The lack of understanding of zero in this thread has been most amusing!
Using the logic in this thread coming from some people 1-2=-2... the problem is they think 0 represents nothing, which clearly isn't the case 1-2=-1 the zero counts!
Basically if your going to use a numerical system then you need the 0, without it mathematical logic breaks down. I don't really care where you put the 0 it just needs to be there. In this case floors are being represented by digits so 0 needs to be included. The most logical place to put the 0 is the ground floor.