Section 35 ILCS 5/1003 - Interest on Deficiencies(a) In general. If any amount of tax imposed by this Act, including tax withheld by an employer, is not paid on or before the date prescribed for payment of such tax (determined without regard to any extensions), interest on such amount shall be paid in the manner and at the rate prescribed in Section 3-2 of the Uniform Penalty and Interest Act for the period from such date to the date of payment of such amount, except that if a waiver of restrictions under Section 907 on the assessment and collection of such amount has been filed, and if notice and demand by the Director for the payment of such amount is not made within 30 days after the filing of such waiver, interest shall not be imposed on such amount for the period beginning immediately after such 30th day and ending with the date of notice and demand.(b) Interest treated as tax. Interest prescribed under this Section on any tax, including tax withheld by an employer, or on any penalty, shall be deemed assessed upon the assessment of the tax or penalties to which such interest relates and shall be collected and paid on notice and demand in the same manner as tax. Any reference in this Act to the tax imposed by this Act shall be deemed also to refer to interest imposed by this Section on such tax.(c) Exception as to estimated tax. This Section shall not apply to any failure to pay estimated tax required by Section 803.