GST ON INTEREST

My assessee has sold goods for Rs. 10,000 vide invoice no. 25 dated 1.8.17 and charged GST in the invoice. As per terms, payment is to be made within 60 days but the payment was received on 31.12.17 i.e. after the stipulated time. Thus he has issued a debit note to the customer for amount of interest @ 12% for delayed payment in January 2018 for Rs. 300/-. But the payment of such interest was received by him in April 2018.

question is :
in which month return he have to show debit note january or April ???