect webserver.domain.com:443 | openssl x509 -noout -dates
...
notBefore=Nov 1 11:23:30 2008 GMT
notAfter=Nov 1 11:23:30 2009 GMT
Easy :-).