20 CFR §404.1019

1. (a)Your work as a member of a uniformed service of the United States is covered under Social Security (unless creditable under the Railroad Retirement Act), if—
   1. (1)On or after January 1, 1957, the work is service on active duty or active duty for training but not including service performed while on leave without pay; or
   2. (2)On or after January 1, 1988, the work is service on inactive duty training.
2. (b)You are a member of a uniformed service if—
   1. (1)You are appointed, enlisted, or inducted into (or a retired member of)—
      1. (i)One of the armed services (Army, Navy, Air Force, Marine Corps, or Coast Guard); or
      2. (ii)A component of one of the armed services, including any reserve component as defined in Veterans' Benefits, 38 U.S.C. 101 (except the Coast Guard Reserve as a temporary member);
   2. (2)You are a commissioned officer (including a retired commissioned officer) of the National Oceanic and Atmospheric Administration or the Regular or Reserve Corps of the Public Health Service;
   3. (3)You are a member of the Fleet Reserve or Fleet Marine Corps Reserve;
   4. (4)You are a cadet at the United States Military, Coast Guard, or Air Force Academy, or a midshipman at the United States Naval Academy;
   5. (5)You are a member of the Reserve Officers Training Corps, the Naval Reserve Officers Training Corps, or the Air Force Reserve Officers Training Corps, when ordered to annual training duty for 14 days or more including periods of authorized travel to and from that duty; or
   6. (6)You are selected for active military or naval training under the Military Selective Service Act or are provisionally accepted for active duty in the military or naval service and you are ordered or directed to a place for final acceptance or entry upon active duty and are on the way to or from, or at, that place.