I am trying to run this worksheet change event for two different columns(A) and (I)...
Private Sub Worksheet_Change(ByVal Target As Range)
Dim A As R
edited after OP's comment
expanding on @Jeeped solution, you can avoid looping:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rng As Range
Set rng = Intersect(Range("A:A, I:I"), Target) ' define range of interest
If Not rng Is Nothing Then ' check it's not "nothing"
If WorksheetFunction.CountA(rng) = rng.Count Then 'check for all of its cells being not empty
On Error GoTo safe_exit 'add error control
Application.EnableEvents = False 'don't do anything until you know something has to be done
rng.Offset(, 1).Value = Date 'write Date next to all relevant changed cells
End If
End If
safe_exit:
Application.EnableEvents = True
End Sub