Cannot connect to the real github.com in Chrome
You need to delete the expired certificate from your login keychain, and restart Chrome.
Here is the DigiCert link that explains how to do it.
https://blog.digicert.com/expired-intermediate-certificate/
On July 26,2014 at 12:15 PM, some customers and users on sites secured by DigiCert reported that Mac OS X systems were experiencing certificate chain errors due to what appears to be a bug in how the Apple OS handles expired intermediate certificates in the SSL trust chain. Customers on Windows and other platforms have not reported any issues. The problem actually affect any client platform with a locally cached intermediate certificate.