ActiveCell.Offset(1, 0).Select

Sub a()
If Selection.Row > 10 Then
a = Selection.Offset(0, 1)
Else
a = Selection.Offset(0, -1)
End If
' a.Formula = Selection - лишняя строка, закомментировал
' a.Replace What:=",", Replacement:="" - заменил на другую
a = VBA.Replace(a, ",", "")
End Sub

Dim val

Private Sub Worksheet_Change(ByVal Target As Range)
For Each Cell In Target
 If Not Intersect(Cell, Range("C4:C1000")) Is Nothing Then
    If val = Target Then Exit Sub
  With Range("E" & Cell.Row)
     .Value = Now
     .EntireColumn.AutoFit
     End With
    End If
   Next Cell
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    val = Target
End Sub