The difference between a.m. and p.m. is in the time of day. The designation a.m. refers to any time that is after midnight and prior to noon. Any time that is between noon and midnight is designated p.m.
The designation a.m., which is short for ante meridiem in Latin, means "before the sun passes the meridian." An example of a.m. usage is 10:00 a.m. The designation p.m. is short for post meridiem in Latin. An example of p.m. usage is 3:00 p.m. Noon refers to 12:00 p.m., and midnight refers to 12:00 a.m.
The U.S. Army uses a 24-hour format to indicate time rather than a 12-hour format, so a.m. and p.m. are not needed.