Err:8
http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release.gpg The following signatures were invalid: KEYEXPIRED 1515625755 Hit:9
http://us.archive.ubuntu.com/ubuntu xenial-backports InRelease Reading package lists... Done W: An error occurred during the signature verification. The repository is not up dated and the previous index files will be used. GPG error:
http://repo.mongodb. org/apt/ubuntu xenial/mongodb-org/3.4 Release: The following signatures were inv alid: KEYEXPIRED 1515625755 W: Failed to fetch
http://repo.mongodb.org/apt/ubuntu/dists/xenial/mongodb-org/3 .4/Release.gpg The following signatures were invalid: KEYEXPIRED 1515625755 W: Some index files failed to download. They have been ignored, or old ones used instead.