Thanks for your reply. But this is not possible. The fields and data are coming in to us as a feed. So, it is not possible to change the field layout.
Basically I need to read the Status field and get the corresponding CallDate to get the difference. The CallDate for Status = Started and CallDate for Status = Ended needs to be considered. The CallDate for Status= In Progress need to be ignored.
So for Jill, the CallDate 4/1/2018 and 4/24/2018 are relevant and the datediff is supposed to give us 23 days as the total processing time. Similarly, for John, it will be 4/12/2018 to 5/2/2018 = 20 days processing time.
Thanks a lot Ankit. This is exactly what I was looking for!