How did ancient folks reference BCE times? | AskHistorians Archive Observer

Someone else might be able to chip in on the origins and spread of BC/AD (and the growing use of BCE/CE, which started sometime in the 19th century but only really became widespread from the 1980s or 1990s) specifically. The really short version is "this is a reference point for time that Christian civilizations had in common, and it's one of the only ones they had in common, so they eventually ran with it."

As far as the use of time periods and year references prior to that, or outside of the Christian cultural sphere, there's at least as many different methods as there are cultures. I'll sort of begin to kind of scratch the surface here: the tl;dr version is "they referenced time periods in a whole lot of ways, often several at once, and we still do so today."

Major religions like to build calendars around the establishment date of their religion (in the Islamic calendar we're currently in the year 1438) or their accepted date for the creation of the world (in the Hebrew calendar, it's 5777). Christian dates use the same theme, with some extra fun and confusion from slight ambiguities around the dates of Jesus' birth and death, and more-than-slight ambiguities over clergy's attempt to calculate the exact date of the world's creation.

(As an aside: while the clergy-calculated creation dates were obviously off-base, winding up with times in the 3000s to 5000s BC as the universe's starting point, the work they did to come to those points, with the knowledge and evidence they had available, was legitimately impressive. Building a chronology largely from first principles and scattered, often-contradictory texts like the Bishop Usshers of the past did is hard.)

On more down-to-earth levels, there are a lot of less lofty reference points used. Referencing things by the current leader was very common, particualrly when they were still practically running things: "in the tenth year of Elizabeth's reign" or "during the consulship of P. Cornelius Rufinus and M. Curius Dentatus" place things in time fairly well for people who knew who they were and could fit them into the general sequence of things. The Hellenistic calendar juggled regnal references like this and Olympiads, four-year periods starting in 776 BCE. These were practical. They worked on the sorts of time scales most people were interested in, precisely enough that you could date something specific within them. If you've ever talked about something that happened back in the Clinton years, you've done this.

On even more down-to-earth levels, a lot of people simply wouldn't have cared as much about the specific year on a calendar, and would date things much more contextually: "the year of the two bad floods," or "the year of the plague," or "the year where we had that really good harvest," or "the year after that really good harvest where people spent two solid months hammered because oh god awesome beer everywhere," etc. People still uses this one casually: we all talk about "back when I was in high school" or "during the war," and everyone in my neck of the woods knows exactly what I'm talking about when I refer to that godawful nor'easter-every-single-Wednesday winter. It's nowhere near a formal calendar, but like with regnal references it's practical: good enough for people in the conversation to place something in sequence.

Of course, there's larger-scale versions like "the post-9/11 era," which don't have any official status but which are seem as a transition point.

Another set of common themes runs them off an event that's important to the culture if not its whole world: when a state was founded, or the first occurrence of some big event that's important to them. This is a similar idea to the religious calendars I mentioned above - and sometimes they overlap a lot - but it's meant as specifically relevant to a particular people. Romans would reference their assumed founding year of the city in 753 to date things. North Korea's calendar had a reset so 1912 - the birth year of Kim Il-Sung - was their new Year One, placing us a few days into the year 106 there. This replaced their previous calendar, which used the establishment of one of the first Korean kingdoms as its starting point.

Combinations were common too! As an example, here's Dionysus of Halicarnassus trying to puzzle out the founding year of Rome in his Roman Antiquities:

As the reigns, therefore, of the kings amount to two hundred and forty-four years or sixty-one Olympiads, it follows necessarily that Romulus, the first ruler of the city, began his reign in the first year of the seventh Olympiad, when Charops at Athens was in the first year of his ten-year term as archon.

You see several things at once here: he references "the seventh Olympiad" as an absolute calendar reference, references a point in Charops' archonship as a second reference point based on leaders, and a span of 61 Olympiads beyond this point, giving readers another timespan reference they'd be used to parsing in their heads.

The United States still actually does a variation of this today: legislation and proclamations are dated in both the Christian calendar and a separate national-founding one: "this third day of April, in the year of our Lord two thousand seventeen, and of the Independence of the United States of America the two hundred and forty-first."