A.M. and P.M.
Meaning of A.M. & P.M.
AM is the abbreviation for "ante meridiem", i.e. 'before
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
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
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.