No update time on medias and comments for Instagram Graph APIs

Question

On Instagram I'm calling this api to get medias:

https://graph.facebook.com/{id}/media?fields=id,media_url,timestamp&access_token={token}

And this api to get comments on each media:

https://graph.facebook.com/{media_id}/comments?fields=id,text,timestamp&access_token={token}

Both calls return the field timestamp, which is the creation time. There is no update time: what's the best way to retrieve all medias that have been updated? (You cannot edit the media itself but you can edit information like the caption field. However, the timestamp doesn't appear to be updated by that).

And what's the best way to retrieve new comments? If I write a new comment on an old media how do I know about that without querying each media? Is there a simpler way?