When you look at a calendar, you may notice the letters BC and AD attached to certain years. These two labels are connected to the life of Jesus Christ and have long shaped the way much of the world measures time.

BC stands for “Before Christ.” It refers to the years before the birth of Jesus. For example, 300 BC means 300 years before Jesus was born. Time in BC counts backward. The year 1 BC comes right before AD 1. There is no year zero in this system.

AD stands for a Latin phrase, Anno Domini, which means “In the year of our Lord.” It refers to the years after the birth of Jesus. So AD 100 means 100 years after Jesus was born. Unlike BC, AD years count forward, just like we normally count numbers today.

The BC and AD system was first created in AD 525 by a Christian monk named Dionysius Exiguus. He was trying to create a new calendar for the Christian church. At that time, many people used calendars based on the reigns of Roman emperors. Dionysius wanted a system centered on the birth of Jesus instead. Over time, this dating system spread across Europe and later around the world.

Interestingly, the calendar was not exactly accurate. Jesus was born during the rule of Herod the Great. Most historians believe Jesus was actually born around 4 or 5 BC, not in AD 1.

Jesus started his ministry at around the age of 30, or his early 30s, according to the Gospels. His ministry lasted around three years, with his death in either 30 or 33 AD.