a.m is short for ante merridean (latin translated as before noon), pm for post merridean. So twelve noon can not be either am or pm - it is m. One minute after noon is 12:01 pm. Midnight being 12 hours after noon could be described as 12:00 pm as it is 12 hours after mid-day but one minute later is 00:01 am. It is simpler for digital watches and clocks to call midnight 0:00 am. 12:01-12:59 are somtimes described as am, followed a minute later by 1:00 a.m, which doesn't make much sense.

All of which is probably exactly what Nigel said up there in the first place.

Use the 24 hour clock. Its much simpler.