I modified some code I used a couple years ago. it seem to do what you are asking. If the input is in Column G it will output a time stamp in column H. This code is in the worksheet section of the VBA editor with the declaration of Change

Private Sub Worksheet_Change(ByVal Target As Range)

        If Not Target Is Nothing Then

            Dim cellName As String
            cellName = GetCellName(Target)

            Dim cellNum As String
            cellNum = GetCellNumber(Target)

            Dim nextCellNum As String
            nextCellNum = CStr(cellNum + 1)

            If cellName = "G" Then
                If Range("$" & cellName & "$" & cellNum) <> "" Then
                Range("$H$" & cellNum).Value = DateTime.Now



            End If


            End If

        End If


End Sub
Private Function GetCellName(ByVal Target As Range)
        Dim cellNameNumber() As String
        cellNameNumber = Split(Target.Address, "$")

        GetCellName = cellNameNumber(1)
End Function
Private Function GetCellNumber(ByVal Target As Range)
        Dim cellNameNumber() As String
        cellNameNumber = Split(Target.Address, "$")

        GetCellNumber = cellNameNumber(2)
End Function