im sure theres a better way but you could use the following code in the Worksheet_SelectionChange event if your only using one sheet, its worked for me in the past but didnt really like this option

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim vartimer As Variant
   ' The next line sets the time in hours, minutes and seconds
   vartimer = Format(Now + TimeSerial(0, 30, 0), "hh:mm:ss")
   If vartimer = "" Then Exit Sub
   ' The action of the timer is to call a nother macro called test 
   Application.OnTime TimeValue(vartimer), "test" 
End Sub