When converting a.m./p.m. to 24-hour time for the hours between 12:00 a.m. and 12:59 a.m., just subtract 12 from the time. For instance, 12:45 a.m. minus 12 will equal 0045, and 12:59 a.m. minus 12 will equal 0059. Times between 1 a.m. and 12:59 p.m. are a straight conversion, such as 9:00 a.m. equals 0900 and 11:00 a.m. equals 1100. For the conversion between 1 p.m. and 11:59 p.m., add 12, such as 8:34 p.m. plus 12 equals 2034, and 11:59 p.m. plus 12 equals 2359 (also seen as 20:34 and 23:59).

To convert in the opposite direction from the above, add 12 for the hours between 0000 (midnight) and 0059, such as 0034 plus 12 equals 12:34 a.m., and 0059 plus 12 equals 12:59 a.m. To convert from 0100 to 11:59 and from 1200 to 12:29, there is a one-to-one conversion, the former to a.m. and the latter to p.m. For example, 0100 equals 1 a.m., 0345 equals 3:45 a.m., 1235 equals 12:35 p.m., and 1259 equals 12:59 p.m. To convert between 1300 and 2359, subtract 12, such as 1424 minus 12 equals 2:24 p.m., and 2359 minus 12 equals 11:59 p.m.