If you get an error like
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://download.opensuse.org Release: The following signatures were invalid: KEYEXPIRED 1397815516 W: Failed to fetch http://download.opensuse.org/repositories/isv:/ownCloud:/community/xUbuntu_12.04/Release W: Some index files failed to download. They have been ignored, or old ones used instead.
You simply need to reimport the new key… copy and paste the command below
wget -qO - http://download.opensuse.org/repositories/isv:ownCloud:community/xUbuntu_12.04/Release.key | sudo apt-key add -
Thanks to fkunz for the fix