Does anyone know how to calculate the hotel check-in time? I transferred to the airport at 8 pm on the 4th and arrived at my hotel room, but I want to check out at 8 pm on the 5th.

The time period of hotel settlement is from noon on the same day 12 to noon on the next day 12. Some hotels and guesthouses have membership discounts, which can generally be postponed until the afternoon 14. As you said, this situation is inter-Japanese, and there will be a 2-day charge. But you can check out at noon, leave your luggage at the front desk (generally, there is no separate charge for this), then go out for a stroll, and then you can pick up your luggage. Or you can choose to add an hourly room, usually four hours, and the cost is less than half of the daily room price.