I know it's way too recent for them to say the actual release date for kh3 because they officially announced the release year for the game, but I think it's possible that we will get to know the actual release date of kh3 before 2018, but first, let me use an example of a game that released this year:
kingdom hearts 0.2's release date was announced 6 months before the game came out. (June 2016-January2017)
I know this isn't much of an example, but this can at least show us how far back a video game can announce it's release date.
This obviously isn't concrete because as far as we know, we might get the release date of the game a week before it comes out
If we get a release date for the game in december, it could mean that that the game could come out in June 2018 at the earliest and that is if the game is nearly done by then