Thanks, Eddy,
Avast Home. It's just updated to 4.5.549 (I have rebooted since update).
I did not uninstall/reinstall. This is the first time I have installed it (at least on this machine).
The date and time are currently correct, but it might have been incorrect ( a month out) when I installed it, though I can't be sure. Perhaps that is it.
If you think that this is not normal behaviour, I will investigate further.
If you have any suggestions, I would be grateful.