481
Context: https://en.wikipedia.org/wiki/Emperor_Gaozu_of_Han#Insurrection_against_the_Qin_dynasty
Liu was responsible for escorting a group of penal labourers to the construction site of Qin Shi Huang's mausoleum at Mount Li. During the journey, some prisoners escaped; under Qin law, allowing prisoners to escape was punishable by death. Rather than face punishment, Liu freed the remaining prisoners, some of whom willingly acknowledged him as their leader and joined him on the run from the law.
We can only do that when we know how many years go by, though. Western History can maybe count back safely some centuries before that, but beyond that we find some calendar discrepancies, we're missing some rulers and we know we have incorrect reign lengths, and depending one which one we pick, the whole thing shifts maybe a decade or two (short / intermediate / long chronology in Mesopotamia and Egypt), so that's not ideal. Eastern chronologies aren't any safer beyond 1000 BCE either. As you may say, people learned to write before they learned to count.
Even the Julian / Gregorian calendar counted back and was still off by 3 to 7 years.
I feel like a decade drift isn't so bad a price to pay for easier historical literacy.