date calculations

[thewillemzelluf]

Hi, I have a question about calculating the 'net working time' to determine the productivity. I have 3 fields, Start time | end of time | break time. The sum is end time - start time - pause = networking time in minutes or hours. Does any of you have an idea of how I can make this sum in the editor? (the fields are from the same table).  See the image below for the data types: 

Thanks in advance!

[NZFei]