Why we use B.C. and A.D.

Dear Matthew Alice:

Why do we measure dates from B.C. to A.D.? Where did this system come from? I know it’s measured from the time of Jesus’s birth, but who decided that? And what did people do before then to measure time, or didn’t they care?

— Mike, via email

Of course they cared. We’ve been run ragged by clocks and calendars pretty much forever — at least ever since we started growing crops and living under grand rulers. The A.D.-B.C. system dates from about 525 A.D., invented by a monk who was charged with figuring out a neat way of predicting the date of Easter. Since he was a man of the church, he took as the starting point for his calendar the birth of Jesus, the most significant event in the ecclesiastical year.

This is pretty typical of the many other calendars devised before and since the monk’s. They all begin with a watershed event. The Mayans developed a calendar that went back through thousands of years of mythology, so the cornerstone event wasn’t necessarily an actual occurrence. Egyptians measured years by giving them a number and the name of the pharaoh who was in power at the time. Babylonians used a similar system. The Jewish calendar is measured from a calculation of the Era of Creation. The Muslim calendar begins with the Era of the Hijra, the time Mohammed and his followers migrated from Mecca to Medina. Rome measured time from the founding of the city. So, if you want to develop a Mike calendar — Before Mike-After Mike — go right ahead.