transient?
SLA is a timer. It starts when the ticket is created (created datetime). It ends when it meets the selected SLA time out.
If you change the SLA before it becomes over due it will not compare the time from created to the new SLA, but once a ticket is overdue, its forever overdue.
I'm not aware of a way to show the timer other then looking at the created datetime and looking that time on your computer and doing some quick math.