Calculating Incident Response Time

Our organization is struggling with how to track average resolution of medium and low priority incidents. The issue is, we are not required to work and resolve incidents outside of our business hours of 7am – 5pm. How to others track issues reported during business hours and resolved after close of business or issues reported and closed during non business hours. We are leaning towards excluding these issues or establishing a standard default resolution time. Any suggestions or other methods used? Thanks