The term BCE (Before Common Era) first appeared in 1615, in the writings of German astronomer Johannes Kepler. It wasn’t invented recently as part of any modern movement. The dating system itself, meaning the calendar that BCE references, dates back even further to 525 AD, when a monk named Dionysius Exiguus created the framework we still use to number our years.
Where the Calendar Baseline Came From
Around 525 AD, a monk named Dionysius Exiguus proposed a new way of numbering years. Before his system, Europeans dated events using the Diocletian era, named after a Roman emperor infamous for persecuting Christians. Dionysius wanted to replace that with a system centered on the estimated birth year of Jesus Christ, which he designated as Year 1. Everything after that point became AD (Anno Domini, “in the year of the Lord”), and everything before it was eventually counted backward from that same reference point.
His system caught on across Christendom over the following centuries and became the standard way of marking time in the Western world. When Pope Gregory XIII revised the calendar in 1582 (creating the Gregorian calendar we use today), he kept Dionysius’ numbering scheme intact.
How BCE Replaced BC
The shift from “BC” (Before Christ) to “BCE” started in the 17th century, not the 21st. Kepler, a devout Christian scientist, used the Latin equivalent of “vulgar era” in his astronomical writings. At the time, “vulgar” didn’t mean crude. It meant “common” or “ordinary,” referring simply to the present era of timekeeping. Writers began using “vulgar era” interchangeably with “after the time of Christ” and “in the common era.” Over time, “common era” stuck, was shortened to CE, and its counterpart BCE (Before Common Era) followed naturally.
The terms appeared in English by the 18th century. Their use has grown significantly in recent decades, particularly in academic, scientific, and multicultural contexts, because they describe the same calendar system without requiring a specifically Christian label. The years themselves don’t change: 500 BCE is the same year as 500 BC.
How BCE Years Are Counted
BCE years count backward. The year 1 BCE comes right before 1 CE, and as you go further into the past, the numbers get larger. So 3000 BCE is older than 500 BCE. Think of it like a number line with Year 1 at the center: numbers increase in both directions, forward into CE and backward into BCE. This design means the system can stretch infinitely into the past without running out of numbers.
One detail that trips people up: there is no Year 0. The calendar jumps directly from 1 BCE to 1 CE. This wasn’t an oversight exactly. When Dionysius created his system in the 6th century, the concept of zero hadn’t yet reached Western Europe. By the time zero arrived through medieval Arabian scholarship and the Gregorian calendar was introduced in 1582, nobody corrected it. The missing year creates a one-year gap that still causes problems today, particularly for scientists working with tree ring data, ice cores, and radiocarbon dating where precise year counts matter.
How Astronomers Handle the Gap
To get around the missing Year 0, astronomers use a different numbering system. In astronomical year numbering, 1 BCE becomes Year 0, 2 BCE becomes -1, 3 BCE becomes -2, and so on. NASA uses this system extensively for eclipse calculations and other work that requires clean math across the BCE/CE boundary. The formula is straightforward: any year labeled “x BCE” becomes -(x-1) in astronomical notation. So 44 BCE (the year Julius Caesar was assassinated) is -43 in the astronomical system.
This distinction matters if you’re reading scientific papers or astronomical tables. Historians and astronomers are literally off by one year when referencing the same ancient events, and both groups have to be careful when translating between systems.
Counting Centuries in BCE
Centuries in BCE follow the same logic as CE, just in reverse. The 1st century BCE covers the years 100 to 1 BCE. The 2nd century BCE covers 200 to 101 BCE. This can feel counterintuitive because the century number and the year digits don’t match in the way you might expect: an event in 250 BCE falls in the 3rd century BCE, not the 2nd.
The trick is the same one that applies to CE dates. The year 2024 is in the 21st century, not the 20th, because the 1st century started at Year 1 rather than Year 0. In BCE, just divide the year by 100 and round up. The year 450 BCE sits in the 5th century BCE. The year 1200 BCE is in the 12th century BCE.