When attempting to install a new package or update an existing package if you are presented with the error
E: Release file expired, ignoring http://archive.debian.org/debian-archive/debian/dists/squeeze-lts/Release (invalid since 189d 0h 42min 32s)
You will need to execute the following command as root:
$ echo 'Acquire::Check-Valid-Until "false";' > /etc/apt/apt.conf.d/90ignore-release-date
And then perform…
or if you are using aptitude…
You may need to execute the above command twice to make sure everything is up to date.