Start & End Hours
Start and End Hours in most cases can be leaved as the default setting of 12:00am to 11:59pm. If you change these hours when viewing your calendar on Appointment Reminder only these hours will be visible. This can help prevent users from adding appointments outside of business hours and can make your calendar cleaner and makes finding appointments easier.
In Business Settings (Settings > Business) if you enable "Only Send Reminders During Calendar Hours" then appointment reminders will only be sent during the Start & End Hours you have set for this calendar.
If using Google Calendar you can tick the "Only Sync Appointments during calendar hours" so Appointments from Google will only be sync'd if they are between these two times.