40 ILCS 5/15-113.3
"Service for periods of military service": Those periods, not exceeding 5 years, during which a person served in the armed forces of the United States, of which all but 2 years must have immediately followed a period of employment with an employer under this System or the State Employees' Retirement System of Illinois; provided that the person received a discharge other than dishonorable and again became an employee under this System within one year after discharge. However, for the up to 2 years of military service not immediately following employment, the applicant must make contributions to the System equal to (1) 8% of the employee's basic compensation on the last date as a participating employee prior to such military service, or on the first date as a participating employee after such military service, whichever is greater, plus (2) an amount determined by the board to be equal to the employer's normal cost of the benefits accrued for such military service, plus (3) interest on items (1) and (2) at the effective rate from the later of the date of first membership in the System or the date of conclusion of military service to the date of payment. The change in the required contribution for purchased military credit made by this amendatory Act of 1993 does not entitle any person to a refund of contributions already paid. The contributions paid under this Section are not normal contributions as defined in Section 15-114 or additional contributions as defined in Section 15-115.
The changes to this Section made by this amendatory Act of 1991 shall apply not only to persons who on or after its effective date are in service under the System, but also to persons whose employment terminated prior to that date, whether or not the person is an annuitant on that date. In the case of an annuitant who applies for credit allowable under this Section for a period of military service that did not immediately follow employment, and who has made the required contributions for such credit, the annuity shall be recalculated to include the additional service credit, with the increase taking effect on the date the System received written notification of the annuitant's intent to purchase the credit, if payment of all the required contributions is made within 60 days of such notice, or else on the first annuity payment date following the date of payment of the required contributions. In calculating the automatic annual increase for an annuity that has been recalculated under this Section, the increase attributable to the additional service allowable under this amendatory Act of 1991 shall be included in the calculation of automatic annual increases accruing after the effective date of the recalculation.
40 ILCS 5/15-113.3