The date in the contract is the latest closing date possible, but it could close earlier than that. If it passes ROFR and all parties turn in documents, title search is completed, and final payment is received, it can close as soon as all that happens. Usually the broker and/or title company informs you when closing is done.
If you’re looking at postings here and the closing date is blank and there’s no dates after, it’s because it hasn’t closed yet. People usually start a post on her once the contract passes ROFR but before closing.