Date Calc is an extremely simple piece of software that allows you to calculate exact intervals between any two dates. For instance, if you are counting down the days to your next holiday, just input today's date and the date your plane leaves the airport and Date Calc will immediately tell you how long you have to wait. Of course there are other uses for this, but I guessed you could relate to that one.
The interface is very clear and simple to use. You won't need to scroll through a help guide with this one. As soon as it loads it's obvious what you need to do. What's more, the possibilities are limitless. So if that holiday's not happening for another fifty years, it'll still calculate the number of days you need to wait. In fact, perhaps prisoners would find this useful.