Any member accruing annual leave or sick leave days may, after the effective date of this section, elect to use the days at the time of retirement to acquire additional credited service in this retirement system: Provided, That the accrued annual or sick leave may not be used to purchase health insurance under the Public Employees Insurance Agency until the member reaches the age of fifty-five. The days shall be applied on the basis of two workdays' credit granted for each one day of accrued annual or sick leave days, with each month of retirement service credit to equal twenty workdays and with any remainder of ten workdays or more to constitute a full month of additional credit and any remainder of less than ten workdays to be dropped and not used, notwithstanding any provisions of the code to the contrary. The credited service shall be allowed and not considered to controvert the requirement of no more than twelve months' credited service in any year's period.