first of all:
I use php 4.0.1pl2 and don't have the possibility to upgrade. Please verify with latest version. 

I tried to add 12 months to a given timestamp and figured out that mktime() doesn't include a 29th February into its calculations. Effect is, that following code doesn't jump forward 1 year during each loop. Every leap year 1 day gets lost!

$a = mktime(0,0,0,1,1,1970);		
for ($i=0; $i<20; $i++) {
	$a = mktime(0,0,3600 +$a,1+12,1,1970);
	$b = date("d-m-Y", $a);
	print "Date: $b<br>";
}