Calculating Days ..

What I would like to do is insert a formula into column E that would calculate the number of days from when the count is at 50 to when the next count is i.e. for part 6689841 the time between when the count was 50 and when it was at 20, so the difference between 21/01/2008 and 08/08/2008 in the above example. It does not matter what the count number is (it could be any number except 50), I'm only concerned with every time the count is at 50 how long it is until the next count and to show the result in days.