Calculate the time between Assigned transactions

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I am being asked to report how long an incident has been assigned to each agent that the incident has been assigned to.  For example, an incident is created and then someone assigns the incident to person A, Person A assigns it to Person B, Person B then assigns it back to Person A: I am being ask to calculate the time between assignments.  I have a report that reports only when an incident has been assigned (transactions.trans_type = 4), but how can I report how much time has elapsed between assignment changes (transactions.trans_type = 4).  I was thinking it