macOS Calendar.app Duration-of-Day Preference Nonresponsive

'Day starts at' / 'Day ends at' is for working day. It has effect on Calendar > Window > Availability Panel (⇧ + ⌘ + A). This panel is 'available' for usage only if calendar is connected to supported service (and iCloud is not being one) and for meeting planning.

I believe that this is for work related appointments so that your colleagues can't make appointments outside of the working day and they can see your appointments or free/busy time only during working hours while planning meeting.

'Show ... hours at a time' has effect on Calendar Day view. It determines how many hours are shown as default in Day view.

"Show 24 hours at a time" setting displayed in Day view:

"Show 8 hours at a time" setting displayed in Day view: