Capital gains a/c - open or not?

Sold flat in Sept 2017 (Rs.X) and want to re-invest capital gains by buying a resale flat (Rs.X+Y)
The transaction (registration of property to be purchased) can only be completed in August 2018.
I will be filing the IT return for FY 2017-18 by 31st-July 2018
 

Can I keep holding the money in an ordinary savings account? -- OR -- Do I have to invest the capital gains in a capital gains account before filing the returns?
IF YES, can I avoid putting the sum in a capital gains account  if I pay the total capital gains (Rs.X) to the seller before July 2018 and (Rs.Y) in Aug 2018?
Thanks