18.1. Where the tax payable in respect of the taxable supply, or of the bringing into Québec, of property or a service becomes payable or is paid without having become payable after 31 December 1993, the tax shall be calculated at the rate that would be applicable in respect of the supply or bringing into Québec were it to become payable or were it paid before 1 January 1994, except where, but for this section, the tax payable must be calculated at the rate of 6.5 %.