It appears as though what you want to do is block ICMP flood attacks or Smurf attacks.
If that is the case then your best option is an IPS. The AIP-SSM module is actually an IPS module that can be integrated into the ASA itself.
If however you do not wish to use an IPS then the next best option is threat detection on an ASA, but that was introduced only in 8.0, so you'll have to run 8.x code in order to use it. Which is why, you are unable to find the command in 7.0
Regarding time bases ACLs, there are such things, but they don't work the way you intend them to. A time based ACL kicks in at a certain time and can be removed after a certain period of time.