How to add a formula that subtracts date and times

How to add a formula that subtracts date and times

Hello,

I am new to app development.  I am trying to create my own zoho app that is a call log. 

I have 2 fields that are date and time,  one is start time, one is end time.  I want to track how long I am on the phone. 

Can someone please show me step by step to get my Field, 'Total Time' to show how many minutes and seconds I was on the phone please?

Thank you!