5 ILCS 75/1

(a) A bill passed prior to June 1 of a calendar year that does not provide for an effective date in the terms of the bill shall become effective on January 1 of the following year, or upon its becoming a law, whichever is later. (b) A bill passed prior to June 1 of a calendar year that does provide for an effective date in the terms of the bill shall become effective on that date if that date is the same as or subsequent to the date the bill becomes a law; provided that if the effective date provided in the terms of the bill is prior to the date the bill becomes a law then the date the bill becomes a law shall be the effective date.