5 ILCS 440/1
At two o'clock ante meridian of the second Sunday in March of each year, the standard time in this state shall be advanced one hour, and at two o'clock ante meridian of the first Sunday in November of each year the standard time in this state shall, by the retarding of one hour, be made to coincide with the mean astronomical time of the ninety degrees of longitude West from Greenwich, the standard official time of which is described as United States standard central time, so that between the second Sunday of March at two o'clock ante meridian in each year and the first Sunday in November at two o'clock ante meridian in each year the standard time in this state shall be one hour in advance of the United States standard central time: Provided, however, that nothing in this act shall be so construed as to be in contravention of any federal law or authorized order of the Interstate Commerce Commission with respect to the time zones of the United States. And in all laws, statutes, orders, judgments, rules and regulations relating to the time of performance of any act of any officer or department of this state, or of any county, township, city or town, municipal corporation, agency or instrumentality of the state, or school district or school authority or relating to the time in which any rights shall accrue or determine, or within which any act shall or shall not be performed by any person subject to the jurisdiction of the state, and in all the public schools and in all institutions of the state, or of any county, township, city or town, municipal corporation, agency or instrumentality of the state or school district or school authority, and in all contracts or choses in action made or to be performed in the state, it shall be understood and intended that the time shall be the time prescribed in this section.
If the date on which time is to be advanced one hour, the date on which time is to be retarded one hour, or both, as set forth under Section 260a of the federal Uniform Time Act of 1966 (15 U.S.C. 260a), as now or hereafter amended, renumbered, or succeeded, differs from either or both of those dates as set forth under this Section, then the dates set forth under the federal law shall control and shall apply in Illinois, notwithstanding the dates set forth in this Section.
5 ILCS 440/1