Why do we say A.M. and P.M. when telling time?

The  letters A.M (ante meridian) P.M (post meridian) tell us  what part of the day it is on a 12 hour clock. 

As we all know, there are 24 hours in a day but only 12 hour numbers on a clock.  This is because in  most  countries the hours of the day are usually divided into two parts  for  timekeeping..

We call the  first  part  of the day  a.m. the second part p.m.

The  first clued clocks did  not keep  good time.  They had to be set every  day at noon, when the  sun  was at the  meridian, or the  highest point in the sky.

From  this we began to call the morning  part of the day a.m., meaning ”ante meridian.” or before noon.  P.M. means “post meridian” or after noon.

If every  town set its  clocks  by the sun today, few  places would  be on  the  same time.  When it is noon  in  one town, it is not yet noon in a town a few miles east.  For this reason. the world is divided into time zones.   All the clocks in a time zone are set  the same time.-Dick Rogers