Active duty - means full-time duty in the Armed Forces or as a commissioned officer of the regular or Reserve Corps of the Public Health Service or of the National Oceanic and Atmospheric Administration. It does not include any period during which an individual:
(1) Was assigned full-time by the Armed Forces to a civilian institution for a course of education which was substantially the same as established courses offered to civilians,
(2) Served as a cadet or midshipman at one of the service academies,
(3) Served under the provisions of section 511(d) of Title 10, United States Code, pursuant to an enlistment in the military reserve or national guard,
(4) Served in an excess leave without pay status, or
(5) Served in a status specified in § 3.15 of this chapter.