Point of taxation: payment within the prescribed short period fixes the tax point; otherwise ordinary rules apply. Where an invoice was issued before the cut-off but payment remained unpaid on that day, the transitional rule provides that if payment is made within a short prescribed period from the invoice date the point of taxation is the date of payment; if payment is not made within that period the point of taxation is to be determined as if the transitional rule and the related proviso do not apply, leaving the ordinary point of taxation rules to govern.