Calculating time between process steps

This one totally evades me...

I have a report that lists the times for each step in a process. I want to calculate the time it takes each step to happen so I can run stats to say how long things take and how we have been improving, etc.

An example is attached. This one is a simple example but if I can do it for 4 steps I can do it for more. In the example, a document gets upload, two approvers and then gets published.

I would want to look at time from upload to publish as well as time from upload to approval, etc. This sheet has three docs but my real sheet has hundreds so manually doing it is not feasible.

Any help you could provide would be greatly appreciated.

Thanks!