Anno Domini

The terms AD and BC are labels used to name years in the Gregorian and Julian calendars. AD stands for the Medieval Latin phrase "Anno Domini," which means "in the year of the Lord." In English, people often use the term "before Christ" for the label BC. This system uses the birth or conception of Jesus as a central starting point, known as an epoch. Years marked AD count forward from this point. Years marked BC count backward from it. It is important to note that there is no year zero in this system. Therefore, the year AD 1 follows immediately after 1 BC.

This dating method was invented in 525 by a monk named Dionysius Exiguus. He was an Eastern Roman monk who wanted to create a new Easter table. Before his invention, people used the Diocletian era to track years. Dionysius did not want to use that system because it was named after a tyrant who had persecuted Christians. He replaced the old system by calculating years based on the incarnation of Jesus. In his own records, he stated that the year 525 was 525 years since the incarnation of our Lord.

Dionysius did not leave a clear explanation of how he chose his starting date. This has led to many different theories among historians. One theory is that he used the Gospel of Luke to calculate the date. That Gospel says Jesus was about thirty years old during the fifteenth year of Tiberius Caesar's reign. By subtracting thirty years from AD 29, historians like Tertullian and Eusebius suggested Jesus was born in 2 BC. Another theory suggests he used a 532-year astronomical cycle. A third idea is that he used an older source, such as the Chronograph of 354, which placed the birth in AD 1.

Because the exact date of Jesus's birth is not in the Gospels or secular texts, scholars use different methods to estimate it. Most modern scholars believe the birth actually occurred around 5 BC. They look at historical events mentioned in the accounts of Matthew and Luke to find a window. They also work backward from the estimated start of Jesus's ministry. While the exact year remains a subject of debate, the system Dionysius created became a way to organize all human history. This organization helped people move away from older calendars like the Anno Mundi, which counted years from the creation of the world.

Even though the system was invented in the 6th century, it was not used everywhere immediately. It did not become widely used until the 9th century. An Anglo-Saxon historian named Bede helped spread the use of AD in his work, the *Ecclesiastical History of the English People*, completed in AD 731. Later, the system gained massive popularity through the Carolingian Empire. The leader Charlemagne promoted the use of the AD epoch across his lands.

The scholar Alcuin also helped introduce it during the Carolingian Renaissance. This political support was a key reason the system spread throughout Europe.

As the system grew, different regions adopted it at different times and in different ways. In Catholic countries, the use of AD became more common between the 11th and 14th centuries. Portugal was one of the last Western European nations to switch, doing so in 1422. Eastern Orthodox countries were much slower to change. Russia did not adopt the AD system until 1700, while others waited until the 19th or 20th centuries. People also struggled with when the new year should actually begin. Some used Christmas as the start, while others used the Annunciation on March 25 or even the date of Easter.

Today, many people use different terms to be more inclusive of non-Christian cultures. The terms Common Era (CE) and Before the Common Era (BCE) are often used instead of AD and BC. These labels are considered more neutral, but the numbers remain exactly the same. For example, the year AD 2024 is the same as CE 2024. This shows how a system created by a single monk in the 6th century has become a global standard for measuring time. It connects ancient religious history to modern scientific and astronomical numbering systems used around the world.