Interest on past due balances

[teppie_d]

I am trying to determine the amount of finance charges due based on 18% APR when I have the payment terms in one column (ie Net 15), the invoice date in one column (ie 1/15/12), the balance due in a column (ie $790.00), and the statement date in a column (ie 5/18/12).

Suggestions?

Also, to add a little complexity I would also like to have another column with the projected interest charge if paid according to the customer's average pay days. I have a column with the APD (ie 46.10).