Add functionality to track changes made to a custom table field for reporting

At the idea level there is a status and there are fields we can use with reporting that show us the date when the idea moved to a new status. We have a sub-status field in a custom table that also has a status field but I cannot see any reporting fields that show us the date that status was changed to another state. The addition of this functionality would enable us to track the sub-status changes and manage our workflow more accurately.