The workstep instance due date is always based on the group calendar or the system calendar and the estimated duration defined for the workstep.
If the workstep has a single performer as group:
If the group calendar is defined, then it is used.
If the group calendar is not defined, then the system calendar is used.
If the workstep has more than one performer, then the system calendar is used.
The calculation of the due date of the workstep instance never uses the user calendars, as individual calendars may vary in vacation days and working hours. Note that at the design stage, the application design considers only standard time and duration and not any individual variation.