83(1)This paragraph applies where it is provided under Part 4 that writing-down allowances are to be made in respect of any expenditure during a writing-down period of any specified length.
(2)If allowances were made under paragraph 27(2) of Schedule 14 to FA 1965—
(a)for income tax purposes, for either of the tax years 1964-65 and 1965-66, and
(b)for accounting periods of a company falling wholly or partly within either of those years,
the periods for which allowances were made are added together in calculating the writing-down period, even though (according to the calendar) the same time is counted twice.