see https://snapshot.debian.org/
> To access snapshots of suites using Valid-Until that are older than a dozen days, it is necessary to ignore the Valid-Until header within Release files, in order to prevent apt from disregarding snapshot entries ("Release file expired"). Use aptitude -o Acquire::Check-Valid-Until=false update or apt-get -o Acquire::Check-Valid-Until=false update for this purpose.
(thanks to Axel Gembe for pointing this out)
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