Custom Fiscal Year

My organization does not operate on a calendar year. Because we are involved in education, our fiscal year starts on Sept 1 each year.

My question is, how do I set it up so that when I close this fiscal year, the system knows that the year ends on Aug 31, 2019 and that the new year starts on Sept 1, 2019?

Manager does not care what your fiscal year is. It is a perpetual system. Read these Guides:
https://www.manager.io/guides/5490
https://www.manager.io/guides/7470