Time of supply rules fix which GST rate applies when invoice or payment timing spans a rate change.

Change in applicable tax rate is determined by tying the time of supply to invoice date and payment receipt when rates change. If supply occurs before the change, the time of supply depends on combinations of invoice issuance and payment timing; similarly, for supplies after the change the time is allocated by whether invoice or payment preceded the rate change. A bank-credit rule makes the date of receipt the bank credit date when credit occurs more than four working days after the rate change. "Date of receipt of payment" means the earlier of the entry in the supplier's books or bank credit.

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