You could try something like this
$date = "2003-09-23";
list ($y,$m,$d) = split ("-",$date);
// 1 week into the future
$future_timestamp = mktime (0,0,0,$m,$d + 7,$y);
$future_date = date('Y-m-d', $future_timestamp);
$future_date is now 1 week ahead of $date.