Subject to the provisions contained in this Act, there shall be charged on every company for every assessment year commencing on and from the first day of April, 1964, ¹[but before the first day of April, 1988] a tax (in this Act referred to as the surtax) in respect of so much of its chargeable profits of the previous year or previous years, as the case may be, as exceed the statutory deduction, at the rate or rates specified in the Third Schedule.