apt 0.7.20 does not check when the date command returns an "invalid date" error, which can prevent apt from loading security updates in time zones for which DST occurs at midnight.
https://bugs.launchpad.net/ubuntu/+source/coreutils/+bug/354793
http://www.openwall.com/lists/oss-security/2009/04/08/11
http://www.debian.org/security/2009/dsa-1779
http://secunia.com/advisories/34874
http://secunia.com/advisories/34832