If you happen to have missed it, try this - right click on the avast tray icon and select 'Show last popup message.'
If you haven't had another popup since that one or have rebooted the option will be greyed out as it is in the attached image.
this is the screenshot of the most recent message. Thanks for the tip on how to bring up the pop up again.
You're welcome.
Strange that has a problem with date, as the main issue can be your system time/dat being correct, but your image shows it appears to be correct.
Try an avast Repair, Control Panel > Programs and Features > Avast Antivirus > Change - From the new window select Repair and OK your way out, reboot. This has resolved issues with dates being out of sync.
Obviously if that doesn't work re-registering as suggested by Asyn isn't much hassle. Probably quicker than the Repair.