...
SLA is based on work time calendar, so please create and set up work time calendar of your company or make a default calendar from 00:00 till 23:59. To do it, enter Administration/Add-ons/HelpDesk/SLA settings menu:
You need to create new calendar here and define work and non-work days and work time.
...
After creating and setting up work time calendar you need to select it in SLA field configuration:
Now SLA durations in the new issues will be calculated based on work time calendar.