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