Notwithstanding any other provision of the retirement laws of this state, any active member of the employees’ retirement system who was 60 years of age or greater when his or her employment commenced, and commenced employment before June 22, 1989, and was ineligible by state law from joining the retirement system shall be allowed to purchase service credits for actual past employment. The number of years for which credits may be purchased shall be limited to the difference between the date upon which they commenced employment at 60 years of age or greater and June 22, 1989, when they were required by law to join the retirement system. Purchase of the past service credits shall be at the rate that contributions would have been calculated at the time of past service, plus interest.