Classic Shell's installer is digitally signed so the user can be sure it is not tampered with. But one of the certificates for the countersignatures expired on 11th May, 2015. That is why you are getting the message. It is safe to ignore it and still install it (as long as you downloaded the installer from the official website,
http://www.classicshell.net (which redirects to mediafire.com).
If you set the date back to earlier than May 11, 2015, then it will show no errors and say the digital signature is OK.