stop this countdown timer macro

Sub Countup()
Dim CountDown As Date
CountDown = Now + TimeValue("00:00:01")
Application.OnTime CountDown, "RealCount"
End Sub
Sub Realcount()
Dim count As Range
Set count = [E1]
count.Value = count.Value - TimeSerial(0, 0, 1)
If count <= 0 Then
MsgBox "SORRY - TIME UP!"
Exit Sub
End If
Call Countup
End Sub