What century is it if the year is 500? First century. What about if the year is -500 (500 BCE)? It's the first century BCE. You have now arrived at US numbering of "floors". Now which century is zeroth? It doesn't make sense to ask, just like having a floor numbered "zero". This might be why the ground floor is not special to us.
Are we on the number line, or counting the spaces, or counting additional layers? It's an arbitrary question or distinction, but it seems like it should be labeled somewhere in the elevator!