Historians are usually pretty good at what they do, and aren't just taking ancient calendars at their word. On top of that, we also have astronomical records t

Historians are usually pretty good at what they do, and aren't just taking ancient calendars at their word. On top of that, we also have astronomical records to serve as anchors for other dates. We can figure out when eclipses happened, when each planet was visible, and so on. If ancient texts record an eclipse during the reign of King Foo, and we know when there was an eclipse visible from that area, we can interpret any other events in Foo's time around those definite dates.