How could Jesus be born in 4 B.C.?

The Bible does not contain any year designations, like the year AD 2008. Why? Because the system of counting years used today did not develop until the 500s, and then it didn’t catch on until hundred years after that. The guy who developed the AD system was Dionysius Exiguus (470-544), but more about him and why he felt it necessary to create the AD system later.

While the Bible does not have a system for counting years, it does use an old way of keeping track of years. This method is known as the "regnal system" and is based on indicating an event within the reign of a ruler. For instance, “such-and-such occurred in the fifth year of King whoever.” A great example of this is in Luke 3:1-2. In those passages, Luke attempts to tell when “the word of God came to John [the Baptist] son of Zechariah in the desert.” Luke describes this event within the rule of many people: “In the fifteenth year of the reign of Tiberius Caesar—when Pontius Pilate was governor of Judea, Herod tetrarch of Galilee, his brother Philip tetrarch of Iturea and Traconitis, and Lysanias tetrarch of Abilene—during the high priesthood of Annas and Caiaphas, the word of God came to John son of Zechariah in the desert.”

Luke is very specific because he dates the call of John within the reigns of seven different people. This system worked well as long as people knew when the rulers ruled, but as time passed and people began to forget those rulers, they also forgot the dates when they ruled.

Now to Dionysius. Dionysius was a monk who help track the day when Easter was celebrated. (Which was difficult when you don't have a regular calendar.) In all of his calculations, he decided to change from the regnal system to a system of counting years based on the birth of Jesus. Thus Dionysius began his calendar with Anno Domini (“In the year of the Lord”) 1. He first started using this system in the year AD 525. (That means you will never find any dates using the AD system before the year 525. In other words, the people living in the year 524 didn’t realize they lived in that year.)

Dionysius’ system was mainly theoretical at that time and did not have a broad appeal. But in the early 700s, a man named Venerable Bede wrote a history book that used Dionysius’ system, and, soon afterward, this dating system was widely accepted.

Now about Jesus: Dionysius decided to start the calendar by calling the birth of Jesus "year 1." But as archaeology developed in the 1800s and 1900s, scholars began to be able to date people and events in the distant past with more accuracy. And they concluded that King Herod the Great died in the year 4 BC. Therefore, since Herod was alive when Jesus was born, this pushed the date of Jesus' birth back to at least 4 BC (or maybe even a couple of years earlier). So scholars say that Jesus was born in the year 4 BC (and some say 5, 6, or even 7 BC) because Dionysius got it wrong by at least four years. That isn't too bad, considering he devised the system in the Dark Ages.

Read on to learn more about the leap year (you only think you know the rule). Therefore, the way to keep track of years was established, but some tweaking with the length of the year became necessary. The calendar used by Dionysius is known as the Julian calendar and has three years of 365 days, then one year of 366 days, which we know as the leap year. Problem: the Julian calendar assumes that each year is 365.25 days long, but it is 365.2425 days long, just a bit shorter. So if you add a day every four years, you will not add enough.

By the 1500s, it was apparent that too many days had been added. Therefore, Pope Gregory XIII decided that the day after October 4, 1582 would be October 15, 1582. To make sure this mistake didn’t happen again, they adjusted the leap year rule. No longer would an extra day automatically be added to every fourth year. From then on, an extra day would be added to every fourth year unless the year ended in “00”, then an extra day would not be added unless the year could be evenly divided by 400, when an extra day would be added. So, 1900 was not a leap year but 2000 was.