(1) You may change the day in each year on which a tax period would otherwise end. However:
(a) the day must be no more than 7 days earlier or 7 days later than a day on which one of the tax periods that applies to you would otherwise end if the days were not changed; and
(b) the change must be consistent with the commercial accounting periods that apply to you.
(2) If the day on which a tax period ends is changed, the next tax period starts on the day after that day.