Hi,
Maybe a communication gap here -- just want to check whether you and whocares are actually asking and answering the same question.
At one time -- and it's likely still the case, since I've never seen "official" word that it was changed -- avast would only change the "last-scan" date when you scanned using "local drives". Using "folders", even if you select the entire drive that way, won't update the last-scan info.
Best,
Mike