I have a solution, along the lines of daddylonglegs'. However, I've just noticed that your manual calculations in this workbook are using NETWORKDAYS, which has me a bit confused.

Since daddylonglegs' solution did not consider working days only, and since you appeared happy with the solution he offered you, I'm not quite sure why your new query is asking for that to be a factor.

Can you clarify?

Regards