BC and AD explained

What the labels mean

AD stands for the Latin Anno Domini, “in the year of the Lord”. BC stands for the English “before Christ”. Together they number years forward and backward from a traditional date for the birth of Jesus.

Strict style places AD before the number and BC after it — AD 1066, 44 BC — because Anno Domini is a phrase the year slots into. In practice most publications write both after the number, and QuoAnno does the same on the timeline, where a consistent trailing era label is easier to scan.

There is no year zero

The year before AD 1 is 1 BC. Nothing sits between them. The system was built from Roman numerals, which have no zero, so the count runs …3 BC, 2 BC, 1 BC, AD 1, AD 2…

This makes spans across the boundary one year shorter than subtraction suggests. From 44 BC to AD 2026 is 2,069 years, not 2,070: add the two numbers and subtract one. Within a single era, ordinary subtraction works.

Astronomers avoid the problem by using a different convention, in which year 0 exists and equals 1 BC, so −1 means 2 BC. That is why an astronomical date for an ancient eclipse can look a year off from a historical one. Both are right; they are different scales.

Where the system came from

The monk Dionysius Exiguus devised the Anno Domini count in 525 AD, while drawing up tables for the date of Easter, to replace a system that numbered years from the reign of the emperor Diocletian — a persecutor of Christians. It spread slowly, and owes much of its adoption to Bede, who used it throughout his Ecclesiastical History of the English People in 731 AD.

BC came much later; counting years backward only became standard practice well over a thousand years after Dionysius. And his arithmetic was probably off: most modern scholarship places the birth of Jesus a few years before AD 1, which makes the epoch a convention rather than an anniversary. That does not weaken it — an agreed zero point is useful whether or not it marks what it was meant to.

BCE and CE

BCE (“before the common era”) and CE (“common era”) number years identically. 44 BC and 44 BCE are the same year; AD 1066 and 1066 CE are the same year. Only the label changes, and the aim is a name that does not assert a religious claim in contexts where that is unwanted. Academic and museum publishing has largely moved to them.

QuoAnno uses BC and AD throughout its player-facing timeline. It is the convention most players read a date in fastest, and mixing the two in one interface helps nobody. The underlying numbers are identical either way, so a date you know as 3200 BCE is exactly the date the game will show as 3200 BC.

One more wrinkle: which calendar

An era label says nothing about which calendar produced the day and month. The Gregorian reform of 1582 dropped ten days and changed the leap-year rule, and different countries adopted it across the following three and a half centuries — Britain and its colonies in 1752, Russia not until 1918.

So a seventeenth-century event can carry two different dates depending on whether the source is Old Style or New Style, without either being an error. Where this affects a date here, the event page says so rather than silently picking one.