Date based conditional function

How would I write a function that applies some change only to posts published after a particular date?

For example, one thing I’d like to do is to have my theme automatically insert the “Featured Image” at the beginning of each post but I only want this to apply to every post moving forward because all of my posts prior to this have the image manually inserted at the beginning of the post and without a conditional exception I would either have thousands of posts with two leading images or have to remove thousands of images manually.

The function would go something like this:

“If the post was published after 9/3/2010 then display the featured image, or else do nothing”

How would I write that?