Actually my reply based on your query as per your your notified months... But, Now you can understand the following...
We are follow our business transactions months called "Financial Year" it's from 1st April to 31st March.... Then we assessed the transactions after next FY it's called Assessement Year...
FY April 2016 to March 2017 --> AY 2017 - 2018 FY April 2017 to March 2018 --> AY 2018 - 2019...
you are supposed to maintain books prescribed under section 44AA as per accounting standards... you can follow any year but for income tax act, p.y. is April to march only