Jenkins ver. 2.154

1. visit /pluginManager/advanced
2. change the update site to https://updates.jenkins.io/experimental/update-center.json
   1. Submit
3. change the update site to https://updates.jenkins.io/update-center.json
   1. Submit
4. See Update information obtained: N/A ago
5. Expect not to see N/A ago

Afaiu, the code starts:

https://github.com/jenkinsci/jenkins/blob/7aef24d4542738ce309122ea05a3d194e842c8d8/core/src/main/resources/hudson/PluginManager/check.jelly#L28

https://github.com/jenkinsci/jenkins/blob/22aa2e6e766074d11249893e3f35e0b99e20d3d0/core/src/main/resources/hudson/PluginManager/check.properties#L23