problem with mktime - please help ?

From: Richard (news.richardNO_at_SPAMntlworld.com)
Date: 12/31/03 

Date: Wed, 31 Dec 2003 18:10:03 -0000

$mn = date("F",mktime(0,0,0,date("m")+1,date("d"),date("Y")));

gves January (as it's December now)

but

$mn = date("F",mktime(0,0,0,date("m")+2,date("d"),date("Y")));

gives March !?!!?

Why ?