Tracking date/time when a field changes

I'm developing an app for people to do task assignments in a workflow. It would be great to be able to log the time in a separate field when the value of an index field changes. (I.e., the time when the "status" field changes from pending to completed" for instance). Thanks!

Would that not be as simple as having date fields for each of the events you want to date stamp (e.g. a date field for Status change completed) then, when the status is changed, using form rules update the date field to current time.

I'll submit my request for this same functionality.