Problema.

apt-get -y update && apt-get -y upgrade

Hit http://mirrors.buyvm.net wheezy Release.gpg
Get:1 http://mirrors.buyvm.net wheezy-updates Release.gpg [1,554 B]
Get:2 http://mirrors.buyvm.net wheezy/updates Release.gpg [1,554 B]
Hit http://mirrors.buyvm.net wheezy Release
Get:3 http://mirrors.buyvm.net wheezy-updates Release [136 kB]               
Hit http://mirrors.buyvm.net wheezy/updates Release    
E: Release file for http://mirrors.buyvm.net/debian-security/dists/wheezy/updates/Release is expired (invalid since 5d 23h 38min 57s). Updates for this repository will not be applied.

Tres posibles soluciones.

1. Cambiar de repositorios (/etc/apt/sources.list).

2. Ejecutar el siguiente comando.

apt-get -o Acquire::Check-Valid-Until=false update

3. Ejecutar el siguiente comando.

rm -rf /var/lib/apt/* && apt-get update