Thanks for the note. Indeed, there was a date issue that popped up as 2013 rolled over.
I think what I'll do is just leave it for those affected. While I could do a roll-back, it would mean that some people would have legitimate challenge completions taken away from them, and it would probably cause more confusion than just leaving things alone.
So, if you got a freebie or two, consider it a lucky New Years gift
I *think* the problem has been fixed from this point onward.