Traditionally, the initials "BC" or "AD" are used when writing dates. "BC" stands for "Before Christ" and "AD" stands for Anno Domini (year of our Lord). Why do they use English for one abbreviation, but Latin for the other? Shouldn't coordinate ideas be expressed in parallel form?
Nowadays you'll see "CE" and "BCE," which stand for "Common Era" and "Before the Common Era." Why the switch? Well, there are a couple of reasons:
1. The expression "before Christ" is inaccurate, since nothing existed before Christ. It says so in the Bible (Col. 1:17). Some Christians think it's blasphemous to talk about something existing before Christ.
2. Jesus wasn't born in the year one. He was born around 6 BCE. We know this because King Herod was alive when Jesus was born, and Herod died in 6 BCE. In other words, "B.C." is inaccurate, because it would mean that Jesus was born six years "before Christ."
Some Christians continue to use the "BC" and "AD" designations simply because (1) they're suspicious of anything new or (2) they're afraid that somebody will think that they're ashamed of Jesus. In other words, they think they'll lose their reputation as Staunch Dedicated Christians; they're afraid that their friends and neighbors will consider them to be Wild New Age Beatnik Hippies.
A Christian shouldn't worry about what people think. That's of the flesh.