Make an avg on 2 dates with a distinct variable

[#1]

Hi

I'm new to qlikview and have a problem with a chart. I want to show the average time between 2 dates and use this expression:

round(Avg((end_date+end_time)-(start_date+start_time)),0.01)

It works fine, but my problem is that I want to add: distinct(case_id)

who to do that?