If you’re presuming ground is zero then you’ve told us the answer you want already. Are you numbering the floors or numbering the stories? That’s the real question.

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.