The OP could have also put in an existing Key that they had which would expire on the given expiration date, but without knowing what product they have (Free, Pro, AIS), we don't have enough information.
Since this was a clean install, if the product was the Free version and they got this problem with the dates (and the time/date on the machine is correct), I would suggest a reboot first. If that does not fix the problem, then try an Avast Repair:
- Go to Control Panel > Add/Remove programs > Avast Antivirus.
- Scroll down and choose Repair function in the pop-up window.
- Reboot.
If this fails to fix the problem, then we need to know the answer to the first sentence in my post before recommending an uninstall/clean install.