This Subchapter (Parts 16-23) shall take effect immediately and shall apply to taxable years beginning on or after January 1, 1985, provided that:

(a) the repeal of section 37.11 of this Subchapter and the addition of a new section 21-4.4 of this Subchapter shall apply to returns required to be filed (determined without regard to any extension of time for filing) after December 31, 1985; and

(b) the provisions of this Subchapter, as repealed by amendments filed on December 3, 1985, shall continue to apply to taxable years beginning prior to January 1, 1985.