How to get the Current time difference with inputted time

Hi all,
 

i have an issue, I have two fields: "Start Date" and "Duration". When I input the "Start Date", I want the "Duration" field to automatically calculate the years and months of how long the person has worked until today. How can I solve this? And can I use a business rule to solve this problem? 

i am using version 8.2.0
 

thanks