absence duration calculation

After entering the start date and end date of an absence in Absence records page, we want the total duration to be calculated by summing up the start date and end date duration and not based on the work schedule of the employee. In order to achieve this, we wrote a fast formula and added to Absence plan Conversion Formula but still the duration is getting calculated based on the work schedule of the employee but not as per the rules that we have written in the conversion fast formula.

Can you please help us here.