A member may purchase service credit as provided in subsection (b) of this section: for a period of employment when the member was on an employer-approved leave of absence not exceeding 2 years that has qualified for purchase under regulations adopted by the Board of Trustees; or when membership was mandatory but for which member contributions were not made. A member who purchases service credit under this section shall: complete a claim for the service credit and file it with the Board of Trustees on the form that the Board of Trustees provides; and pay to the Board of Trustees in a single payment the member contributions the member would have made for the period of employment for which service credit is being purchased plus regular interest to the date of payment. A member may pay for service credit purchased under this section at any time before retirement.