Booking Options/Default Time Choice

When the "Default Time Choice" setting is set to "Next Available," if a specific time/duration isn't available for a customer to book, the system will automatically find the next open slot, which often results in shifting the customer to the next day.

To stop this from happening, change that setting to "By Date."

This setting can be found under Account Options > Booking Options > Default Time Choice:

This setting restricts the system to the specific date the customer selected. If they try to book 2 hours and it doesn't fit, the system will simply show it as unavailable rather than jumping them to a different date.