Time Difference Between Dates of Completed Tasks

Content

Hi all,

I am trying to build a report that calculates the time that each incident was in certain stages based on the dates the tasks were completed on. We can then see at which stage the incident is taking longest to complete.

Is it possible to write an expression to calculate the difference between the date completed of specific tasks? Like the difference between the dates completed for tasks 11-14, or would I need to make a dashboard and define everything in the filters as a separate report? I am hoping that I am just being thick and it's really quite easy.