Time of supply rules determine which tax rate applies when a change in rate occurs, based on invoice and payment timing.

Time of supply for changes in tax rate is fixed by rules: for supplies before the change, the time of supply is the earlier of invoice or payment when both occur after the change; invoice date governs if invoice preceded and payment followed; payment date governs if payment preceded and invoice followed. For supplies after the change, payment date governs if payment follows but invoice preceded; the earlier of payment or invoice governs if both preceded; invoice date governs if invoice follows but payment preceded. Payment receipt is treated as bank credit if that credit occurs more than four working days after the rate change; receipt means earlier of books entry or bank credit.

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