(a) For purposes of this article, except as provided in subsection (b), each shareholder's pro rata share of any item for any taxable year shall be the sum of the amounts determined with respect to the shareholder by assigning an equal portion of the item to each day of the taxable year, and then by dividing that portion pro rata among the shares outstanding on the day.
(b) Under regulations prescribed by the Department of Revenue, if any shareholder terminates his or her interest in the corporation during the taxable year and all affected shareholders and the corporation agree to the application of this subsection, subsection (a) of this section shall be applied to the affected shareholders as if the taxable year consisted of two taxable years the first of which ends on the date of the termination.
(c) For purposes of subsection (b), the term "affected shareholders" means the shareholder whose interest is terminated and all shareholders to whom such shareholder has transferred shares during the taxable year. If such shareholder has transferred shares to the corporation, the term "affected shareholders" shall include all persons who are shareholders during the taxable year.