5 U.S.C. § 8301: Uniform retirement date

(a) Except as otherwise specifically provided by this title or other statute, retirement authorized by statute is effective on the first day of the month following the month in which retirement would otherwise be effective. (b) Notwithstanding subsection (a) of this section, the rate of active or retired pay or allowance is computed as of the date retirement would have occurred but for subsection (a) of this section.