I'm not sure why it would give a date error, but as it only happens with Google and Facebook this could be related to an SSL decryption issue. These sites, plus a growing number of others, use a TLS extension called SPDY. Prior to version 5.9, the firewall could not decrypt such TLS traffic.
If running an older version it might be a good idea to first upgrade to 5.10. This requires that SMC is running 5.10 too.
We are using 5.9.2 .
I'd check the certificate, is it issued by the client protection CA defined on firewall or something else. If it's something else, it could even be caused by Avast or Kaspersky etc doing some HTTPS inspection in the PC itself.
Might be best to open a support ticket if it's not caused by some software on the client PC, though in that case 5.10.2 would be preferable to 5.9.2 so you might want to upgrade first.