Insert date whenever cell is updated (not edited)

Hello everyone

I'm using this macro to insert the time and date into another cell whenever a cell within a column is edited. However I want to try the same thing, but with cells that are not edited manually(they are vlookup-ing to another location), so that when the vlookup returns something different than before, the date/time cell is updated.

Formula:

Private Sub Worksheet_Change(ByVal Target As Range)

If Target.Column = 40 Then

    With Target.Offset(, 1)

        .Value = Now

        .Columns.AutoFit

    End With

End If

End Sub

Thanks