As an expert, under Profile - General, you are able to choose between 3 options under "Minimum booking time before Chat occurs". These are Immediate, 24 hours or 48 hours. If you select immediate, Clients will be able to book you for any slot for which you have added availability, regardless of time proximity to that slot. If you select 24 hours, Clients will only be able to book slots at least 24 hours hence and likewise form 48 hours.
This option is easily adjustable and many experts will alter their Minimum booking time before Chat occurs depending on their situation. For instance, some pilots or cabin crew on a layover will arrive at the hotel, log-on to the WiFi and change their minimum time to Immediate before announcing on their Socials that they are available immediately for anyone who wishes to chat. This is often a good way for crew, often bored and locked-in in hotel rooms, can have some social interaction while also earning money. It is important to remember to change the Minimum booking time before Chat occurs setting back to either 24 or 48 hours, otherwise you could end up missing appointments.