Americans use many abbreviations in everyday speech and writing. Shorthands like "yd" for yard and "a/c" for air conditioning are pretty straightforward, but there are some misconceptions about others.
For example, take "a.m." and "p.m." People in the U.S. are constantly thinking and talking about time in terms of a.m. and p.m., but if you ask someone what those letters stand for, you may be surprised how little people actually know about the abbreviations.
Some say it's "after midnight" or "past midday," but that isn't exactly true...
In fact, like many words and phrases in the English language, a.m. is short for a Latin phrase, "ante meridiem" (or "before midday"), while p.m. is short for "post meridiem" (or "after midday").
While there isn't a ton of information out there about when exactly a.m. and p.m. came into popular use in the English language, the 12-hour timekeeping system dates back to ancient Egypt and Mesopotamia. During the Renaissance period, mechanical clocks with 12-hour analog dials appeared in churches and palaces throughout Europe.
Today, most of the world uses a 24-hour system, while many English-speaking countries follow the 12-hour clock. In the places that use the 12-hour standard, there is some debate around what to use for noon and midnight.
By convention, people in the U.S. say 12 p.m. in reference to noon and 12 a.m. for midnight. But some believe that going from 11 p.m. to 12 a.m. and from 11 a.m. to 12 p.m. is confusing. Others would argue that noon and midnight are neither a.m. nor p.m. since noon IS midday, it can't technically be ante meridiem or post meridiem midday. Likewise, midnight is both 12 hours before midday and 12 hours after midday, which makes it confusing to choose between a.m. or p.m.
To avoid this confusion, many prefer "12 midnight" or "12 noon," but that can seem a bit redundant, given that "noon" or "midnight" alone convey the same information.
A lesser-known option is to denote noon with "m" for "meridiem," as in 12 m ― though you run the risk of people interpreting that as "12 midnight."
Given all the confusion and debate, maybe we should go with the 24-hour military time and be done with it? After all, who doesn't love staying up late on Dec. 31 to shout "Happy New Year!" at zero hundred hours? (Or is it twenty-four hundred hours?)