I've a Nintex 2013 Form with:
- a dropdown to select a recurrence type (Monthly, 3-Monthly, 6-Monthly etc) from a lookup list, which also has a number column with the actual number of months.
- Start Date
- Due Date, which needs to calculated on the fly as soon as the Dropdown selected item changes
- Nintex calculated field that uses lookup() to the lookup list to get the number of months for the selected recurrence type