When studying the past, it is important to understand how time has been recorded throughout history. In ancient times, time was recorded based on events that happened in a specific year. For example, the year of the drought or the time of the flood. People kept careful records, but no numbering system like we have today existed.
The first emperor of Rome, Romulus, started keeping a calendar around 753 BC. The Roman calendar was pretty good, but had some problems. Their years only had 355 days. It takes the Earth about 365 days and 6 hours to travel around the sun, so over time, the dates in the calendar didn’t match the seasons,
In 45 BC, Julius Caesar, the emperor of Rome, introduced the Julian calendar. It changed the calendar to be 365 days long. Every four years would be a leap year, like we have today, to account for the extra 6 hours in each year. The Julian calendar was better than the Roman calendar, but it was still off by about 11 minutes. This doesn’t sound like much, but over thousands of years, those minutes add up.
One of the most important uses of the calendar was for observing holidays within the Christian Church. Over a thousand years, the dates in the calendar no longer matched the holidays in the Church. As a result, Pope Gregory XIII asked a monk, Dionysius Exiguus to fix the Julian calendar to make the dates match.
While working on the calendar, Dionysius decided to change the way we counted forward. At the time, years were recorded based on a Roman emperor named Diocletian. Diocletian hated Christians. He persecuted and killed thousands of people.
Dionysius didn’t want his calendar to be based on Diocletian, so he decided to mark time from the birth of Jesus. He used historical records and writings from the Bible to determine when Jesus had been born. He made this year 1 AD and counted forward from there. This became known as the Gregorian calendar after Pope Gregory XIII.
In the Gregorian calendar, time before Jesus’ birth is called BC for “before Christ”. Time after Jesus’ birth is called AD for “anno Domini” which is Latin for “the year of our Lord.” More recently, people have used BCE for “before common era” and CE for “common era” instead of the references to Jesus.