I found I still can book it even when the time is already passed. For example (to reproduce): 

1. go to your demo page here 
2. select any location, worker and service
3. click on the same day booking calendar to book (e.g. if today is 3/18, click on the 3/18)

then you will see if current time is 3pm, 3/18, you still can book any appointment at anytime on 3/18, you can even book at 7:30am, which is already passed. I thought if the time already passed, it should be greyed out ? or where I should change the setting?   Thanks