You can get patch-o-matic from the Netfilter website, but it is not trivial to implement. IIRC it requires a kernel recompilation to allow patch-o-matic support, (though it may just need to be compiled against the kernel headers, I'm not 100% sure). If you are trying to setup a relatively simple time-based ruleset, I would use cron (as Hangdog42 suggested) to just run two or more different iptables scripts. If you have a very complex time-based ruleset, the POM might be worth the effort of setting it up.
Newest patch-o-matic link: