Here's what you could do:
Create a Form with the two fields from your table, then in the form design mode, double-click on the DateFileCheckedOut text box. The properties window will open. Select the "Event" tag, then click on the "After Update" event. Click on the little button with the 3 dots which will pop up on the right. Select "Code Builder" from the "Choose Builder" pop up, then click "OK". The code window will open with the following:
Private Sub DateFileCheckedOut_AfterUpdate()
End Sub
Paste this code between these two lines.
DateFileDue = DateAdd("d", 30, DateFileCheckedOut)
This is what your code should look like after:
Private Sub DateFileCheckedOut_AfterUpdate()
DateFileDue = DateAdd("d", 30, DateFileCheckedOut)
End Sub
John