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As I understand it, the premise of the method is that, at day D, you already have written the contents of tweet Tr (to be posted as a reply to T) and so you can already compute its hash and tweet the hash as part of T on day D. Then on day D+N, you tweet Tr, which hashes correctly to what you said it would, and thus is extremely unlikely to have been tampered with.
Where does the date string contained in Tr come into play?