Check your system date, especially the day, month and year, correct if required.
If it is correct, you might try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
I have seen this correct what seems to be an imbalance in the dates shown between what is reported in the about avast and what is displayed in the Simple User Interface.