Date override in Scheduler not working

I've set up the scheduler and have managed before to set Date overrides. However, this feature seems to have stopped working. I'm just adding dates, and keeping the hours to 9 to 5, click add, see the date added, but when I navigate the scheduler page, I still see the date showing and I'm able to book an appointment. 

What am I missing? Is there a tick somewhere to make this work?