Answer:
Traditionally, 12:00 is neither am or pm, it is either midnight or noon. With the advent of digital clocks, midnight marks the start of the am portion of the day.
The thirtieth edition of the U.S. Government Style Manual (2008) sections 9.54 and 12.9b recommends the use of "12 a.m." for midnight and "12 p.m." for noon.