How to execute code only on posts older than 2 weeks

It looks like a couple of things are out of order.

• First, I’d definitely make sure to declare the $post global
• It’s also probably worth wrapping your logic in an is_single() and/or is_singular() checks
• Last, the section where you’re calculating the time as greater than 2 weeks, the current time and post date are switched. This works for me:

/**
 * Enqueue a stylesheet for posts older than two weeks.
 */
function hide_meta_for_older_posts() {
    if ( is_single() || is_singular() ) {
        global $post;

        if ( time() - strtotime( $post->post_date ) > ( 2 * WEEK_IN_SECONDS ) ) {
            wp_enqueue_style( 'remove-style-meta', plugins_url( 'css/entrymetastyle.css', __FILE__ ), false, '1.0', 'all' );
        }
    }
}
add_action( 'wp_head', 'hide_meta_for_older_posts' );