"What does AM and PM stand for in time measurement?"

Hello zaglawgirl,

Thank you for your question.

A good explanation of AM and PM appears at greeenwichmeantime.com
where the question is asked "What is noon and midnight"?

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

What is Noon and Midnight?

AM and PM start immediately after Midnight and Noon (Midday)
respectively.

This means that 00:00 AM or 00:00 PM (or 12:00 AM and 12:00 PM) have
no meaning.

Every day starts precisely at midnight and AM starts immediately after
that point in time e.g. 00:00:01 AM (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 PM starts e.g. 00:00:01 PM (12:00:01)

AM  abbreviation "ante 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. 
 
PM  abbreviation "post meridiem"

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. 
 

Searching to further define the origin and meaning of ante meridiem, I
found a short description at a Wolfram Research page on astronomy by
Eric W. Weisstein:

http://scienceworld.wolfram.com/astronomy/AnteMeridiem.html

Being before noon. "Ante meridiem" is commonly abbreviated a.m., A.M.,
or AM, and is used in time designations in the United States 12-hour
clock system to indicate the morning hours. Noon itself is neither
a.m. nor p.m., but the minute between 12:00 and 12:01 is p.m., so
12:00 noon is often denoted p.m. on digital clocks in the U. S.

Similalrly, he describes post meridiem as:

Being after noon. "Post meridiem" is commonly abbreviated p.m., P.M.,
or PM, and is used in time designations in the United States 12-hour
clock system to indicate the afternoon hours. Noon itself is neither
a.m. nor p.m., but the minute between 12:00 and 12:01 is p.m., so
12:00 noon is often denoted p.m. on digital clocks in the U. S.

And finally, Bartleby.com has the American Heritage definition online
at http://www.bartleby.com/61/82/A0328200.html :

ante meridiem 

ADVERB & ADJECTIVE: abbr. A.M. or a.m. or a.m. Before noon. Used
chiefly in the abbreviated form to specify the hour: 10:30 a.m.; an
a.m. appointment.

ETYMOLOGY: Latin ante, before + merdiem, accusative of merdis, noon.

Merdis is commonly translated as "midday"


Search strategy "AM and PM"  "ante meridiem"


I trust this answers your question.

Regards,

-=clouseau=-

The am / pm system has been widely used in the US, but the peoples of
Europe, Scandinavia, and most of Asia have long ago moved over to the
24-hour clock system, where 00:00 is midnight, 12:00 is midday, and
24:00 is the end of the day and is equivalent to 00:00 of the
following day.

This is defined in an International standard called ISO 8601, which
also has YYYY-MM-DD for dates, which neatly avoids the ambiguity that
05/04/03 means May 4th in the US and 5th April in parts of Europe.

As to the comment about avoiding ambiguity in time tables where times
at midnight are concerned some European time tables will list trains
arriving at midnight as 24:00 and those departing at the same time as
00:00