5 C.F.R. § 837.503
(a) Title requirements. A reemployed annuitant is entitled, on separation, or conversion to intermittent service, to a supplemental annuity if—
(1) The annuitant performed—
(2)
(b) Computation of supplemental annuity—(1) CSRS.
(c) Creditable service.
(1) All actual reemployment service performed after the date of retirement on a full-time or part-time basis may be credited in the computation of a supplemental annuity provided—
(d) Commencing date.
(1) Except as provided in clause (2) of this subparagraph, the supplemental annuity commences on the earlier of the first day of the month following—
(2) The supplemental annuity of a FERS annuitant, and the supplemental annuity of a CSRS reemployed annuitant who has not elected FERS coverage and who was—