Application VisualBasic Script

Dim NextTick As Date
Sub UpdateClock()
    ThisWorkbook.Sheets(1).Range("A1") = Time
    NextTick = Now + TimeValue("00:00:05")
    Application.OnTime NextTick, "UpdateClock"
End Sub
Sub StopClock()
    On Error Resume Next
    Application.OnTime NextTick, "UpdateClock", , False
End Sub