It is now longer possible to book an appointment on a date in the past.
It is enabled by a flag in settings. Go to:

Frontpage - System - Setting - Global - Choices - Restrict creating booking in the past

When creating a booking for today by right clicking on the time that I want to book at, I'll get an option to Create Appointment.

If I try to book a date that is in the past and right click on the time that I want to book at, the Create appointment isn't active and can't be used.

If trying to move an appointment to a date in the past, the system will give a pop up warning.

However it is still possible to book an appointment at eg. 11 o'clock if the clock is eg. 13.00 on the same date.