It may be an approximation. But if Chronopolis arrived in 12,000BC, wouldn't Lavos then totally come back into existance? And can Chronopolis move around or what? If it stayed in the same position, but 10,000 years into the past, then I guess it wouldn't interfer with any of the Zealian's having to know about it. But as fro Dinopolis, and it seems that they were just bombarded by fighter jets, then you'd think some Zealians would know, and not only interbreed with the Dragonians :O
Maybe the Japanese version has the true dates and not approximations.