Working hours considering overlap times

Hey AIB,

First of all thank you for taking the time for reading and helping me with this! 

Regarding the data, for example, my shift is from 8:00 AM until 17:30 PM, durning that time I am supposed to be working on a specific task, but due to activities related to the work, helping a coworker, breaks, being on the phone, going for lunch, etc. I don't spend the whole shift at those specific tasks that I'm suppose to do.

Therefore I would like to calculate the amount of time that I am actually doing that specific task, that is why I am trying to calculate the duration of each activity (End - Start time) and then sum all the durations from one shift. And based on that time and considering the amount of SKU's processed, determine:

- The "real" working hours

- The productivity (SKU's / Working hours) 

I hope that clarifies the problem that I'm facing.

Once again thank you for your help!