Time discrepancies don’t affect the NFS protocol itself, they affect the behaviours which depend on timestamps. Before NFSv3, the NFS server was solely in charge of maintaining timestamps; NFSv3 introduced client-controlled timestamps, but they’re not used in all cases and don’t solve all the problems anyway.
The traditional issue is that when a client updates a file, the timestamp is set by the server. If the server’s time isn’t the same as the client’s, the client will then see a timestamp which it doesn’t expect; this causes problems with programs which rely on timestamps, such as Make, many mail clients, or even download programs. With Make for instance, if the server is in the client’s future, then built artifacts will appear newer than they really are, and might not be rebuilt when necessary.
Client-controlled timestamps fix some of that, but not all instances, in particular if different clients are working on shared files and aren’t synchronised themselves.