Why do we say A.M. and P.M. when telling the time?
There are 24 hours in a day, but only 12 figures on the clock. This is because in most countries the day is divided into two parts of 12 hours each. We call the first part of the day a.m., meaning “before noon,” and the second part p.m., which stands for “afternoon.”
The first crude clocks did not keep good time and had to be set every sun-shiny day at noon, when the sun was at the “meridian,” or its highest point in the sky.
From this we began to call the first part of the day a.m., meaning “ante (before) meridian,” or before noon; p.m. means “post (after) meridian,” or after noon.
Some people tell time as if every clock had 24 hours marked on its face instead of 12.
They count the hours in the day from 1 to 24 and say “hours” instead of “o’clock.” They tell the time with four numbers. Thus 1 a.m. is 0 1 0 0, or “oh-one hundred hours.” 1 o’clock p.m. is “1 3-hundred hours –1 3 0 0 hours.” The last two numbers stand for the minutes. Thus, “1 3 1 5 hours” is fifteen minutes after the hour, or 1:15 p.m. – Dick Rogers