While most civilians in the U.S. are perfectly happy measuring their time in 12-hour chunks, there are plenty of folks both here and abroad that prefer the more exact notation of the 24-hour clock.
Luckily, iOS makes it easy to switch between the two. Simply fire up the Settings app, tap on General, tap on Date & Time, and turn on 24-Hour Time.
To go back to your a.m. and p.m.