how to manage gaps in Valid-Until of ftp.us.debian.org

Hi:

I maintain a local mirror of the standard debian.org repositories for
squeeze and wheezy. I am finding lately that there are significant gaps in
the Valid-Until entry for my mirror of the wheezy-updates repo.

I can trace this back to the fact that Valid-Until entry differs

ftp://128.61.240.89/debian/dists/wheezy-updates/Release (15:00 UTC)
ftp://64.50.233.100/debian/dists/wheezy-updates/Release (09:01 UTC)
ftp://64.50.236.52/debian/dists/wheezy-updates/Release  (09:01 UTC)

What should my expectations be for mirroring ftp.us.debian.org? If I were
to mirror on the 64.50 entries every morning at 10h UTC, would I be
guaranteed to have only a 1h gap in my local mirror?

My local mirror's previous entry expired at 08:58 UTC which is suggestive
of a 3 minute period during which ftp.us.debian.org did not provide a
bugfix repository which self-declares as valid.

It seems to me that there should never be gaps in the Valid-Until provided
by a primary mirror. That is, if the maintainers of the debian.org
repositories intend to update every 7 days, that the previous entry should
be valid for 8 days. But I'll allow I haven't thought about repo management
a whole lot so there may be good reasons to prefer a gap in validity to a
repository declaring itself as valid even if it is slightly out-of-date.

I'm not sure if this problem affects other repos at debian.org. I tried
some simple google searches to see if this particular problem has been
reported before, but I only saw evidence of what appears to be transient
issues in the "Valid-Until" rather than something systematic like the above.

--
Tom Downes
Associate Scientist and Data Center Manager
Center for Gravitation, Cosmology and Astrophysics
University of Wisconsin-Milwaukee
414.229.2678