Sub Timer()
    Const COUNTDOWN As Integer = 5

    Dim testButton As Excel.Button
    Set testButton = Application.ActiveSheet.Buttons("TestButton")
    testButton.Select

    For i = COUNTDOWN To 0 Step -1
        ActiveCell.FormulaR1C1 = i
        testButton.Caption = i
        Call Application.Wait(Now + TimeValue("00:00:01"))
   Next i
End Sub