Working hours for regular business hours
When a calendar with regular business hours is used for calculating the due date then, the working hours are calculated as:
(Regular End Time - Regular Start Time) minus (Regular Mid-day Break End Time - Regular Mid-day Break Start Time)
For example, consider a calendar with

Regular Start Time: 09:00

Regular End TIme: 18:00

Mid-day Break Start Time: 13:00

Mid-day Break End Time: 14:00
The working hours are calculated as (18:00 - 09:00) - (14:00 - 13:00), equalling 8 hours.