Average 12 months Account formulas

Hi everyone, I suspect that this is fairly simple, but I haven't been able to find a solution as yet.

I need to add formulas for Debtor Days, Creditor Days, and Inventory Days, all of which use 12-month averages in the calculation. The also all use the number of days in the month.

Does anyone have an example of the syntax needed to include both of these?

Thanks