Date Formulas

Hi All

I am trying to install an automated field for a module with the following parameters:

Our standard business hours are 08:30 to 17:30 (9 hours).
We have self imposed SLA deliverable restrictions for different jobs, guaranteeing we will complete an order within X number of business hours.
We want a field to be able to look at the Order Created time, the current system time and subtract this figure from the established SLA restriction automatically.
This is to show the time remaining before an SLA breach occurs (very important).

For example:

An order is received at Thursday 20th 16:30 with a 6 hour SLA restriction. This means we must complete this call by Friday 21st 13:30. So upon looking at this order at Friday 21st 11am I want the field to tell us that we are 2.5 hours from an SLA breach. The problem I'm am facing is trying to use existing fields in formulas or just trying to get around the Business Hours usage instead of normal 24 hours usage.

Am I making sense? Is this something people have attempted before?

Thanks all!