SOLUCIONADO Código PHP:
queda asi la cosa:
//Sumem dies a una data
function sumadies($data,$ndies){
$any = substr($data,0,4);
$mes = substr($data,4,2);
$dia = substr($data,6,2);
$dia = $dia + $ndies ;
$nueva = mktime(0,0,0, $mes,$dia,$any);
$novadata = date("d/m/Y",$nueva);
return $novadata;
}
//Resta dies a una data
function restadies($data,$ndies){
$any = substr($data,0,4);
$mes = substr($data,4,2);
$dia = substr($data,6,2);
$dia = $dia - $ndies ;
$nueva = mktime(0,0,0, $mes,$dia,$any);
//$nueva = mktime(0,0,0, $mes,$dia,$any) - $ndies * 24 * 60 * 60;
$novadata = date("d/m/Y",$nueva);
return $novadata;
}