How to setup task deadline using Working days setup in Calendars

I want to create this function where the deadline of my tasks for my sales process, I want it to follow the working days & working hours that I have set-up using calendars. 

However, when I checked initially it doesn't by default use the working days/working hours that I set up in calendars. Its instead just taking calendar days.

Does anyone know how I can create maybe a business process to these two requirements: 

1. When creating a task deadline, the system calculates the end date factoring the holidays setup in calendars. For example the task has a SLA/deadline to be completed in 5 days and the task was created this Monday. But this Tuesday its a holiday in the calendar, so the deadline for this task is not set for Friday but instead the following Monday. 
    
2. When tracking total task duration can this track the setup working hours in calendar? Because right now when the partner tries this, its counting the entire 24 hour day instead of the 8 working hour that has been set-up.
    

If anyone knows or has experience with setting up the business process would appreciate it