Import date of a certificate into keychain?
Keychain Dump

There is no automatic history of the Keychain kept on Mac OS X. You can see everything stored within a keychain file using the following command:

security dump-keychain ~/Library/Keychains/login.keychain


authd.log

A rotating log of authentication related actions is kept. This may have logged the granting of rights needed to import the certificate. It is unlikely but the only place Mac OS X, by default, logs authorization and authentication related activity.

You can see the log using Console.app within the /var/log/authd.log range of files.