prova così
Codice PHP:
function addDays( $date, $offset = 0 ){
$giorno=86400;
list( $day, $time ) = explode( ' ', $date );
$day = explode( '-', $day );
$time = explode( ':', $time );
$timest = mktime( $time[0], $time[1], $time[2], $day[1], $day[2], $day[0] )+($giorno*$offset);
return $timest;
}