31 March 2019
As per the Section 73 Reg person can pay the GST amount with interest referred in sec 50 with out any penalty before issue of SCN. as per section 50 interest should be payable from due date of return. for example due date for 31-.3-2017 was extended is 31-03-2019, However return filed was 19-04-2018. so the interest should be calculated from 31-03-2019 or from Return filing date 19-04-2018 ( which date should be considered as due date) if the payment made on 10-04-2019 ( for int cal )
31 March 2019
If Payment made on or before 31st March 19 then You can as per sec 50 & Interest.
If it's not paid on or before 31st March 19 then due date will be taken as per previous date which 20th of the next month...