wordpress_post_expirator.php

<?php

/*
This function assumes a custom field named 'expiration' with a human friendly date/time.
*/

function is_post_expired($post_ID = null){
    
    if(!$post_ID) global $post;
    
    $post_ID = $post_ID ? $post_ID : $post->ID;
    
    //Human Friendly Expiration Date
    $expiration = get_post_meta($post_ID, 'expiration', true);
    
    //Adjust server time for your timezone
    date_default_timezone_set('American/New_York');
    
    $expiration_timestamp = strtotime($expiration);
    $time_left = $expiration_timestamp - time();

    if($time_left < 0):
        if(expire_post($post_ID))
            return true;
    endif;
        
}

function expire_post($post_ID){
    
    $args = array(
        'ID' => $post_ID,
        'post_status' => 'draft'
    );
    if(wp_update_post($args))
        return true;
}
?>