Francois Demers

AM stands for "Ante Meridiem" and "Post Meridiem", Latin for "Before" and "After" the middle of the day which is deemed to be the instant when the Sun is due South in the sky. There is archaelogical evidence for this convention dating back to 1500 BC, when solar clocks were just about the only way to measure the passage of time with socially-useful accuracy. The convention eventually fell into disuse at the beginning of the 17th century, when labelling from 00:00 to 23:59 became more popular, except in British colonies. Any time-labelling system is a convention. The "am pm" convention is, by far, the most-used in the USA. Claiming that there is no such thing as 12AM and 12PM is the same as claiming there is no such thing as dollars (the name of the currency is a social convention), pounds, or miles (the names of units of mass and distance are social and legal conventions) . In this, as in other linguistic problems, usage rules.

I am oficially introducing myself as a non-scientist interested in science.