Credit note and debit note rules govern invoice corrections and timing for GST return adjustments. Issuance and reporting of credit notes and debit notes correct misstatements in tax invoices. A supplier may issue a credit note where an invoice overstated taxable value or tax, goods are returned, or supplies are deficient; credit notes must be declared in the return for the month of issue but not later than the September following the end of the financial year of the supply or the date of the annual return, whichever is earlier, and tax adjustments are prescribed with a proviso barring output tax reduction if tax incidence and interest were passed to another person. Debit notes (including supplementary invoices) must be issued when an invoice understated taxable value or tax and declared in the return for the month of issue with prescribed tax adjustments.