Previous 2Ks were released in late September, early October -- 2K17 was released in mid-September, a full 2 weeks before 2K16 was released. The devs commented in quite a few articles how tight the cycle had been and how it was much harder than usual (Forbes, I believe, is the series of articles I'm thinking of).
If 2K18 is releasing in September, I hope it's late September (2K16 was released on 9/29) but I'm not hopeful this is the case -- I imagine early tip-off numbers for 2K17 and preorders were off the chart, which nearly guarantees 2K18 releasing in a similar manner.