在PHP中获取最后一天的某一天

I'm wondering if there's an easy way (like using strtotime) whereby I can get the unix time for the last occurrence of a day/month combination. For example, if I was to ask for "1st of September" today (9th May 2012) I would get 1314835200 (1st Sep 2011), but if the code was to run again this October, I would get 1346457600 (1st Sep 2012), and the same if I ran it 1 year from now.

Being able to do it forwards as well as backwards would be a massive bonus.

$month = 9;
$day = 1;

$timestamp = mktime(0, 0, 0, $month, $day);
if ($timestamp > time()) {
    $timestamp = mktime(0, 0, 0, $month, $day, date('Y') - 1);
}