excel vba countdown timer

Select AllPublic interval As Date
Sub timer()
 
    interval = Now + TimeValue("00:00:01")
    
    If Range("A1").Value = 0 Then Exit Sub
    
    Range("A1") = Range("A1") - TimeValue("00:00:01")
    
    Application.OnTime interval, "timer"
     
End Sub
Sub stop_timer()
 
    Application.OnTime EarliestTime:=interval, Procedure:="timer", Schedule:=False
 
End Sub