The Wikimedia projects are always going through cleansing and cleaning up of content live on the projects. Typically if you see an event with an old timestamp, it is likely that this is a “reverted” revision, meaning somebody removed something that is live and replaced it with a past revision. For example, someone removed a vandalism edit and 'reverted' it back to the previous state of the article. These revisions maintain their original timestamp in order to preserve the original metadata attached.