Change how many minutes before a timeslot is unable to be booked on same day.

Step One.
Navigate to the settings option, and select the 'Booking Form' wizard.

Step Two.
This setting is found on the 3rd tab 'Slots'.

Step Three.
Scroll down to 'Minute Cut Off' and enter the amount of minutes you want before a slot cannot be booked on the same day. e.g 30 minutes will mean that a 7pm slot will become unavailable at 6:30pm online.

Step Four.
Click Save at the bottom of the box.