23For the purposes of paragraph 18 and 21, where a payment period begins before the commencement date and ends on or after that date (“the straddling period”)—
(a)so much of the straddling period as falls before the commencement date, and so much of that period as falls on or after that date, are to be treated as separate taxable periods, and
(b)where it is necessary to apportion an amount for the straddling period to the two separate taxable periods, it is to be apportioned—
(i)on a time basis according to the respective length of the separate taxable periods, or
(ii)if that method would produce a result that is unjust or unreasonable, on a just and reasonable basis.