It sounds like you want to use the DateAdd function, based on the due date.
You could put the calculation in a calculated field on a form, or you can put it as a calculated field in a query.
For these examples, let's say the due date field is called 'DueDate'.
In a query you would put the calculation on the Field row. It would be something like:
Two Weeks Past Due: DateAdd("d",14,[DueDate])
In a form you would put the calculation in the Control Source of a text field. It would be something like:
=DateAdd("d",14,[DueDate])
Let me know if you have difficulty finding the 'Field' row in a query or the 'Control Source' property on a form's text field.