The date you are looking at is when the last manual system scan was run, avast! doesn't have a set daily scan with the free version, though one can be set up using the windows task scheduler.
The best option is to activate the avast Screen Saver, open Control panel/Display properties, then select the Screen Saver tab and make sure that avast! antivirus is selected from the dropdown option menu and OK it. It will now run a sequential scan every time the screen saver runs.