Sum of Total Days Since Last Purchase

I'm trying to solve the "single date field" problem, but I don't think it's possible because it would require an LOD calc and a table calc to work together.

The Need

I want to be able to look at each purchase made by a customer within a month and calculate the number of days since their last purchase to ultimately show the average number of days since the last purchase for people that had orders in a month.

Example

I would want this to show me that for purchases made in October it had been 98 days (summed) since the last purchase.

I can get this result for each individual as a table calc showing the Name and Date in the view, but I need a rollup of the values to reflect monthly totals for the entire data set.

Thanks for the help!

-Wesley