Time of supply rules for tax rate changes determine whether invoice date or payment date governs tax liability. Change in tax rate fixes the time of supply by reference to when supply, invoice issuance and payment occur. If supply was before the rate change, the time of supply is the earlier of invoice or payment where both follow the change; otherwise invoice date governs if issued before and payment after, and payment date governs if paid before and invoiced after. If supply occurs after the rate change, timing rules reverse: payment after with prior invoice yields payment date; both prior yield the earlier of payment or invoice; payment prior and invoice after yields invoice date. Date of receipt of payment is the earlier of book entry or bank credit.