Pub. L. 83-773, tit. V, sec. 511

Sec. 511. (a) A Reserve officer whose name is on a recommended list may be promoted to fill a vacancy at any time, but shall be promoted, irrespective of the existence of a vacancy, on the date upon which he completes four years of promotion service in grade and seven total years of service, if he is in the grade of first lieutenant; seven years of promotion service in grade and fourteen total years of service, if he is in the grade of captain; or seven years of promotion service in grade and twenty-one total years of service, if he is in the grade of major. (b) A Reserve officer on active duty who is recommended for promotion to a grade higher than that in which he is serving, shall, before being promoted, elect to serve on active duty in the grade in which he is then serving or be released.