I have 3 different custom post types (news, job, result ,) When I post something new under any of them, I want "new" label or gif for a limited time in title . Am already try that one which am found in old ans... But not working for me

add_filter( 'post_class', function( $classes ) 
{
    if ( is_singular() )
        return $classes;

    // now minus last mod time in seconds
    $diff = time() - mysql2date( 'U', $post->post_date );

    if ( DAY_IN_SECONDS <= $diff ) $classes[] = 'new-post';
        return $classes;
});

Css

.new-post {
    padding-left: 20px;
    background-image: url(new.png) left top no-repeat;
}