This is an idealised thought experiment, not a forecast of when a real blend is mixed. It assumes the gases lie in separate layers with nothing stirring them, so only diffusion evens them out. A real fill is turbulent, and real blends mix much sooner.
What it does show is how slow diffusion is at cylinder pressure. Enter the cylinder’s inside height, the pressure, the temperature and the gases, and the calculator uses Fick’s second law to estimate how long it takes until every gas is within 0.1 percentage point of the final mix (you can change that limit). Usually that is days to weeks.