Work hours definition impact SLA calculations. As an example, a Priority 3 ticket with 24 hours fix time spans 24 hours of "work hours".

This is to say, if each work day has 8 hours, the Due time calculated by the desk will be 3 work days from time of ticket creation. 

Work hours or Shift hours which define support desk working hours are setup in System Settings / Work Hours tab and are self explanatory.

Example entries for defining work hours: note the Day Start Time and End time in the screenshot below.

Note: for a weekly off day define start and end time as same, which is to say 0 work hours. Due to bugs with how daylight saving works, define time as 03:00 or more. See entry for Sunday below: