This should take us back to school and something most understand, but wonder what it actually means. The 12-hour clock can be traced back as far as 1560 BC.
Outside of English-speaking countries, the terms a.m. and p.m. are
seldom used and often unknown. In most countries, computers by
default show 24-hour time.
The letters after numbers when talking about time: a.m. stands for
“ante meridiem”, which is Latin for “Before Midday”; p.m. stands for
“Post Meridiem”, which is Latin for “After Midday”.
Depending on the style guide referenced, the abbreviations are
variously written in small capitals, uppercase letters ("AM" and
"PM"), or lowercase letters ("am" and "pm" or "a.m." and "p.m.").
Guides also suggest the use of a space between the number and the am
or pm abbreviation. It is time for me to be done with this.