Substract and Average Time

Hello, 

I need to know how I can make this on PowerBi: 

• I will receive a template table, so I will apply my measure to it each time I received a new Table
• I need to calculate the difference between two columns, the format is date(mm-dd-yyyy) - hours - minuts. So I need to calculate the difference between my column "Time Started" and "Date Submitted", which means time ended. This column needs to be called "Time Spent"
  Can I rename this column to Time Ended, and it will stay as "Time Ended" even if I change the data source?.

Also, I want to know the time spent per each category of users. So I will calculate the average time, that the user spent. So, it will be the "Average Time" for each category of users. May be I could used a Slicer, it could be nice.

Thank you for your help.