Is this maybe a problem with time zones? I just hit
root@cyberdyne2:~$ date +%W
28
root@cyberdyne2:~$
where my OS X Calendar says, we're in week 29. Which is kinda funny, because the same command under OS X returns 28 as well.
Edit:
and Dec 31 2012 was week 53, according to my linux server.
root@cyberdyne2:~$ date -d20121231 +%W
53
root@cyberdyne2:~$