A member who returns to active state service following an employer-approved leave of absence because of his or her serious illness or injury may elect to receive service credit for that period of uncompensated absence at any time prior to retirement by making contributions as specified in Sections 21050 and 21052. The purchase of additional service credit pursuant to this section shall not reduce the amount of service credit that the member is eligible to purchase pursuant to this chapter. A member may purchase service credit pursuant to this section for a leave of absence that occurred either before or after the effective date of these provisions.