SLA's - First response Sent DELAY

Hi there,

We use SLA's to monitor which Cases are being worked on in a 4 hours delay.

Now to push this a little further, we would like to capture the time between Case Creation and First Response Sent.

Scenario:

SLA Item Failure: 4 hours

Business Hours are from 8am to 5pm

If a case is created on Monday at 3pm, and the First Response is sent Tuesday 9am, the delay should be 3 hours. So the SLA was fulfilled.

If a case is created on Monday at 3pm, and the First Response is sent Tuesday 11am, the delay should be 5 hours. So we failed to fulfill the SLA. But failing by 1 hour is different than failing by 3 weeks...

So my question is: is there a way to capure the delay in a field in CRM? We want to analyze our average response time.

Thanks!