> I've had this thought before but it seems like the natural key for a web resource has to be the URL (location) plus the time that the resource was accessed for practical reasons.
No browser currently implements them, but a viable resolution mechanism probably involves keeping a default store of Memento Time-Gates (http://www.mementoweb.org/guide/quick-intro/) and querying them to see if any of them have a copy of your resource for that date.
These are called Dated URIs/DURIs: http://tools.ietf.org/html/draft-masinter-dated-uri-10
No browser currently implements them, but a viable resolution mechanism probably involves keeping a default store of Memento Time-Gates (http://www.mementoweb.org/guide/quick-intro/) and querying them to see if any of them have a copy of your resource for that date.