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Hello.
In the Booking Calendar at the WP Booking Calendar > Settings General page in “Availability” section you can define for the options “Unavailable days from today” value “1 day” (or “2 days” depends from your needs).
In this case, system will block today date, and users of your website can not submit the bookings for today date. Please note, this option based on the days and not on the 24 hour increment.
Kind Regards.