As in, we use B.C and A.D, but obviously the Romans, Greeks, Egyptians etc etc. wouldn't have that, so how did they record the years and where was the starting point? Furthermore, how would we know exactly when things happen and put it on our own timeline?
Sorry if this question is badly worded.
This is one of our most frequently asked questions; there are some great older answers regarding the Greeks and the Meso-Americans, among others, in our FAQ section on this topic.