Not sure about antedating or postdating the note, but you have to consider, what is actually the time of publication. In a centralized service. it's either the time the client sends the request, or more likely the time, when the server receives the request. You can however publish the same note for example a week apart to two different relays. What is the time of publication of that note in that case?

You could argue that it would be the first publishing recorded by any relay anywhere. Then the question is, how would the other relays know that?