With the myriad of copyright law changes and extensions that have taken place over the past century, it is almost impossible to tell whether or not a work is protected under copyright, even under the best of circumstances.
It is a great tool and it illustrates how complicated and convoluted copyright law has become thanks to the patchwork of changes that have taken place. It becomes clear when using this slider, especially when you look at the asterisks and other caveats, that there is a real need for simplification of copyright law in the U.S.
To make matters worse, the slider only covers some of the potential situations. If you take a look at the table provided by Cornell, you can see that there are many possibilities not covered by the slider.
Still, the slider is a great tool for most situations and serves as a great introduction to the public domain and when works expire.