Last-Modified gives you the time when the content was BELIEVED to be last modified (or the time the current content is created). If you want the exact time when the original content (which is probably gone already) was first created, I don't believe HTTP specification provides that. However you have to re-think whether that time is meaningful.

Well, if you actually have access to the server, that will be a different story, and you can just use whatever command the OS provides you to check, if the content is not created on fly.