.
This version should answer all your questions :
Code:
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