Change the starting hour and ending hour of Scheduler?

Yes, this is possible. Please see the documentation for more details:

https://doc.daypilot.org/scheduler/hiding-non-business-hours/

Thanks - note: I found this did NOT work:

dp.businessBeginsHour = 9;
dp.businessEndsHour = 18

However, THIS works
dp.dayBeginsHour = 9;
dp.dayEndsHour = 18;

This is confirmed by other users of the forum. So, not sure if there is a problem w/ businessBeginsHour or it needs something else to be called to work.

OK, so you mean the Calendar component.

In the Calendar, there are actually two ways to achieve that - using businessBeginsHour/businessEndsHour + heightSpec and using dayBeginsHour/dayEndsHour:

https://doc.daypilot.org/calendar/business-hours/
https://doc.daypilot.org/calendar/height/
https://doc.daypilot.org/calendar/overnight-scheduling/