In the year 1950 and thereafter the review of assessments by boards of review in all cities and townships shall be completed on or before the first Monday in April, any provisions of the charter of any city or township to the contrary notwithstanding: Provided, That the legislative body of any city or township, in order to comply with the provisions hereof, may, by ordinance, fix the period or periods for preparing the budget and for making, completing and reviewing the assessment roll, any provisions of the charter of such city or township or any law to the contrary notwithstanding.