In Greenwich (civilian) time, the typical clock has to go around twice in the day. The abbreviations "A.M", and "P.M." are used to differentiate what is the relation of the time shown on the clock to the MERIDIEM.
The word "meridiem" is Latin for "midday". This means that we would tell time according to when in the day the time occurs, whether it is before or after midday.
Hence, the "A" in the abbreviation A.M. stands for the Latin "Ante" which means "prior to", or "before". The "P" in the abbreviation for P.M stands for "Post" , which is the contrary of "before", or "after."
Therefore, A.M - Ante Meridiem means before midday while P.M., or Post Meridiem means "after midday". Granted, we quit using P.M. right at midnight. Therefore, A.M begins the hour after midnight, at 1:00 AM. In Zulu /UTC (military/aviation) time, midday is referred to as 1200, midnight is referred to as 0000, and 1 A.M. is referred to as 0100, and it continues the same pattern throughout the day.
“AM” stands for the Latin phrase Ante Meridiem —which means “before noon”—and “PM” stands for Post Meridiem : “after noon.”