It depends what your field types are. I assume Term is a text field since you can have 0 or N/A in it. Might be better off going with a numeric field and treating blank as a 0. I am going to go with it being a text field though. There might be a better way to do this but this works.
I would just make your "Expiration Date" field a formula date and put this in it. If term = blank then Expiration date is just blank. If it has a term then it adjusts the date based on that term.
If([Term]="", ToDate(""), AdjustMonth([Effective], ToNumber([Term])))
I think you might be better off making it so that instead of going by if term is 0 have a checkbox or something that makes it explicit that there is no term. Then this formula would not have to interpret what a user might randomly put into a text box.
