(a) The board of trustees shall establish rules and regulations setting forth the amount of service in a year which is equivalent to one year of creditable service.

(b) In no case shall more than one year of service be creditable for all service in one calendar year, provided that creditable service performed as a member of the General Assembly shall be in addition to all other service.

(c) One year of creditable service shall be given for each year of service in the Georgia National Guard, Georgia State Guard, or the General Assembly, provided that no credit shall be given for such service unless the individual was a member of the retirement system at that time; provided, further, that no more than five years of creditable service may be so obtained.