Why Won't It Update This Table Entry?

Hi there. My mother (who hasn't programmed in say 25 years) has been driving herself absolutely up the wall trying to figure something out. So I'm asking here since my programming knowledge does NOT include Access. We have a form for keeping track of previous appointments and scheduling the next appointment. It's a simple form consisting of a "Date Last Visit" text box, an Interval integer text box, and a Date Next Due text box.

What we're looking for, is to have it so that when the user clicks on or tabs to the Date next Due text box, it reads what's in the Date Last Visit variable and changes Date Next Due to Date last Visit + 3 months. So... here's what we have.

Code:Private Sub Date_Next__Due_GotFocus() 'MsgBox (Interval) If IsNull(Date_Last_Visit) = False Then Date_Next_Due = DateSerial(Year([Date_Last_Visit]), Month([Date_Last_Visit]) + Interval, Day([Date_Last_Visit])) End If 'MsgBox (Date_Next_Due) End Sub

Now if I uncomment that bottom msgbox, it shows the correct Date_Next_Due... But it's not displaying it on the Form, nor is it updating it in the table! We've had this working before, but due to an accidental deletion and some other messing around, I CAN"T FIGURE IT OUT! So why isn't it updating?

If there's anything else you need to know, please ask. We really need this to work. Thanks so much in advance!