Do you really mean “posts”? A timer per post within a topic will be more difficult.
Hmm. The only topic timer that would change how the topic is shown is the delete timer. I am not sure that’s the result you intend. It is kind of as if the topic never existed - for example likes on those posts no longer count and the user won’t be able to see their posts in the topic. Is that what you want? I had the impression a category with “past events” that is muted by default so it’s hidden from topic lists like /latest might be closer to what you want. But I am not sure what you mean by “seen as a new topic”. By default a topic is considered as “new” when it was posted in the past 2 days. There is a user preference where users can change that for themselves. Additionally new replies are considered to be “new” for the user when they are watching or tracking the topic.