Show part of template ONLY if post older than…

I have a somewhat simple problem. Basically, what I’m trying to do is come up withe PHP required to say “If this post is in category X, and was posted after the date Y, then show the following, else do nothing.”

I realize the second part can be achieved by either checking to see if the current post number is higher than a certain number, or if the post date is higher than a certain date. I’m not sure which is more effective.

My problem is that I’m transferring an old blog to a new blog, and I want certain things to show up only on new posts, without effecting the old.