Possible new feature for apt

[Joey Smith <joey@joeysmith.com>]

I don't know if this has been discussed before, or maybe
even solved before, so I thought I'd ask here before filing
a feature request in the BTS.

I'd like to see apt able to check a mirror to see how 'fresh'
it is. My initial concept was something like this:
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Expand the "Releases" file to include a version number of some kind
Add a new kind of deb line to sources.list that says where to get
 the authoritative version of Releases
 "deb-chk http://central.repostory /location"

When apt-get update is run, warn about sources that have a stale
 Releases version
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Is this something that might be possible? Am I just missing something
that might already be implemented?