The special ad hoc adjustment is effective on the date of the first payment of the retiree's retirement annuity, retirement benefit, service pension or disability pension benefit due after December 31, 1988. The municipal retirement system shall pay this as soon as practicable after that and shall include in the first payment any omitted amount payable between the effective date of the adjustment and the date of the first payment.
53 P.S. § 896.303