[The European Commission has a calculator](http://ec.europa.eu/dgs/home-affairs/what-we-do/policies/borders-and-visas/border-crossing/schengen_calculator_en.html) as well, available in a “pop-over” from this [page about Border Crossing](http://ec.europa.eu/dgs/home-affairs/what-we-do/policies/borders-and-visas/border-crossing/index_en.htm). It comes with no guarantees (from the user's guide: “The calculator is a helping tool only”) and presumably gives the same results as other calculators but it's from a somewhat official source so hopefully it's maintained by people who know the rules inside out.