Not only that, but it would be great if it worked better. I once was told how it's figured out (something like 25% weight on the most recent paid, and diminishing by half on each previous invoice) but I have some customers where I know it's way off.
 
For instance, I have one customer who almost always takes the 10 day discount, and aways pays within 20 days (at least for the last couple of years), and it shows 54 days. Another has been paying in 45-60 for the last several months and it shows 26 days.