Starting Meetings Only at :00 or :30?

Hello!  In my Bookings calendar, I offer a few different meeting length (15min, 30min, 1 hour) and then a 45 minute presentation/introduction option. 

In any case, I'd like to configure Bookings so regardless of what the person chooses, it's starting the meeting at the top of the hour or at the :30 mark.  I have a number of cases lately where someone is booking something at 1:25 or 1:55, for example, which isn't ideal. 

I can't really find a toggle button for such a use case. Any workarounds or ideas?  We're pretty locked down in terms of software options, so third party tools aren't a viable solution.