checks current time against time of post and expires the post if it's older than today.

check-time-wordpress

// setting posts with current date or older to draft
if (!wp_next_scheduled('sfn_expire_hook')){
    wp_schedule_event( time(), 'hourly', 'sfn_expire_hook');
}
add_action( 'sfn_expire_hook', 'sfn_show_expire');
add_action( 'sfn_expire_hook', 'sfn_show_expire');
function sfn_show_expire(){
    global $wpdb;
    $server_time = date('U');
    // below echoes date so we know what's up with the server date
//    echo date( 'Y, m, d - g:i' );
    $result = $wpdb->get_results("SELECT * FROM $wpdb->posts WHERE post_type = 'show' AND post_status = 'publish'");
    if( !empty($result)) foreach ($result as $a){

        // getting the show time
        $show_time = get_the_time('U', $a->ID);

        // adding 3 hours to the post time so things don't expire right away
        $show_time_three = strtotime( '+3 hours', $show_time );

        // if the show time is more the 3 hours older than the server time
        // lets set the show to draft since it's old
        if ( $server_time > $show_time_three ){
           $my_post = array();
           $my_post['ID'] = $a->ID;
           $my_post['post_status'] = 'draft';
           wp_update_post( $my_post );
        }
    } // end foreach
}
// this simply serves to run the above function for testing don't do this live b/c it will run every time the site loads and that's a bad thing
//add_action( 'init', 'sfn_show_expire' );
//
// need to uncomment the clearing hook below if you're changing the cron settings
// wp_clear_scheduled_hook( 'sfn_expire_hook' );
//
// the code below tells you when the hook is sheduled
// $crontest = wp_get_schedule( 'sfn_expire_hook' );
// echo $crontest;