12 September 2017
Because ultimate purpose of GSTR-3B is to collect taxes, and technically also when you submit GSTR-3B after submission "payment of tax" tab gets activated and without making payment of same return cannot be filed and if same is filed (along with payment of taxes) after due date i.e. 20th September then there is interest of 18%p.a on same which is calculated on submitted tax amount.
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