Having played since Vanilla this is no different really and if you think about it prepatch is everything except the portal being turned on. The Outlands will be live but won’t be accessible till they turn the portal on.
Pre patch is probably sometime between mid May and early June with between 2 weeks to a month before full release. The latter seems to be a better timeline since everyone can level and they can iron out any bugs that arise.
As far as giving an exact date that’s just opening yourself up for drama if you miss it and they won’t give an expected date range till they know the game is close to being ready for said timeframe.