Doing Calculations on Row level

Hi Everyone - I have been struggling to get the calculation to work as I can only calculate within columns. What I'm trying to achieve is adding a custom column then doing a calculation on row level to determine a value. I have a data set with a time, where a case is logged on a specific time. I want to the subtract a time from another, so that I get a value in minutes (e.g. row 2<time> - row1<time>) so that I can determine the time between cases logged.