Why do some dates have A.D. and B.C.?
Here's the answer:
The earth has been around for a few thousand years. That’s a lot of time to keep track of! So people decided to split all the years on Earth into two parts. When we write out or talk about the first part, we use the letters B.C. We use A.D. for the second part.
But what do B.C. and A.D. mean? A long time ago, people decided to keep track of time based on one of the most amazing things to happen in history: Jesus’ birth. This was such a big and important event that everything before Jesus' birth would be marked as B.C., which stands for "before Christ." All the years that we mark as B.C., or "before Christ," count down to the day we believe Jesus was born. So, 2 B.C. would mean two years before the date when people believed Jesus was born. (If you’d like to read more about Jesus' birth, see Luke 2:1–21.)
A.D. is used to mark all the years after the date Jesus was born. A.D. stands for Latin words, "anno domini," which mean "in the year of our Lord." So, A.D. 2 would mean two years after the date people believed Jesus was born.
We know now that Jesus was probably born in 6–4 B.C., but that’s ok. Using the letters B.C. and A.D. are still a great way to keep track of time!
"Today in the town of David a Savior has been born to you. He is the Messiah, the Lord" (Luke 2:11).