Stories retrieved via the api should have a last change timestamp

I'm working on a tool that uses the api data to store a local cache of the stories. Instead of retrieving and storing all the stories all the time, I'd like to be a little more intelligent and only update locally the stories that have changed.
To do so now I would need to compare every story locally with the story I obtain via the API and only update the ones that changed.
At the very least the API could supply a "last-changed-at" time stamp which I could store and use to update only the changed stories. Ideally we'd have access to a last-changed-at time stamp and to the history of the story, so that I don't have to compute the changes myself.

Thank you