A credit note can be issued for any supplies. However, in order to declare the details of the credit note and thereby claim a reduction in output tax liability, it must be issued and declared in a return for the month during which such credit note has been issued but not later than September following the end of Financial Year in which such supply was made, or before filing of the annual return for that Financial Year, whichever earlier and the tax liability shall be adjusted in such manner as may be prescribed: E.g. Assuming that annual return for the year 2017-18 is not yet filed, a credit note can be issued in respect of any of supplies made during the year 2017-18, up to 30th September 2018. However, if the Annual Return is filed for financial year 2017 18 on 30th June 2018, then the details of the credit note for the year 2017-18 cannot be declared in the returns file after 30th June 2018.
(ICAI FAQ PUBLICATIONS 06-09-2017 TAX INVOICE, CREDIT AND DEBIT NOTES: FAQ NO. 24)