The User Management area allows the administrator to define for a user which days/times they prefer to work. Scheduled Preferences is set for any preferences a user has with scheduling appointments.
Go to Administration > User Management and select a user. In the User Details window, click on the Sched. Preferences button.
Select a day and enter the start/end times. PIMSY uses military time here so for 8am to 5pm enter 8:00 and 17:00.
Be sure to Save your changes. Any days on the calendar specified for a User will appear in green color.