That's a good example! Elevators sometimes have a 0 for the ground floor, but they often have an "E" in German-speaking countries or a "C" in English-speaking countries.
In this example, people also call the floor with index 1 the "first floor," although they don't call the ground floor the zeroth floor, as you say.
Since floors can go below ground, the first underground floor is floor -1, so everything works out. There's a floor for every number, unlike with 1-based floor numbering, where floors go from -1 to 1.
