What I would do is an onchange microflow that checks if the time picked is between opening hours and show a message if it is not.
I assume you know how to check if a date time is between a certain range?
Not quite sure what you want to achieve, but this you take a look at the timepicker app in the appstore?
Thanks Ronald. We are using that, but I only want to show the list of times between 8am & 5pm in the dropdown. Currently it shows everything from 00:00 to 23:59