Sea of Thieves Release Window Revealed
According to a Tweet by Xbox Australia, Sea of Thieves is set for a February 2017 release.
IGN was able to get a picture of the Tweet before it was deleted. It seems likely that this will be the actual release window, though Microsoft has not officially confirmed it.
Aaron Greenbrug, Xbox's global marketing chief, later conformed that this release date is indeed unconfirmed as of now. Still, it is unlikely that Xbox's Australian division leaked an inaccurate date. So for now, take this as a general release window until Microsoft provides any more specifics.