apt line - specific date

 deb http://snapshot.debian.net/archive/YYYY/MM/DD/debian unstable main contrib non-free
 deb http://snapshot.debian.net/archive/YYYY/MM/DD/debian-non-US unstable/non-US main contrib non-free
 deb-src http://snapshot.debian.net/archive/YYYY/MM/DD/debian unstable main contrib non-free
 deb-src http://snapshot.debian.net/archive/YYYY/MM/DD/debian-non-US unstable/non-US main contrib non-free

Useful when you want to get debian archive on specific date.