Estimated is just Amazon covering themselves if there is a delay. They do it with every purchase.
I expect the 14th to be accurate to within a day or two anyway if not spot on.
Given that, I'd expect a demo to be available around 19th-20th September and a few days earlier for the PSN judging by last year.
I suspect the slight delay compared to last year maybe to let Konami have the chance to make the rosters as accurate as possible, doesn't take more than a day or two to put players in the right team does it. But I wouldn't hold my breath on that.