Historically, people created a fresh calendar after a significant event. Sometimes that was the coronation of a new king, and everything was counted as “in the tenth year of King Henry’s reign” and so on.

The Roman calendar was originally structured on the phases of the moon.

The Romans used AUC (“Ab Urbe Condita”, From the Founding of the City), dating everything from the founding of Rome in 753 BC. Note: not necessarily historically accurate.

Many Jews still use AM (“Anno Mundi”, In the Year of the World), dating everything from 1 Tishrei 1 AM, or Monday, 7 October 3761 BC… which is about a year before the creation of time and space on the Jewish traditional date of Creation, 25 Elul AM 1. Try to figure that one out!

The Jewish calendar's reference point is traditionally held to be about one year before the Creation of the world.

There are two problems with the standard calendar terms BC and AD, “Before Christ” and “Anno Domini – In the year of the Lord”.

The first is that many non-Christians object to referring to Jesus as either the Christ or the Lord, and aren’t impressed with using his birth as the basis for a global calendar.

The second is that Jesus was not born at midnight between 1 BC and 1 AD. Under our calendar, he was probably born in the spring of 6 BC, rendering the BC and AD terms ludicrous. Our calendar is now so thoroughly established, however, that it is easier to rename than to renumber. So now people are starting to write about our Common Era (CE) and, before that, Before Current Era (BCE).

Of course, we could just keep the initials BC and AD, and rethink the meaning of them to, say, “Before Current” and “After Dat”. I mean, really, who cares?