For Q4 of FY 20 = 17th July 2020. For Q1 of FY 21 = 03rd August 2020.
Incase of FORM GSTR-3B :
For July 2020 : 27th September 2020.
9% p.a. Interest thereafter till September 30, 2020. 18% p.a Interest thereafter w.e.f. October 01, 2020.
In short if the Return for the period February 2020 to July 2020 is filed within 30th September 2020 ( 30/09/2020 ), then reduced rate of Interest @ 9% p.a. will be applicable. Refer Notification No. 51/2020 - Central Tax, dated 24.06.2020.