The Calendar table contains the records of the working days including working hours as well as holidays.
When creating a task you can specify the start date/time and end date/time for the task. The system automatically calculates the task duration excluding non-working hours or holidays/weekends.
See blog article for detailed explanations.
Any support materials, such as spreadsheets and real-life work samples would be very helpful as they would help us better understand your business process.