The information entered in the “Check-in” and “Check-out” fields is shown at the top of the booking form. This is just a way of sharing information with the customer. You do not manage actual times using this option.
You can edit the “Check-in” and “Check-out” labels from YITH > Booking > Settings > “Customizations”.
For example when you rent out bikes, you don’t need the check-in and check-out labels, but you want to show when to pick up and return the bike.