I am pretty sure it is the release date.
With a Tour beginning April/May
summer break
Autumn Tour in US or Vice Versa
It is the optimal schedule. That was the only positive thing about the Tracker Tour.
No waiting time from Release until Tour Start.
September Release Date with April Tour Start make no sense. It is too much time in between
Anyway I think March 2018 it will be released otherwise they produce a Slow Motion Album...