Under this article, for the purpose of determining years of service, the board shall fix and determine by appropriate rules and regulations the definition of "service," the amount of service to be credited to members of the fire department, in any month or year, provided in no case shall less than 20 days of service (18 days of 24-hour shifts) rendered in a calendar month be credited as a month of service. A member of the fire department who rendered service as a volunteer firefighter on the fire department prior to becoming a paid member of the fire department shall be credited with one year of service for every four years of volunteer service or any portion thereof in such proportion, and such member of the fire department shall also be credited with service as a paid member of the fire department prior to January 1, 1958. The board shall credit each member's service account with the number of years and months of service to which the member may be entitled for service rendered by such member to the city prior to the effective date of the ordinance from which this article derives and periodically thereafter and shall keep a record thereof. (Code 1979, § 15-130)