A. M. e P. M. :


What is Noon and Midnight?

A.M. and P.M. start immediately after Midnight and Noon (Midday) respectively.

This means that 00:00 A.M. or 00:00 P.M. (or 12:00 A.M. and 12:00 P.M.) have no meaning.

Every day starts precisely at midnight and A.M. starts immediately after that point in time e.g. 00:00:01 A.M. (see also leap seconds)

To avoid confusion timetables, when scheduling around midnight, prefer to use either 23:59 or 00:01 to avoid confusion as to which day is being referred to.

It is after Noon that P.M. starts e.g. 00:00:01 PM (12:00:01)

A.M. abbreviation ante meridiem 

P.M. abbreviation post meridiem

Main Entry: an·te me·ri·di·em 
Pronunciation: 
"an-ti-m&-'ri-dE-&m, -dE-"em 
Function: adjective 
Etymology: Latin
Date: 1563
Meaning: being before noon
Abbreviation a.m.

Main Entry: post me·ri·di·em 
Pronunciation:
-m&-'ri-dE-&m, -"em 
Function: adjective 
Etymology: Latin
Date: 1647
Meaning: being after noon
Abbreviation p.m.

Fonte:http://wwp.greenwichmeantime.com/info/noon.htm