We see them all the time at museums, in history books, and on the clock: A.D., BCE, UTC, and EST — but what do they all mean? These abbreviations denote dates and times, and in some cases, may describe the exact same thing (A.D. and CE, for example). This might sound confusing, but once you learn their origins and what they stand for, they’re a lot easier to remember.
A.M. and P.M.
A few countries, namely the United States, Canada, and Australia, use the 12-hour clock instead of the 24-hour clock. This results in a need to differentiate the two halves of a 24-hour period (morning versus night), so the abbreviations “a.m.” and “p.m.” were created. The origin of these abbreviations is simple — “a.m.” stands for the Latin phrase ante meridiem, meaning “before noon,” while “p.m.” is short for post meridiem, the Latin phrase for “after noon.” The 24-hour system, which is used throughout the rest of the world, has no need for such abbreviations.
B.C. and A.D.
B.C. and A.D. are the oldest date abbreviations still in use. “B.C.” stands for “before Christ,” as in, before the birth of Jesus Christ, a central figure in Christianity. “A.D.” stands for anno Domini, meaning “in the year of the Lord” in Latin. There is no year zero in this system — 1 A.D. was the year Christ was born, and 1 B.C. was the year before he was born.
It is unknown when this naming system first came into use, but the terms were adopted separately — “A.D.” was used centuries before “B.C.” ever was. By the eighth century, “A.D.” was used regularly in England in church documents and charters, and its use quickly spread throughout western Europe in the ninth century. “B.C.” took much longer to catch on. There were variants of the “before” part until the 18th century, when “B.C.” finally became mainstream.
Although the B.C./A.D. system took a while to become standardized, it smoothed out a lot of calendar confusion. Before this dating system, years were centered around significant events, such as the rules of emperors and kings, leaving room for a lot of misinterpretation and errors. The B.C./A.D. system was the first time that every single year was accounted for in such a uniform way.
BCE and CE
A modern answer for B.C./A.D. is BCE/CE, which counts years in essentially the same way. These secular abbreviations stand for “Before Common Era” and “Common Era,” but are tied to the same periods of the Christian dating method.
The BCE/CE system has been around since the 18th century, used by various English scholars and writers. In the 17th century, before the new system caught on, "Vulgar Era" was used instead of "Common Era." From Latin, “vulgar” meant “common,” not “crude.” However, “Vulgar Era” didn’t stick, and “BCE” and “CE” became increasingly popular.
So, which set of abbreviations is the correct set to use? There is no right answer, but certain style guides will dictate which version to use. The BCE/CE system has become more popular in the last few decades as a way to be more inclusive in secular writing, but B.C./A.D. is still widely accepted.
UTC and GMT
A map of the world’s time zones will include ranges of numbers (from one through 14), plus or minus a certain number of hours from the UTC 0 zone. "UTC" stands for Coordinated Universal Time. This standard zone for regulating clocks is located on the prime meridian, which is the zero-degree longitudinal line. Iceland, England, Spain, Liberia, and Ghana are located within this zone. Moving to the east, each time zone goes up +1 from UTC, and moving west, each zone goes down -1 from UTC. For example, New York City is within the UTC -4 time zone (and also the EST zone, but we'll get to that later).
UTC goes by a few other names, including the obsolete “Greenwich Mean Time” (GMT) and “Zulu Time” in the American military. It was called Greenwich Mean Time from 1884 to 1972 (when UTC took its place) because of the location of the Royal Observatory Greenwich in England, from which modern time was standardized. The word “mean” came about because GMT was the average of the time when the sun crossed the prime meridian at the observatory in Greenwich. “Zulu Time” (still in use in the military) is based on this system — GMT was “zero meridian,” and the letter “z” in the military is “Zulu.”
U.S. Time Zones
Each time zone within the United States has its own UTC designation, but it also has its own local time zone name. There are six time zones within the United States; from east to west, they are Eastern Standard Time (EST), Central Standard Time (CST), Mountain Standard Time (MST), Pacific Standard Time (PST), Alaskan Standard Time (AKST), and Hawaii-Aleutian Standard Time (HST). The Hawaii-Aleutian zone gets its name from Hawaii and the Aleutian Islands in Alaska, both of which fall under this zone, while the rest of Alaska is under Alaskan Standard Time.
There are an additional three time zones in U.S. territories, including Atlantic Standard Time (AST) in Puerto Rico and the U.S. Virgin Islands, Samoa Standard Time (UTC-11) in American Samoa, and Chamorro Standard Time (UTC+10) in Guam. The latter's name comes from the Chamorro people, an Indigenous group from the Mariana Islands, where Guam is located.
The next time you’re looking at a confusing list of acronyms attached to a calendar invite, remember these origins and decipher the correct date and time.
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