How to do average turn around time with criteria

I currently have a spreadsheet tracking the performance of a purchasing
group. There are 3 columns of interest to me in this formula. Column D is
the purchasing group, some are names, some are acronyms, but the groups I'm
concerned with are all named "Purchasing____" followed by a number, so I have
been using the "Purchas*" value to capture all of those. Column J is the
date a purchasing group has received an order, and Column P is the date the
purchasing group sends out its purchase order. This data is being captured
in a separate workbook, and there is also a date criteria. So I have set up
hidden cells in the workbook capturing the data, with the corresponding
column head and values I'm looking matching up with column D, J, and P. J
and P have two hidden cells each, to represent the upper and lower bounds of
the date ranges I'm looking for.

I'm looking to come up with a function that will go to every entry in column
D that is a "Purchasing____" (hence the "Purchas*"), find the NETWORKDAYS
between J and P, and then average them by month (hence the date constraints).
Any help would be much appreciated, thanks.