code: protected final long nextScheduledTaskNano() {
          Queue<ScheduledFutureTask<?>> scheduledTaskQueue = this.scheduledTaskQueue;
          ScheduledFutureTask<?> scheduledTask = scheduledTaskQueue == null ? null : scheduledTaskQueue.peek();
          if (scheduledTask == null) {
              return -1;
          }
          return Math.max(0, scheduledTask.deadlineNanos() - nanoTime());
      }
docs: Return the nanoseconds when the next scheduled task is ready to be run or {@code -1} if no task is scheduled.