I couldn't find a module or feature in D7 that keeps track of the date/time when a user updates their profile (any of the fields). I'm not using profile2 and I'm not planning to switch to that, otherwise I could probably do it with the node revision.
Is there another way of doing it other than adding a "last_update" date field to the user and then using a hook_user_update() or hook_user_presave()?