Section 35 ILCS 735/3-9 - Application of provisions(a) The provisions of this Act shall apply to the rates of interest for periods on and after the effective date of this Act. Interest for periods prior to the effective date of this Act shall be computed at the rates in effect prior to that date.(b) Penalties shall be imposed at the rate and in the manner in effect at the time the tax liability became due.(c) Interest shall not be paid on claims filed after the effective date of this Act except such interest which is paid in accordance with this Act.(d) Payments received from a taxpayer shall be applied against the outstanding liability of the taxpayer, or to an agreed portion of the outstanding liability, in the following order: the principal amount of the tax, then penalty, and then interest.