Is 12 midnight AM or PM?

a.m.

URL below: "This is a tricky question. The answer is that the terms 12 a.m. and 12 p.m. are wrong and should not be used. To illustrate this, consider that "a.m" and "p.m." are abbreviations for "ante meridiem" and "post meridiem." They mean "before noon" and "after noon," respectively. Noon is neither before or after noon; it is simply noon. Therefore, neither the "a.m." nor "p.m." designation is correct. On the other hand, midnight is both 12 hours before noon and 12 hours after noon. Therefore, either 12 a.m. or 12 p.m. could work as a designation for midnight, but both would be ambiguous as to the date intended."

Here's the site for the quote above.

http://tf.nist.gov/general/misc.htm

I believe conventional usage is that 12:00 pm is understood to mean "Noon", and 12:00 am is understood to mean "Midnight," inspite of the strict meaning of am/pm per the quoted reference. I think the reasoning to follow is that 12:01 (or 12:00:01 am) is clearly one minute (or one second) after midnight, so we say 12:00 is midnight, and similarly, 12:01 pm being one minute past noon, implies 12:00 pm is noon.

12 midnight is the beginning of the new day--a.m.

For the reasons given above, "noon" and "midnight" are the preferred usage in most stylebooks.