Option Explicit

Sub starttimer()
On Error Resume Next
    Application.OnTime Now + TimeValue("00:00:01"), "nexttick"
End Sub

Sub nexttick()
On Error Resume Next
    If Sheet1.Range("S9") = 0 Then
        Sheet1.Range("S9").Value = "0:0:10"
        Exit Sub
    End If
    Sheet1.Range("S9").Value = Sheet1.Range("S9").Value - TimeValue("00:00:01")
    starttimer
End Sub

Sub resettimer()
    Sheet1.Range("S9").Value = "0:0:10"
End Sub
Sub stoptimer()
On Error Resume Next
    Application.OnTime Now + TimeValue("00:00:01"), "nexttick", , False
End Sub