(35 ILCS 5/202.5)
    Sec. 202.5. Net income attributable to the period beginning prior to the first day of a month and ending after the last day of the preceding month.
    (a) In general. With respect to the taxable year of a taxpayer beginning prior to the first day of a month and ending after the last day of the preceding month, net income for the period after the last day of the preceding month, is that amount that bears the same ratio to the taxpayer's net income for the entire taxable year as the number of days in that taxable year after the last day of the preceding month bears to the total number of days in that taxable year, and the net income for the period prior to the first day of the month is that amount that bears the same ratio to the taxpayer's net income for the entire taxable year as the number of days in that taxable year prior to the first day of the month bears to the total number of days in that taxable year.
    (b) Election to attribute income and deduction items specifically to the respective portions of a taxable year prior to the first day of a month and ending after the last day of the preceding month. In the case of a taxpayer with a taxable year beginning prior to the first day of a month and ending after the last day of the preceding month, the taxpayer may elect, instead of the procedure established in subsection (a) of this Section, to determine net income on a specific accounting basis for the 2 portions of the taxable year:
        (1) from the beginning of the taxable year through
    
the last day of that apportionment period; and
        (2) from the first day of the next apportionment
    
period through the end of the taxable year.
    The election provided by this subsection must be made in the form and manner that the Department requires by rule, and must be made no later than the due date (including any extensions thereof) for the filing of the return for the taxable year, and is irrevocable.
    (c) If the taxpayer elects specific accounting under subsection (b):
        (1) 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;
        (2) for purposes of apportioning business income of
    
the taxpayer, the provisions in Article 3 shall be applied on the basis of the taxpayer's full taxable year, without regard to this Section;
        (3) the 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 period bears to the total number of days in the taxable year;
        (4) for purposes of this subsection, net income may
    
not be negative for either of the two portions of the taxable year and positive for the other; if net income for one portion of the taxable year would be positive and net income for the other portion would otherwise be negative, the net income for the entire taxable year shall be attributed to the portion of the taxable year with positive net income and the net income for the other portion of the taxable year shall be zero; and
        (5) the net loss carryforward deduction for the
    
taxable year under Section 207 may not exceed combined net income of both portions of the taxable year, and shall be used against the net income of the portion of the taxable year from the beginning of the taxable year through the last day of the preceding month before any remaining amount is used against the net income of the latter portion of the taxable year.
(Source: P.A. 100-22, eff. 7-6-17.)