5 ILCS 75/2

A bill passed after May 31 of a calendar year shall become effective on June 1 of the next calendar year unless the General Assembly by a vote of three-fifths of the members elected to each house provides for an earlier effective date in the terms of the bill or unless the General Assembly provides for a later effective date in the terms of the bill; 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.