Well, there's no direct command to return the week number.
One workaround you could use is starting with
Code:
$duration($calc($ctime - $ctime(January 1 $asctime(yyyy) 00:00:00)))

and then round the week number up or down. However, that still leaves you with the problem of in which week Jan 1st falls (week 52 or 1). So yeah, it's not as easy as you'd like, to my knowledge.