Hi Mike
As Andrew suggests, the only workable database that contains both the invoice date and the payment date in one record is the SalesAnalysis Transactions database. The records are not at the invoice level, but at the detail (item) level, and if SA is not set to collect the miscellaneous charges and taxes, the total of all of the details will not exactly agree to the invoice.
You have to make sure all AR cash batches are posted, and run the function Utilities | Set Invoice Paid Indicator (pick a starting date about 1 year in the past), so that the paid date field is refreshed in the table.
This table only really works for a weighted Average Days-to-Pay figure, which is more accurate anyway. Also, the paid date won't be populated until the invoice is fully paid, which also makes for a more accurate report.
The SA Detail table is really easy to work with - it can connect to all of the naturally related master tables, such as customer and item.
Steve