macOS Calendar.app Duration-of-Day Preference Nonresponsive macOS 11.3.1, Calendar 11.0, mid-2015 iMac
I have Calendar's preferences set to display a day beginning at 5:00 a.m. and ending at 9:00 p.m. thus:

However, both Calendar's Day and Week tabs display a calendar starting at 12:00 a.m. and ending at 12:00 a.m. This is 24 hours rather than the 17 hours I've set.
Restarting the app and rebooting the computer and rebooting the computer in safe mode each has no effect. In all other respects, Calendar operates as I expect.
How might I get the computer to display the specified day duration?
'Day starts at' / 'Day ends at' is for working day. It has effect on Calendar &gt; Window &gt; 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.
&quot;Show 24 hours at a time&quot; setting displayed in Day view:

&quot;Show 8 hours at a time&quot; setting displayed in Day view: