autosave and close error

I have mutliple team members using one spreadsheet and on the chance someone goes to lunch and leaves the spreadsheet open, no one else can't be accesssed.
What I need is a macro that autosaves the shared workbook, then closes it if left idle for a period of time (say 5 mins).( For your information: I would like to autosave and close the shared workbook ). I am using the following code, but the problem is every 20 s my excel is closing and restarting again automatically. Does any one knows why it is restarting ?
thanks
Option Explicit
Private Sub Workbook_BeforeClose(Cancel As Boolean)
ActiveSheet.AutoFilterMode = False
Call Disable

End Sub
Private Sub Workbook_SheetCalculate(ByVal Sh As Object)
Call Disable
Call SetTime
End Sub
Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target _
As Excel.Range)
Call Disable
Call SetTime
End Sub
Private Sub Workbook_Open()
UserForm1.Show
Call Tempo
MsgBox "This workbook will auto-close after 20 seconds of inactivity"
Call SetTime
End Sub

module code:
Dim DownTime As Date

Sub SetTime()
DownTime = Now + TimeValue("00:00:20")
Application.OnTime DownTime, "ShutDown"
End Sub

Sub ShutDown()
ThisWorkbook.Save
ThisWorkbook.Close
End Sub

Sub Disable()
On Error Resume Next
Application.OnTime EarliestTime:=DownTime, Procedure:="ShutDown", _
Schedule:=False
End Sub

Any help would be appreciated

thanks for you rreply.
Its showing error as
Run time error 1004
Method 'ontime of object' Application failed
and highlighting Application.OnTime DownTime, "Shutdown", , False
thanks