199.3. Where a service is supplied to a person for consideration that includes two or more payments that are attributable to different parts (each of which is referred to in this section as a “billing period”) of the period during which the service is or is to be rendered under the agreement for the supply and tax in respect of the supply, calculated on a particular payment, at any time becomes payable by the person or is paid by the person without having become payable, for the purpose of determining an input tax refund of the person in respect of the service,(1) the person is deemed to have received, at that time, a separate supply of the service rendered or to be rendered during the billing period to which the particular periodic payment is attributable; and
(2) the tax calculated on the particular payment is deemed to be tax payable in respect of the separate supply.