A.M. and P.M.

Meaning of A.M. & P.M.

AM is the abbreviation for "ante meridiem", i.e. 'before noon'.

Consequently PM stands for "post meridiem",  i.e. 'after noon'.

Every time before 12 noon is an a.m. time and after 12 noon you see time data enriched with a p.m.

It doesn't matter whether you use capital letters or lower case letters - every form can be found in daily life.

In the US it's very common to use the 12-hour display instead of the more meaningful 24-hour display. So the indicator a.m. resp. p.m. is needed.

But some problematic areas exist.

Noon & Midnight

How would you indicate 12 o'clock NOON (sharp) and 12 o'clock MIDNIGHT (sharp). The answer is that nobody would use a.m. or p.m., because acertain confusion would be included automatically. So just say "Noon" or "Midnight", whatever is meant.
Anyway that is not really appropriate for time tables, e.g. at airports. To write 'noon' would look strange.
That  means that such timetables, especially such that deal with sensitive time information,  are usually written in a 24-hour format.

International Timetables
To avoid any ambiguity one should use the 24-hour format. Science and military do it the same way, there usually doesn't exist a 12-hour format.

And in case that a 12-hour format is used in any timetable, a common way is to avoid 12 o'clock sharp. Instead it'll be: 12.01 a.m. (standing for 00.01) or 11.59 p.m. (23.59) and so on.

 

AM or PM without time
If you hear somebody saying "I call you tomorrow a.m." ... then it is meant the he calls in the morning before noon.