code: def timer_tick(self):
          
          # TODO: should exceptions thrown from this be caught and ignored
          self.process_timers()
  
          delta = datetime.timedelta(milliseconds=self.base_interval_msec)
          self._timeout = IOLoop.current().add_timeout(delta, self.timer_tick)
docs: Callback executed every self.base_interval_msec to check timer
          expirations.