c. The officers and employees of such organization shall be credited with such periods of prior service as shall be certified by their employer for service rendered to it, or its predecessor, or the state, or in any other capacity approved by such employer and the comptroller. Service for such employer after the date on which it commences to participate in the police and fire retirement system and on account of which such employer pays contributions shall be considered as member service. An officer or employee of such employer who, as of the date he or she is so approved for membership in the police and fire retirement system, is already a member thereof, shall not have his or her total credit reduced by such approval. Any reserve held on account of any such officer or employee in the pension accumulation fund shall be used as an offset against the deficiency contribution payable thereafter by such employer on account of such officer or employee for any prior service credit and any such previous credit. Except as otherwise provided in this article, a police officer or firefighter of such employer who, by reason of his or her service, is a member of any other governmental retirement system shall not participate in the police and fire retirement system on that part of his or her compensation so covered. The term "governmental retirement system," as used in this subdivision, shall include any retirement system wholly or partly maintained by this state, by a municipality of this state, by another state or political subdivision thereof, by the United States government, or by any foreign country or political subdivision thereof. The provision in subdivision b of this section limiting participation in the New York state police and fire retirement system by reason of membership in another governmental retirement system shall not diminish or in any other way affect the prior or continual membership in the New York state police and fire retirement system, or any rights or benefits heretofore or hereafter arising therefrom, of any officer or employee of a public or quasi-public organization who (1) is in the service of such employer at the time this act takes effect, or was in such service prior thereto, and (2) by reason of such service is or was a member of any retirement system maintained by the United States government.