Creditable service on which the allowance of a member is based consists of: eligibility service, except as provided in subsection (b) of this section; and credit for unused sick leave as provided in § 20-206 of this article. Except as provided in paragraph (2) of this subsection, a member’s creditable service for a period of regular part-time employment equals the member’s actual employment during that period. Subject to paragraph (3) of this subsection, a member’s creditable service for a period of regular part-time employment equals the member’s eligibility service for that period if: the medical board determines that the member’s part-time employment is a result of a medically determined disability that prevents full-time work; and the member then elects to make the contributions for full-time employment required by § 22-214 of this title. Except to the extent that a disability was aggravated after a member’s enrollment, paragraph (2) of this subsection does not apply to a disability incurred before a member’s enrollment.