There are two basic approaches to do this.
1. There is the Patch-o-Matic netfilter extension 'time' that allows you to match packets based on time of arrival. If you are not familiar with POM, it requires a kernel recompilation in order to incorperate the desired netfilter patches. Check out the HOWTO and the description of the 'time' extension:
2. Second way use to use two seperate iptables scripts that are run via cron jobs. Think "script A is run at 8 am, script B is then run at 6pm". Probably easier to implement if you just want something simplistic. If you have something very complex in mind, then POM will probably be the better option.