40 ILCS 5/13-402
For the purpose of computing the length of service for the retirement annuity, surviving spouse's annuity, and child's annuity, and calculating the minimum service requirement for payment of military service under subsection (b) of Section 13-403, service of 120 days in any one calendar year shall constitute one year of service and service for any fractional part thereof shall constitute an equal fractional part of one year of service unless specifically provided otherwise. For all other purposes under this Article, including but not limited to the optional plans of additional benefits and contributions provided under Sections 13-304, 13-304.1, and 13-314 of this Article, 26 pay periods of service during any 12 consecutive months shall constitute a year of service, and service rendered for 50% or more of a single pay period shall constitute service for the full pay period. Service of less than 50% of a single pay period shall not be counted.
40 ILCS 5/13-402