RedHat patch date?

Other than looking at the dates when pkgs were applied in /var/log/yum.log* how can I tell when the last time a RedHat system was patched? My systems don't have regular access to the web.

Thanks in advance.

---------- Post updated at 02:37 PM ---------- Previous update was at 02:27 PM ----------

One other thing I need is the installation date of the system.

I found rpm -qi basesystem for the install date. Is that the best option?