Date object September is missing?

programming flash
1

Hi guys!

Can somebody explain why the month September is not working in my example?

var $d:Date = new Date();
$d.setMonth(7);
trace($d); // Tue Aug 31 12:10:29 GMT+0200 2010
$d.setMonth(8);
trace($d); // Fri Oct 1 12:10:29 GMT+0200 2010 ??? makes no sense