There is not a way to set the hours your business is open. You can simply create shifts for the times you want to schedule employees to work.
Also employee work time preferences are set on a grid that shows 24 hours in the day and can be set down to `15 minute increments. Note that any preferences that are set for times outside your scheduled shifts are simply not used.
Some schedule views such as the Availability / Coverage and Graphical have a option to show only your open hours of the the display.
business hours open hours hide times a change time view closed hours