You can active this feature either with the most-right icon in the iconbar or via Settings->Show Toolbar. If it is activated you will see a slider below the PSE and an LCD which displays the number 2002. If you move the slider you will notice that some elements disappear if you move it to the right and reappear if you move it to the right. Furthermore the number will change constantly.
The number represents the data you are looking at. If you move the slider to e.g. 1856 you will only see the elements which where known in the year 1856.
You will notice that some elements are not even shown in the year 2002. This is because some of the elements have not been discovered yet but are predicted to exist.