See who granted a role in Postgres
PostgreSQL doesn't register the timestamp when a privilege was granted. The semantics would be unclear: what should the time be when you restore a dump? The time the privilege was originally granted or the time of the restore?

Anyway, you can get the rest of the information from the system catalogs:

SELECT roleid::regrole, member::regrole, grantor::regrole
FROM pg_catalog.pg_auth_members;