There is no way to determine how old things are based on where they are found. It's based on an imaginary assumption, and these facts and more show how that assumption holds no validity at all.
Actually, it's a rounded amount. You can tell somewhat on how old things are based on scientific studies. Not an exact amount, no way. But a rounded between a million or so years. A lot, yes. But the fact is that you can use fossils to find a possible time period.
You can't just find a fossil and say, "Oh my God! Triassic Era!!!"