(a) The retirement board shall fix and determine, by appropriate rules and regulations, how much service in any year is equivalent to a year of service, but in computing that service or in computing the compensation it shall credit no period of more than a month’s duration during which a member was absent without pay nor shall more than one year of service be credited on account of all service in one calendar year.
(b) Notwithstanding any other section of law, no member of the retirement system shall be permitted to purchase service credit for any portion of a year for which he or she is already receiving service credit in this retirement system.