Hey Jesus, what year is it? (Photo: biblevector/Flickr and Kate Wheeling/Pacific Standard)

I'm writing this on May 17, 2015. At its core, that date—any date really—is just a code. It's a three-part system allowing those in various locations and points of time to distinguish when an event occurred or will occur. The first two parts—the month and date—have had a legion of originators, from Cro-Magnon astronomers marking phases of the moon on their eagle bones, to Mayan mystics tracking the movements of the stars from their forest canopies. The 365-and-change-day calendar we use is the result of scientific sweat, an attempt to bring us to a Verifiable Truth regarding how long it takes the Earth to complete one rotation around the sun.

But while months and days are based on the planet's gravitational forces, and thereby grounded in reality, the third aspect of our dating code is a total mess. The Earth is way past its 2,015th birthday. Why do we consider 2015 A.D. the year we're currently living in?

Spoilers: The planet's been around longer than any of us—or any of our ancient relatives—can remember. Tests date the Earth to about 4.54 billion years old, but a whole lot of that time didn't really have anything of substance—to us humans, at least. Starting a calendar 4.54 billion years ago doesn't make much intuitive sense. Rather, we need to find another, closer Year One to begin things. This, as you'd imagine, is where things get chaotic.

“At no point in world history has there ever been a single uniform dating system that's unanimously agreed to be shared by everyone. In the Middle Ages and Antiquity, there were multiple eras jostling for recognition.”

“At no point in world history has there ever been a single uniform dating system that's unanimously agreed to be shared by everyone,” says Dr. Carlos Noreña, a scholar of ancient history at University of California-Berkeley. “In the Middle Ages and Antiquity, there were multiple eras jostling for recognition.”

The key wasn't what Year One was, as much as getting everyone on the same page. (Let's not even discuss Year Zero, seeing as this jockeying for Year One position occurred before the concept of zero had even been invented.) If we wanted to allow for commerce, trade, and simple communication across cultures to develop, we needed to be living in the same year.

The Greeks were among the first to try to get everyone running on the same year. They looked into their archives, decided Year One should be an event of cultural importance, and settled on the first year that the Olympic Games were held. For us, that's 776 B.C., but for the Greeks—and other countries and cultures under their dominance—that was Year One.

The Roman Empire had its own system. They looked into their archives, also decided Year One should be an important event, and decided it should be the founding of the city of Rome, an event that supposedly took place in what we now consider 753 B.C. To the Roman Empire, that was Year One ab urbe condita, or A.U.C. As the Empire spread, so did the concept. “The Romans didn't impose their dating system,” Noreña says. “But because they were so powerful and influential, people picked up their calendar and dating system because it was convenient.”

While these were the dominant systems, there was a hodge-podge of various cultures with different Year Ones. The Byzantine Empire started its first year in what was considered the year of creation (our 5509 B.C.) and simply counted up. The Church of Alexandria began its Year One in what is now 284 A.D., to coincide with the rise of Roman emperor Diocletian into power. The Sumerian calendar re-started Year Ones with the rise of each new king. The Mayan's wheel-like Long Count calendar began at what is now 3114 B.C., ultimately completing one cycle and starting anew on the not-at-all-end-of-the-world December 21, 2012.

Into this calendar chaos, a humble monk by the name of Dionysius Exiguus stepped in. “He said, wait a minute, wait a minute, why are we dating things by the founding of the city of Rome?” Noreña says. “That doesn't make any sense. Surely we can find a better event to start counting from. Of course, his choice was the birth of Christ.” Dionysius took his abacus, crunched some numbers, and figured out when Jesus was born. He wrote a letter to a bishop named Petronius detailing his plans for Year One, designating it as anno Domini (A.D.), which translates to “the year of our Lord.”

That said, Dionysius forgot to carry some ones. “The Gospel of Matthew claims [Jesus] was born in the time of Herod the Great, who died in 4 B.C.,” writes David Ewing Duncan in his book Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year. “This means the birth must have occurred before this date. Other Gospels and historical sources suggest dates ranging from 6 or 7 B.C. to A.D. 7, though most historians lean towards 4 or 5 B.C. This means the year 1996 or 1997 was probably the true year 2000 in the anno Domini calendar, if one does the arithmetic without a year 0.”

While Dionysius' concept didn't take off immediately, he got some help from a monk pal who used A.D. in a few of his textbooks, which were then taught by Christian missionaries. And then, in our year 731 A.D., the historian Venerable Bede used A.D. in his Ecclesiastical History of the English People. That bout of publicity coincided with the spread of Christianity and—just as the Romans spread their Year One during their era of supremacy—the concept of 1 A.D. as Year One continued. The last big holdouts were Portugal, which adopted the system in 1422, and the Russian empire, which gave in by 1700. By the 19th century, most everyone was on board.

It's worth pointing out the concept of B.C.—that is, “before Christ”—wasn't introduced until 1627, by a French astronomer. He reasoned that the world existed before 1 A.D., so decided to figure that in by counting backwards. Due to its logic, B.C. was an easier, and quicker, sell.

That takes us to the present. The most recent dating battle has been a semantic one over the rise of C.E./B.C.E., which uses the same Year One starting point, but removes the religious implications by referring to Common Era. “This is a little bit silly for two reasons,” Noreña says. “One, they use the same year, so it's the same system. And two, when most people see it, they think it stands for Christian Era and Before Christian Era, so it doesn't really solve the problem people wanted to solve.”

If Jesus was never born, meaning Christianity never existed, meaning Dionysius never stuck his nose into our calendar back in the 6th century, what year would it be? There are a few possibilities:

We'd still be using the Roman Empire's Year One. It was the most widespread at the time, giving it a head-start over any other Year Ones. In that scenario, we'd currently be living in the year 2768 A.U.C. No Jesus means no Christianity, leaving another religion to fill the void. One distinct possibility would be Islam. “If Islam had become ascendent in the Western world and Europe, which it almost did, we'd probably be on that dating system,” Noreña says. (Feel free to take a minute or 60 to consider the theological implications of having Islam without first having Christianity.) The Islamic calendar—still in use in some Muslim countries—begins in 622 A.D., the year of the Prophet Muhammad's emigration from Mecca to Medina, an event known as the Hijra. In this scenario, we'd be using iPhones and entering the world of consumer space travel while living in the year 1393 A.H., or after Hijra. This is the rest of the field. Some other religion could've risen, spreading its dating system as Christianity did. Say, when did Xenu come to Earth and plop aliens into volcanoes? Or, maybe a pre-existing calendar like the Byzantine would have undergone a resurgence and dominated the landscape again.

As the world continued to “shrink” due to the establishment of trade routes and expansion of population and as once-insular communities started opening up and exploring, a single Year One would have inevitably dominated. The specifics of which one are not particularly important. “It could've gone in any number of different ways,” Noreña says. “The fact that we're living in 2015 is a historical accident.”

The Sociological Imagination is a regular Pacific Standard column exploring the bizarre side of the everyday encounters and behaviors that society rarely questions.