40 ILCS 5/7-111
"Prior Service": The period beginning on the day a participating employee first became an employee of a municipality, or of an instrumentality thereof, or of a municipality or instrumentality that was superseded by the employing participating municipality, or of a participating instrumentality, and ending on the effective date of participation of the municipality or participating instrumentality, or upon the latest termination of service prior to such effective date, but excluding (a) the intervening periods during which the employee was separated from the service of the municipality and all instrumentalities thereof, or of the participating instrumentality, (b) periods during which the employee was employed in a position normally requiring less than 600 hours of service during a year, and (c) periods during which the employee served in a position normally requiring performance of duty less than 1000 hours per year, if the participating municipality or participating instrumentality adopted, prior to its effective date of participation, a resolution or ordinance excluding persons in such positions from participation.
40 ILCS 5/7-111