There are a few company settings that determine what hours show on the Calendar. If you use the Calendar, the primary reason for setting your work hours is because the start time is where all jobs without appointment times will appear.
To adjust all of these settings, go to Settings > Company Information > Settings. This article explains the functionality of the settings in the orange box:
If Use 24 Hour Calendar is enabled, it will hide the next three settings. The Calendar will display hours from 12:00 AM to 11:59 PM. Any jobs that do not have appointment times will appear at 12:00 AM. Timesheets will default to a Sunday to Sunday view.
If the 24 Hour Calendar is disabled, you can set the Start and End Time of Work Day. These should be your normal business hours from the time your first crew members clock in to when the last jobs are done. This allows you to schedule jobs and calendar events any time during those hours. Any jobs that do not have appointment times will appear at the Start Time of Work Day.
The Timesheet Start Day of Week determines the default start day on Timesheets (Accounting > Timesheets). Timesheet dates can be customized but when you first open the page it will default to the day set in the Company Settings.