50 ILCS 505/4

(50 ILCS 505/4) (from Ch. 85, par. 5604) Sec. 4. Any bill approved for payment pursuant to Section 3 shall be paid within 30 days after the date of approval. If payment is not made within such 30 day period, an interest penalty of 1% of any amount approved and unpaid shall be added for each month or fraction thereof after the expiration of such 30 day period, until final payment is made. (Source: P.A. 84-731.)