Richard, I think you are looking for data about both the variance and the standard deviation of the array of dates. This would yield, for example, a bell-shaped curve, like one gets when administering an intelligence test to a sample population. Two thirds of the dates fall within +/- 1 standard deviation of the mean date, ~95% of the dates fall within 2 standard deviations, and so on.
I think this site does a good job of making the calculations and equations understandable --> http://davidmlane.com/hyperstat/A16252.html . hth, David
