Relevant date for filling Refund (GST)

Hello Sir,
I have a query regarding relevant date for filling of application for GST Refund, for this purpose of relevant date explained in explanation to section 54 clause (2), sub clause (e) shall be read as:
“In the case of refund of unutilised input tax credit under sub-section (3), the end of the financial year in which such claim for refund arises;"
Which is substituted by the Central Goods and Services Tax (Amendment) Act, 2018, w.e.f. 1-2-2019. Sub-clause (e) read as:
In the case of refund of unutilised input tax credit under clause (ii) of the first proviso to sub-section (3), the due date for furnishing of return under section 39 for the period in which such claim for refund arises.
Which means relevant date for claiming the inverted duty refund has been changed from "two years from end of financial year to two years from the date of filing of return i.e is 3B. But it is note worth that amendment came in to force from 1st Feb-2019.

Kindly clarify what should be the date for filling refund for the month of Sept to March-2018.
Is it be 31st march 2020 or
is it be the due date for furnishing of return under section 39 for the period in which such claim for refund arises