This is the only consistent thing I can think of...

=INT((13-WEEKDAY(A2,2)+DAY(A2))/7) this is correct unless A2 is both the 1st or 2nd day of the month and is either a Saturday or Sunday, then it needs to subtract 1

So because 1/1/2012 is a Sunday, technically we didnt have a week 1, because the work week runs Monday-Sunday and we rarely ever work Saturday or Sunday. So in that case I would need 1/8/2012 to return week 1, and Jan 1, 2012 would be week 0, as it wouldn't be necessary...seems complicated