docs: def next_future_periodic_delta(self) -> Optional[float]:
          """Give the amount of seconds before the next periodic task is due."""
          try:
              next_event = self._scheduler.queue[0]
          except IndexError:
              return None
  
          now = time.monotonic()
          next_event_time = next_event[0]
          if next_event_time < now:
              return 0
  
          return next_event_time - now