10 U.S.C. § 12646 — Commissioned officers: retention of after completing 18 or more, but less than 20, years of service

(a) If, on the date a reserve commissioned officer would normally be discharged or transferred out of active status, they have at least 18 but less than 19 years of service (counted under section 12732), they can't be discharged or transferred without their consent before the earlier of: the date they reach 20 years of service; or the third anniversary of when they would otherwise have been discharged or transferred. (b) The same rule applies to an officer with at least 19 but less than 20 years of service, except the deadline is the earlier of: reaching 20 years of service; or the second anniversary of their original discharge or transfer date. (c) An officer kept on under (a) or (b) counts as an extra position beyond what's otherwise authorized. (d) Subsections (a) and (b) don't apply to: officers discharged or transferred for physical disability, for cause, or because they've reached the mandatory discharge age; or commissioned warrant officers. (e) A reserve commissioned officer on active duty (not just for training) or full-time National Guard duty (not just for training), who is within two years of qualifying for retirement under section 7311, 8323, or 9311, and who would otherwise be removed from active status under section 8373, 14513, or 14514 of this title or section 740 of title 14, may — at the Secretary's discretion — be kept on that duty for up to two more years. This extension is allowed only if: by the end of that extra period, the officer will qualify for retirement under section 7311, 8323, or 9311; and before that period ends, the officer won't reach the age requiring mandatory transfer or discharge. An officer kept on under this provision can't be removed from active status while still on that duty.