The getClusterTimeForReplicaSet() function returns a "clusterTime" of 0 if the mongod is not running in a replica set. However, now that the VectorClock is always present, even on standalones and in unittests where it is never ticked/advanced, it should be possible to remove the getClusterTimeForReplicaSet() function and its few uses, and just use the actual VectorClock clusterTime instead (since it should always be 0).