[AccessD] A2002: DateAdd From a Combo box

Hi Y'all

I am storing some DateAdd functions in a 2 column table to give users a list
of choices for automatically creating a new "next due date"
Values in the table look like this

DateAddDescription   DateAddvalue
-----------------------------------------------------
Every Day                     DateAdd("d",1,NextDueDate)
Every Week                  DateAdd("d",7,NextDueDate)
Every Month                DateAdd("m",1,NextDueDate)
Every 3 Months           DateAdd("m",3,NextDueDate)
Every 6 Months           DateAdd("m",6,NextDueDate)

When a user selects one of these items from the combo I'd like the actual
Date Add function selected, to be applied to the actual "next due date" on
the form 
Assume the "next due date" is blank in a new record, once a user selects a
value from the combo, say....the 4th one
      DateAdd("m",1,NextDueDate)
I'd like the new due date on the form to be "2013-2-28"
Make sense?
Many thanks in advance
Darren