Guy on Nostr: Suppose I close my client software, two weeks passes, and near the beginning of the ...

Suppose I close my client software, two weeks passes, and near the beginning of the two weeks the key rotation occurs for someone I follow. Now there are a bunch of notes more than ten days old according to the new key. They will be labeled post rotation noise by my client, right? How can I know whether those events that come to me are legitimate?