15:30 is the military version of saying 3:30 p.m. Those in the military pronounce it as "fifteen-hundred-thirty hours." Military time runs on a 24-hour clock, whereas the standard 12-hour clock uses a.m. or p.m. to designate before or after 12 noon.
Military time, or astronomical time, divides the day into 24 hours. Midnight is 00:00 hours and refers to the start of a new day, and 24:00 (twenty-four hundred) hours is the correct verbiage for midnight at the end of a day. The last minute of a day is 23:59 hours. One hour past midnight is 01:00 ("zero one hundred") hours.