According to Illinois Criminal Code:
"720 ILCS 5/3-5
General Limitations.
(a) A prosecution for: (1) first degree murder, attempt to commit first degree murder, second degree murder, involuntary manslaughter, reckless homicide, concealment of homicidal death, treason, arson, aggravated arson, forgery, or (2) any offense involving sexual conduct or sexual penetration as defined by Section 12-12 of this Code in which the DNA profile of the offender is obtained and entered into a DNA database within 10 years after the commission of the offense and the identity of the offender is unknown after a diligent investigation by law enforcement authorities, may be commenced at any time. Clause (2) of this subsection (a) applies only if the victim reported the offense to law enforcement authorities within 2 years after the commission of the offense unless a longer period for reporting the offense to law enforcement authorities is provided in Section 3-6.
(b) Unless the statute describing the offense provides otherwise, or the period of limitation is extended by Section 3-6, a prosecution for any offense not designated in Subsection (a) must be commenced within 3 years after the commission of the offense if it is a felony, or within one year and 6 months after its commission if it is a misdemeanor."
From the above, the prosecution in your misdemeanor must be COMMENCED within 18 months of the violation.