When the server receives a logical time off the wire which is older than its current time, it does not need to advance the clock. As such, the server does not need to validate the time's signature.

This will have the fortuitous side effect of preventing unsigned "internal" clusterTimes from being validated, if an instance of the mongo shell acquires one after having authenticated as __system to perform administrative work. If it reauthenticates, and gossips an old time, the void signature will not fail validation unless it is somehow newer than what the server knew.