Ruby On Rails, week number is not the correct one (-1)

Question

I need the current week number, if I'm not totally mistaken it's week 51 right now? However when testing it in the console I get this.

Time.now
=> 2013-12-19 11:08:25 +0100
Time.now.strftime('%U')
=> "50"

Date.today
=> Thu, 19 Dec 2013
Date.today.strftime("%U").to_i
=> 50

Why is that?