(a) A member may purchase service credit as provided in subsection (b) of this section:

(1) 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

(2) when membership was mandatory but for which member contributions were not made.

(b) (1) A member who purchases service credit under this section shall:

(i) 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

(ii) 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.

(2) A member may pay for service credit purchased under this section at any time before retirement.