In ascertaining for the purposes of this Part of this Act whether there is any difference with respect to rents or rates between one rental period and another (whether of the same tenancy or not) or the amount of any such difference, any necessary adjustment shall be made to take account of periods of different lengths; and for the purposes of such an adjustment a period of one month shall be treated as equivalent to one-twelth of a year and a period of a week as equivalent to one-fifty-second of a year.