Could you explain the meaning of A.M. and P.M., and when this division
of time began ?

Dear zymurgy,

"A.M." is an abbreviation of the Latin "ante meridiem", meaning
"before midday". It describes the part of a day before noon.

"P.M." is an abbreviation of the Latin "post meridiem", meaning "after
midday". It describes the part of a day after noon.

This division of the day in two halfs goes back to the times of
Ancient Rome, when the days were divided this way.


Sources:

ThinkQuest: Ancient Roman Time-Telling Devices
http://library.thinkquest.org/26602/timedates.htm?tqskip1=1#clocks

Capitolinum.org: Roman numeration
http://www.capitolium.org/eng/ludi/numeri.htm

Mores: Telling Time in Ancient Rome
http://www.dl.ket.org/latin3/mores/techno/time/tellingtime.htm


Search terms used:
introduced "a m" "p m" "ante meridiem" post
://www.google.de/search?q=introduced+%22a+m%22+%22p+m%22+%22ante+meridiem%22+post&hl=de&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&start=0&sa=N
romans "ante meridiem" abbreviations
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=romans+%22ante+meridiem%22+abbreviations&meta=


Hope this answers your question!
Regards,
Scriptor

Thanks!  I had "brain freeze" there for a while, and your answer was
right on.  Nice to know that I can ask any question!  (I always wanted
a job with the group "They said" and be able to share wisdom...) Glad
that I found that group of people to help me ! :)