Calculating 'mean time between failures'

There are many assets amongst this dataset, what i have done is to filter to just 1 asset type. The required result would be to have the completed date of the last known failure and the origination date of the newest failure to be a measured time laspe by days i.e. last known failure completed date 1/12/17 , newest failure origination date is 12/12/17 - days between failure would be 11 days as i wouldn't want to include the origination failure date. 

This where i found the original measure - https://community.powerbi.com/t5/Desktop/repair-metrics-MTTR-MTBF/td-p/191653

Hope this makes more sense.

Thanks

kevin.