(a) Except as provided by Section 823.101(2), military service creditable in the retirement system is active federal duty in the armed forces of the United States, other than as a student at a service academy, that was performed:
(1) as a direct result of being inducted or of first enlisting for duty on a date when the federal government was actively inducting persons into the armed forces under federal draft laws;
(2) as a reservist or member of the national guard who was ordered to duty under the authority of federal law;
(3) during a time when the federal government was actively inducting persons into the armed forces under federal draft laws; or
(4) as a result of voluntarily entering on active duty.
(b) A member may not establish more than five years of service credit in the retirement system under this subchapter for military service. Service may be established in one-year increments except as otherwise provided by this subchapter.

(c) The board of trustees may adopt rules expanding the military service creditable in the retirement system in order to comply with the requirements of federal law.
(d) Military service that is terminated by sentence of a court-martial is not creditable under this section.