I see - including the hash of the latest block in the post tells you it can't have been written before today (which comes for "free"), however to prove it couldn't have been written *after* today, you'd have to write something like the note id + hash into a bitcoin OP_RETURN

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Exactly. Although a single OP_RETURN can prove any number of stamps using a Merkle tree. I believe this is what open timestamps does.