Section 35 ILCS 5/402 - Methods of Accounting(a) Same as federal. For purposes of the tax imposed by this Act, a person's method of accounting shall be the same as such person's method of accounting for federal income tax purposes. If no method of accounting has been regularly used by such person, base and net income for purposes of this Act shall be computed under such method as in the opinion of the Department fairly reflects income.(b) Change of accounting method. If a person's method of accounting is changed for federal income tax purposes, for purposes of this Act it shall be similarly changed.