Finding the average date an event happens

I'm trying to create a report and find the average date range that a product of ours is sold. I think I need to be able to use a measure so that the date range can be filtered and used for any product or groups of products that we have.

We have a very seasonal business, so what I'm trying to define for our products is the average date range that __% of that product is sold between. For example, if we sell 100 of product A in a given year. I want to know what date range were the middle 80 sold (units 11-90). 

In the end I am looking to probably use a 3 year average date range. Right now I'm stuck with even finding the date range for 1 year though.

Thanks,

Marcus

marcush,

Could you please share dummy data of your table and post expected result here?

Regards,
Lydia