Off topic:apt-get update showing error

Question

[harry]

$sudo apt-get update
[sudo] password for aclab:
Hit:1 http://in.archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 https://linux.teamviewer.com/deb stable InRelease
Get:3 https://download.docker.com/linux/ubuntu bionic InRelease [64.4 kB]
Get:4 http://in.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:5 http://in.archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Hit:6 https://mirror.oxfordnanoportal.com/apt trusty-stable InRelease
Get:7 https://cloud.r-project.org/bin/linux/ubuntu bionic-cran35/ InRelease [3,626 B]
Err:7 https://cloud.r-project.org/bin/linux/ubuntu bionic-cran35/ InRelease
  The following signatures were invalid: EXPKEYSIG 51716619E084DAB9 Michael Rutter <[e-mail]>
Get:8 http://in.archive.ubuntu.com/ubuntu bionic-updates/main i386 Packages [1,162 kB]
Get:9 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Get:10 http://security.ubuntu.com/ubuntu bionic-security/main amd64 DEP-11 Metadata [48.9 kB]
Get:11 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 DEP-11 Metadata [59.5 kB]
Get:12 http://in.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages [1,761 kB]
Get:13 http://security.ubuntu.com/ubuntu bionic-security/multiverse amd64 DEP-11 Metadata [2,464 B]
Get:14 http://in.archive.ubuntu.com/ubuntu bionic-updates/main Translation-en [373 kB]
Get:15 http://in.archive.ubuntu.com/ubuntu bionic-updates/main amd64 DEP-11 Metadata [295 kB]
Get:16 http://in.archive.ubuntu.com/ubuntu bionic-updates/restricted i386 Packages [17.4 kB]
Get:17 http://in.archive.ubuntu.com/ubuntu bionic-updates/restricted amd64 Packages [196 kB]
Get:18 http://in.archive.ubuntu.com/ubuntu bionic-updates/restricted Translation-en [26.3 kB]
Get:19 http://in.archive.ubuntu.com/ubuntu bionic-updates/universe i386 Packages [1,547 kB]
Get:20 http://in.archive.ubuntu.com/ubuntu bionic-updates/universe amd64 Packages [1,692 kB]
Get:21 http://in.archive.ubuntu.com/ubuntu bionic-updates/universe Translation-en [355 kB]
Get:22 http://in.archive.ubuntu.com/ubuntu bionic-updates/universe amd64 DEP-11 Metadata [288 kB]
Get:23 http://in.archive.ubuntu.com/ubuntu bionic-updates/universe DEP-11 64x64 Icons [488 kB]
Get:24 http://in.archive.ubuntu.com/ubuntu bionic-updates/multiverse i386 Packages [14.4 kB]
Get:25 http://in.archive.ubuntu.com/ubuntu bionic-updates/multiverse amd64 Packages [36.2 kB]
Get:26 http://in.archive.ubuntu.com/ubuntu bionic-updates/multiverse Translation-en [7,308 B]
Get:27 http://in.archive.ubuntu.com/ubuntu bionic-updates/multiverse amd64 DEP-11 Metadata [2,464 B]
Get:28 http://in.archive.ubuntu.com/ubuntu bionic-backports/universe amd64 DEP-11 Metadata [9,288 B]
Fetched 8,701 kB in 5s (1,743 kB/s)
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://cloud.r-project.org/bin/linux/ubuntu bionic-cran35/ InRelease: The following signatures were invalid: EXPKEYSIG 51716619E084DAB9 Michael Rutter <[e-mail]>
W: Failed to fetch https://cloud.r-project.org/bin/linux/ubuntu/bionic-cran35/InRelease  The following signatures were invalid: EXPKEYSIG 51716619E084DAB9 Michael Rutter <[e-mail]>
W: Some index files failed to download. They have been ignored, or old ones used instead.

Can someone help me why I getting these errors

Thanks in advance to solve this issue.