Change in tax rate timing rules: invoice and payment sequencing determines the time of supply under GST. Change in the rate of tax alters the time of supply: when supply precedes the rate change, the time of supply depends on the relative timing of invoice issuance and payment receipt (earlier of invoice or payment if both post-change; invoice date if invoice pre-dates and payment post-dates; payment date if payment pre-dates and invoice post-dates). When supply follows the rate change, analogous rules apply tying time of supply to invoice and payment chronology, with a proviso and an explanation defining receipt of payment as earlier of book entry or bank credit.