Current open time in business days & SLAs

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I know this is far fetched, but was hoping someone had figured out a work around.  My business has implemented a new metric where each of our categories of work have different SLAs in business time (example: Category 1 should be solved within 3 business days, Category 2 solved within 10 business days).

I've been able to create reporting after the incident is solved, but am trying to build a tool for leadership to manage currently open incidents to view which ones are past the designated SLAs and which ones have yet to meet it.

Since using rel_date_diff within an expression is not possible, has anyone found a way to use business days within an expression with currently open incidents?  Ideally, rel_date_diff(sysdate(),incidents.created,1) used in an expression would be the answer, but with the system constraint, I haven't found a way around it.