In view of section 34 which states as under :
(2) Any registered person who issues 2[one or more credit notes for supplies made in a financial year] in relation to a supply of goods or services or both shall declare the details of such credit note in the return for the month during which such credit note has been issued but not later than September following the end of the financial year in which such supply was made, or the date of furnishing of the relevant annual return, whichever is earlier, and the tax liability shall be adjusted in such manner as may be prescribed:
Provided that no reduction in output tax liability of the supplier shall be permitted, if the incidence of tax and interest on such supply has been passed on to any other person.
In view of the above clause, the discounts given after September month of subsequent year of sale the GST credit on that discount component would not be allowed. However, if credit note to be raised for discount without giving impact of GST then there no time limit. Thanks.