I'm not sure @Roger is correct with this one.
When a guest books for 28 days or more, it falls under the Long Term cancellation policy.
This is what happens below
If the guest books a reservation and decides to cancel the reservation DURING their stay, the guest must use the online alteration tool in order to agree to a new checkout date. Regardless of the checkout date chosen, the guest is required to pay the host for the 30 days following the cancellation date, or up to the end date of the guest’s original reservation if the remaining portion of the original reservation is less than 30 days.
You may of course give an additional refund on top, should you be inclined, via the resolution centre.