For safety critical systems, one strategy is to store at least two copies of important data and compare them regularly. If they don't match, you either try to recover somehow or go into a safe state, depending on the context.
Seems like chronometers would be a case where two are better than one, because the mistakes are analog. If they don't exactly agree, just take the average. You'll have more error than if you were lucky enough to take the better chronometer, but less than if you had taken only the worse one. Minimizing the worst case is probably the best way to stay off the rocks.
