40 ILCS 5/8-121

"Withdrawal from service" or "withdrawal": Discharge or resignation of an employee. For the purpose of improving sums to the credit of municipal employees for annuity purposes at the 3%, 3 1/2% or 4% per annum interest rate herein provided, re-entrance into service within 12 months after withdrawal from service by persons who have not received a refund under this Article shall be considered continuation in service; and for refund purposes a withdrawal from service shall not be final until 30 days after the effective date of the withdrawal.