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 "allow clients to book until X hours before the appointment time" drop down:
You have now set how close to the appointment time a client can book.