Averaging the # of days open

Content

I have created a report where I am looking at incidents from a subset of our customers - I rollup this report by Customer Name and then Incident Severity and then calculate the number of days/hours to resolve using date_diff() - In order to have this display in Days/hours I am using the following definition:

time_format(date_diff(incidents.closed,incidents.created), 'DD Days HH24 Hours')

I then wish to average the number of days open per severity but the average is greyed out when using the time_format. I can average the date diff - but this gives me seconds and I want days/ hours.