BC (Before Christ) and AD (Anno Domini, Latin for “in the year of our Lord”) are dating designations used to mark years based on the traditionally estimated birth year of Jesus Christ.
AD counts years forward from Christ’s birth, while BC counts years backward. This system was developed by a monk named Dionysius Exiguus in the 6th century (around AD 525) when he created a new calendar table to replace the *Diocletian era (which honored a persecutor of Christians).
BCE (Before Common Era) and CE (Common Era) are secular alternatives to BC and AD.
BCE matches BC (years before year 1), and CE matches AD (years after year 1). These terms were promoted in the 17th century but became more widespread in the 20th century for academic and interfaith reasons, as they avoid explicit Christian references. Scholars like Johannes Kepler used similar terms early on, but broader adoption grew through modern secular and scholarly communities.
*Diocletian was Roman emperor from 284 until his abdication in 305.
The Diocletianic Persecution (303–312), the empire’s last, largest, and bloodiest official persecution of Christianity, failed to eliminate Christianity in the empire. After 324, Christianity became the empire’s preferred religion under Constantine.
