Time of Supply

I am Supriya (94 Points)

07 February 2018  
If a service is received from USA (import) for $100 (at Ex rate:64 i.e. Rs.6,400) on 20-7-2017.
A part payment is for $90 (at Ex rate:63 i.e. Rs.5,670) is made on 18-8-2017 and exchange gain earned is Rs.90. 
For remaining $10 the payment is not made till 7-2-2018
GST has to be paid on RCM basis.What will be the time of Supply? 
For $90 i.e. payment made on 18-8-2017 the T.O.S is 18-8-2017
and for $10 the T.O.S is 19-9-2017
Is it right?