Wordpress - Shortcode how many years passed since 1984

Question

I'm trying to show how many years have passed since 1984 on a shorcode.

The php is right, i tested, but when i try to put inside shortcode(wordpress) does not work.

function wpcs_vc_shortcode( $atts ) {
      $d1 = new DateTime('Y');
      $d2 = new DateTime('1984');
      $diff = $d2->diff($d1);
      echo $diff->y;
}
add_shortcode( 'year_ago_slider', 'wpcs_vc_shortcode');