Larry Masinter has published an Internet Draft entitled '"duri" and "tdb": URN Namespaces based on dated URIs'.

The draft defines two persistent namespaces of URNs based on preprending a date to a URI. The result is a namespace in which names can readily be assigned but which also offers the persistence of reference required by URNs.

Two schemes are defined: "duri," which is used to refer to URI-identified resources, and "tdb," used to refer to abstractions that are not network resources but which are described by them. Masinter gives some examples:

For example, urn:duri:2001:http://www.ietf.org is a persistent identifier to 'http://www.ietf.org' as of the very first moment of the year 2001...

So "urn:tdb:2001:http://www.ietf.org" can be used to designate the Internet Engineering Task Force organization, at least as it was described by or referenced by its home page at the first instant of 2001.