CommitmentThe real close time is drawn from a 28–32 minute range and kept encrypted; only this hash of it is published while the window is open. When the window closes the time and salt are revealed and anyone can recompute the hash, which is what makes last-block sniping pointless, because nobody knows when the last block is.
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The countdown is the advertised close. The real one is committed in advance and revealed only after the fact.