Look at the user-supplied notes on this page:
http://www.php.net/date
the easiest way to do this is:
list($year, $month, $day) = split("-", $activation);
$renewal = Date("Y-m-d", mktime(0,0,0,$month, $day, $year + 1));
now $renewal will contain a date 1 year ahead. Using the date and mktime() functions you avoid any anomolies with leap years or other such problems. Hope that helps!
Chris King