Calculating Time Left to Process using Relative Date Difference

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We need a field that shows the remaining time to process on unresolved service requests. The formula created by our system integrator uses absolute time. It is written: if(incidents.status_type=2,0,response_reqs.rslv_within-( round(((date_diff(sysdate(),incidents.created)))/60,0)))

We need to capture the remaining time to process as relative time removing weekends and holidays. Because the rel_date_dif expression cannot be used in a nested formula, I'm having difficulty displaying the true remaining time on a service request before it becomes out of standard. I have tried:
rel_date_diff(incidents.rel_due,sysdate(),incidents.rr_id)