What to do if you get this „GPG error: http://repo.mutability.co.uk wheezy Release: The following signatures were invalid: KEYEXPIRED“
List all keys
sudo apt-key list
Check which key has expired
sudo apt-key list | grep expired
e.g.
# pub 2048R/4D731812 2014-12-28 [expired: 2015-12-28]
Update
sudo apt-key adv –recv-keys –keyserver keys.gnupg.net 4D731812
Double check
sudo apt–key list