Post Gst sale return clause compare to mvat. within six month rule

In GST in term of Section 34 of CGST Act, any credit/debit note against relevant Invoice can be issue till September following the end of FY in which such supply is made.
ie for Fy 2019-20 , the credit note can be issued till September 2020.

Note : Goods can be return after September also but neither the Supplier can reduce is output tax liability nor the recipient can reduce his ITC .