If you want to be able to control your booking window (the earliest time or the latest time that someone can book with you) then follow this guide.
First, log into your Start Booking account and go to your account center - https://app.startbooking.com/#/account
Once you've arrived at the account center, scroll down to the section called Settings > Booking window.
You'll see options for None, Hours, Days, Weeks, Months for both the earliest and latest options. Once you select one of those options, the form will update and ask you for the quantity of Hours, Days, Weeks or Months.
As an example, if you don't want same day bookings, set your booking window to look like this:
That's it, now your customers can only book within the timeframe that you have set.