Siverius

$currentYear = date("Y");
$currentDate = new DateTime();

$bday = get_field('Birthday', $user); // maybe it is like '06-08'
$bday = new DateTime($currentYear . '-' . $bday);

if ($bday <= $currentDate) {
    $interval = $bday->diff($currentDate);
    echo $interval->format('%R%a дней');
}

$date = date('Y-m-t',strtotime($yourDate)); // last day of month
echo date('Y-m-d', strtotime($date. ' + 5 days')); // add 5 days to date