Screen says 'Enter the time periods within which to block these sites (optional)'. But leaving it empty makes the plugin to skip everything in the list. So not optional at all.
It is optional, because you can enter a time limit instead of time periods. If you don't enter either time periods or a time limit, then, of course, nothing will be blocked. You have to specify one or the other. Otherwise, how would the extension know when you want the sites blocked?
