Purchase of Military Time. Any fulltime employee presently employed in the position of Police Lieutenant may, on a one time only basis, prior to January. 1, 1982, elect to receive credit, for retirement purposes only, for time served in the armed forces of the United States on active duty for other than training purposes, and for which he received an honorable discharge. The maximum amount of military service for which he may receive credit is thirtysix (36) months and such credit shall be given only upon payment to the retirement system of a contribution computed in the following manner: Induction rate for patrolman in effect at the time of making the election multiplied by the existing contribution rate, multiplied by the number of years of military service, with interest at the rate established by the Director of Finance, computed from date of hire. Said contribution shall be made In one installment, payable in no event later than 30 days following the date of retirement. In the event an employee does not deposit the contribution required hereunder at the time of making the election, the contribution shall be increased 3/4 of 1% per month, from the date of election to the date of payment, and shall be compounded annually on the amount due. No credit shall be granted for any military service for which the applicant is receiving a pension or which has been used in establishing entitlement to a pension from any other source. Said service shall not be used for the purpose of meeting minimum requirements for retirement, including, but not limited to, deferred or voluntary retirement. The employee shall be required to submit a certificate or other document from the military authorities indicating the character of service and nature of separation.