Hi Andrew
I think the amount should be calculated on the fly, based on a date range selected by the user - the date represents the date an invoice was fully paid, not the invoice date. It should also be weighted, based on the invoice amount.
Here's how I do it - I multiple the number of days it takes to pay an invoice (date paid minus invoice date) times the dollar amount of the invoice, for each invoice whose paid date falls within the range. Then I add up the results for each invoice into one grand total. Then I divide the grand total by the total dollar amount of all of the invoices paid. That's the weighted average days-to-pay.
An example - a $100 invoice was paid in 10 days, and a $1,000 invoice was paid in 20 days. $100 * 10 = 1,000 plus $1,000 * 20 = 20,000 - the grand total is 21,000. Divided by $1,100, the days-to-pay is 19.1 (not 15 days which would be the answer ignoring the invoice value)
Steve