You can easily change how close to an appointment time a client can book.
By default your account is set to allow clients to book up to 24 hours before an appointment.
In order to change this setting simply:
Click on "Settings" in the left hand navigation
Click on "Booking Settings"
Change the number of hours in the "How close to the appointment time can a client book online?"
You can then select how long you would like to allow people to book before their appointment, and select "ok"
Click "Save" in the top right corner to implement those changes
You have now set how close to the appointment time a client can book.