12-hour versus 24-hour time

24-hour time runs from 00:00 to 23:59. 0000 is midnight at the start of the day (12:00 AM). 1200 is noon (12:00 PM), not midnight. 2400 is sometimes written for midnight at the end of a day; it is not a standard civil clock time.

The military-time converter on this site labels those cases explicitly.