Time of supply for services determines tax point when rate changes, based on invoice or payment timing. Change in the effective rate of tax for services modifies the time of supply by reference to the timing of provision, invoice issuance and payment. If the service was provided before the rate change, the tax point depends on combinations of invoice and payment dates (earlier of invoice or payment where both occur after the change; invoice date if invoice before but payment after; payment date if payment before but invoice after). If provided after the change, analogous rules apply, and 'date of receipt of payment' is the earlier of book entry or bank credit, subject to the four working day bank-credit proviso.