Correct. The protocol itself allows the note creator to specify the timestamp.

It’s possible to narrow the creation date considerably by including information from and encoding information back into the Bitcoin blockchain. 

 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? 

 Correct. There‘s only relative timing by referencing and signing.

Bitcoin is the only good distributed timestamp server I know of. 

 I've been thinking a bit about this.

The possibility of backdating could expose #nostr to innumerable possibilities of spam, confusion, astroturfing, etc.

We need to fix this.

nostr:note1qvtvq0jwf45ncpekxx86p2ukwaxjk7yeuk6dmt5wghhxh2d3fdrq6dzhx7