There is another post on the same subject and it is said to be a bug as this should instead be some other alert and currently although it is set to a specific time it does not honor the settings at this time, I found it (and this post) looking for the solution as well. I assume an update will come out within the next few weeks (or less) that will address the problem.
This is the other topic with a little more info and responses. http://forum.avast.com/index.php?topic=137369.0