Lets see if I'm understanding correctly. You have a control that you enter the invoice date (I'll call it txtInvoiceDate) and you have another control that has the amount of time that the customer has to pay the bill, which you call Term (I'll call the control txtTerm). You want to be able to enter a date in txtInvoiceDate and the number of days in which the company has to pay the bill in txtTerm and have a third control (I'll call it txtDueDate) be Term days after Invoice Date. Am I correct?
If so, in the Control Source of txtDueDate put the following:
- =[txtInvoiceDate] + [txtTerm]
You will then be able to enter a date in txtInvoiceDate and the number of days for the term in txtTerm, and then when you select txtDueDate, the due date will appear.