root@grey:~# apt-get -o 'Acquire::Check-Valid-Until=false' update
Ign http://archive.debian.org jessie InRelease
Get:1 http://archive.debian.org jessie Release.gpg [2420 B]
Hit http://archive.debian.org jessie Release
Ign http://archive.debian.org jessie Release
Ign http://archive.debian.org jessie/main powerpc Packages/DiffIndex
Ign http://archive.debian.org jessie/non-free powerpc Packages/DiffIndex
Ign http://archive.debian.org jessie/contrib powerpc Packages/DiffIndex
Hit http://archive.debian.org jessie/contrib Translation-en
Hit http://archive.debian.org jessie/main Translation-en
Hit http://archive.debian.org jessie/non-free Translation-en
Hit http://archive.debian.org jessie/main powerpc Packages
Hit http://archive.debian.org jessie/non-free powerpc Packages
Hit http://archive.debian.org jessie/contrib powerpc Packages
Fetched 2420 B in 12s (197 B/s)
Reading package lists... Done
W: GPG error: http://archive.debian.org jessie Release: The following signatures were invalid: KEYEXPIRED 1587841717
root@grey:~#