40 ILCS 5/15-113.12
A participant may establish earnings credit for periods of voluntary pay reduction, taken in lieu of furlough, beginning on or after July 1, 2015 and ending on or before June 30, 2017. To receive this credit, the participant must: (1) apply in writing to the System before December 31, 2018; and (2) make, on an after-tax basis, employee contributions required under Section 15-157 based on the voluntary reduction in pay, plus an amount determined by the Board to be equal to the employer's normal cost of the benefit, plus compounded interest at the actuarially assumed rate from the date of voluntary reduction in pay to the date of payment. The participant shall provide, at the time of application, (i) written certification from the employer providing the total voluntary reduction in pay per pay period for each pay period with a voluntary reduction in pay and (ii) written certification from the employer stating that the voluntary reduction in pay was taken in lieu of furlough.
40 ILCS 5/15-113.12