https://github.com/metabrainz/mmd-schema/blob/master/schema/musicbrainz_mmd-2.0.rng#L42-L46 allows adding a `created` attribute to the `metadata` element that holds the timestamp of the last update to the data.

This should probably be either

• the last update time of the core itself
• the last update time of any of the documents in this particular result set