Sales Return Time in GST

DUE DATE GSTR 1(JAN _ MARCH) >> 30-04-2018 (QUARTERLY DEALER) 

DUE DATE GSTR 1( MARCH2018) >> 10-05-2018 (MONTHLY DEALER)

it is 6 months, but you have a way out on practical basis . just make normal invoice and sell it to the supplier againist its proper GSTN with GST without adding any profit.

this way your supplier will get his goods back along with its input tax credit. 

But, there will be a problem if the goods are in good quantity as this will amount to interstate sales and will add to your sales.

However, legal way is to issue Credit / debit notes within the time will solve the problem.