Time of supply rules govern tax rate change; invoice or payment timing determines which rate applies. Time of supply for changes in tax rate is determined by the timing of supply, invoice issuance and receipt of payment relative to the rate change. For supplies before the change, the time of supply is: earlier of invoice or payment if both occur after the change; invoice date if invoice before and payment after; payment date if payment before and invoice after. For supplies after the change, mirror rules apply: payment date if payment after but invoice before; earlier of payment or invoice if both before; invoice date if invoice after but payment before. 'Date of receipt of payment' means earlier of books entry or bank credit, except bank credit occurring after four working days from the rate change is treated as the date of receipt.