Put it in your function that is updating the post date.
Instead of
wp_update_post( $my_post );
use
$post = wp_update_post( $my_post );
if (is_wp_error($post)) {
echo 'WP ERROR: <pre>'; print_r($wp_error); echo '</pre>'; die;
}
This should cause the error to be displayed it there is an error.