Section 35 ILCS 5/202.3 - Net income attributable to the period prior to July 1, 1989 and net income attributable to the period after June 30, 1989(a) In general. With respect to the taxable year of a taxpayer beginning prior to July 1, 1989 and ending after June 30, 1989, net income for the period after June 30, 1989 shall be that amount which bears the same ratio to the taxpayer's net income for the entire taxable year as the number of days in such year after June 30, 1989 bears to the total number of days in such year, and the net income for the period prior to July 1, 1989 shall be that amount which bears the same ratio to the taxpayer's net income for the entire taxable year as the number of days in such year prior to July 1, 1989 bears to the total number of days in such year.(b) Election to attribute income and deduction items specifically to the respective portions of a taxable year prior to July 1, 1989 and after June 30, 1989. In the case of a taxpayer with a taxable year beginning prior to July 1, 1989 and ending after June 30, 1989, the taxpayer may elect, in lieu of the procedure established in subsection (a) of this Section, to determine net income on a specific accounting basis for the 2 portions of his taxable year: (i) from the beginning of the taxable year through June 30, 1989, and(ii) from July 1, 1989 through the end of the taxable year. If the taxpayer elects specific accounting under this subsection, there shall be taken into account in computing base income for each of the 2 portions of the taxable year only those items earned, received, paid, incurred or accrued in each such period. The standard exemption provided by Section 204 shall be divided between the respective periods in amounts which bear the same ratio to the total exemption allowable under Section 204 (determined without regard to this Section) as the total number of days in each such period bears to the total number of days in the taxable year. The election provided by this subsection shall be made in such manner and at such time as the Department may by forms or regulations prescribe, but shall be made not later than the due date (including any extensions thereof) for the filing of the return for the taxable year, and shall be irrevocable.