Differences between AD and BC

AD vs. BC

The designations AD and BC are used to label years in the Julian and Gregorian calendars. AD is the abbreviation for Anno Domini, a Latin phrase meaning "in the year of the Lord," while BC stands for "Before Christ." This system centers the calendar on the traditionally reckoned year of Jesus's birth. Years are counted forward from this point as AD and backward as BC.

The Anno Domini dating system was created in 525 by a monk named Dionysius Exiguus. He intended to replace the Diocletian era system, which was based on the reign of a Roman emperor who persecuted Christians. The use of AD became gradually more common in Europe between the 8th and 14th centuries. The English historian Bede, in the 8th century, popularized the system and was the first to use the BC designation to date events before the start of the era.

In modern times, the alternative notations of CE (Common Era) and BCE (Before the Common Era) are often used. These terms are numerically equivalent to AD and BC; for example, 2024 CE is the same year as AD 2024. The use of CE and BCE began in the 17th century and became more common in the late 20th century, particularly in academic and scientific writing, to be more inclusive of non-Christians.

A key feature of the AD/BC system is the absence of a year zero. The year 1 BC is immediately followed by AD 1. This is because the concept of zero was not in use in Europe when Dionysius Exiguus developed the calendar.

Comparison Table

Category	AD	BC
Full Name	Anno Domini	Before Christ
Meaning	"In the year of the Lord"	Years preceding the birth of Jesus
Religious Association	Explicitly Christian	Explicitly Christian
Secular Alternative	CE (Common Era)	BCE (Before the Common Era)
Year Counting	Counts forward from year 1	Counts backward from year 1
Placement	Traditionally written before the year (e.g., AD 1066)	Written after the year (e.g., 44 BC)
Year Zero	Not included in the system	Not included in the system

Venn diagram comparing Differences between AD and BC

Origins and Usage

Dionysius Exiguus devised the Anno Domini system to calculate the dates for Easter for future years. His intent was to establish a chronology based on the birth of Jesus, whom he referred to as "our Lord." He calculated the year of Jesus's birth as 753 Ab urbe condita (from the founding of the city of Rome) and designated the following year as AD 1.

Modern historians and scholars place the birth of Jesus between 6 and 4 BC, indicating a miscalculation by Dionysius. The exact method he used to determine the date is not known for certain, though it is thought he may have based it on statements in the Gospel of Luke.

The adoption of the AD system was a slow process. The historian Bede was influential in its spread through his work Ecclesiastical History of the English People in 731. By the 11th century, it had become a standard for dating events in most of Christian Europe.

Modern Usage and Alternatives

The use of BCE and CE has grown in many academic, scientific, and international contexts. Proponents argue that these terms are more neutral and appropriate for interfaith and secular dialogue, as they do not presuppose faith in Jesus as "Lord." Major style guides, including the Chicago Manual of Style, recognize and provide guidance on their use.

Despite the rise of BCE/CE, the AD/BC notations remain in widespread use, particularly in religious contexts and by those who see the alternative as an unnecessary attempt to obscure the Christian roots of the Gregorian calendar. Ultimately, both systems are numerically identical and based on the same historical starting point calculated by Dionysius.