Section 35 ILCS 5/401 - Taxable Year(a) In general. For purposes of the tax imposed by this Act, the taxable year of a person shall be the same as the taxable year of such person for federal income tax purposes. The taxable year of any person required to file a return under this Act but not under the Internal Revenue Code shall be his annual accounting period if it is a fiscal or calendar year, and in all other cases shall be the calendar year.(b) Change in taxable year. If the taxable year of a person is changed for federal income tax purposes, the taxable year of such person for purposes of the tax imposed by this Act shall be similarly changed. In the case of a taxable year for a period of less than 12 months, the standard exemption allowed under section 204 shall be prorated on the basis of the number of days in such year to 365.(c) Termination of taxable year for jeopardy. Notwithstanding the provisions of subsections (a) and (b), if the Department terminates the taxable year of a taxpayer under section 1102 (relating to tax in jeopardy), the tax shall be computed for the period determined by such action.