function start_modify_html() {
ob_start();
}
 
function end_modify_html() {
$html = ob_get_clean();
$html = str_replace( '2021', '2022', $html );
echo $html;
}
 
add_action( 'wp_head', 'start_modify_html' );
add_action( 'wp_footer', 'end_modify_html' );