Hi,
It appears that the SLA will only react to the operational hours if it is measured in 'hours' rather than 'days'.
For example, if I set the resolution time to 23hrs 59minutes it will set the DueBy date based on actual working hours. If i then change the resolution time to 1day, the DueBy date is set at exactly 1 day away, regardless of the operational hours.
This means that we can only set an SLA of a maxium 23hrs 59mins as any more than this does not work correctly.