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