Gst interest calculation date from 31.3.19 or return filed date

I have filed my GST Return For January 2018 on 20 -02-2018 Itself i failed too show one sale bill in that and i am going to show the Sales Bill amount in mArch 2019 GSTR - 3B and pay the tax along with interest now i want to know i want to calculate interest from 31.03.2019 (i.e GST Date Extened ) or 20.02.2018 which i have filed the return do the needfull