Time of supply determined by invoice or payment timing when tax rate changes; earlier of invoice or payment governs. Time of supply is determined by the relative timing of invoice issuance and payment receipt. The time of supply is the earlier of invoice date or payment date where both occur after the rate change. If the invoice was issued before the change but payment is received after, the time of supply is the invoice date. If payment was received before the change but the invoice is issued after, the time of supply is the date of receipt of payment.