Just a side note--I realize the problem
is solved, so this is more of a response
to runrig's 'arbitrary' comment--by definition,
the first week of the year is whatever
week includes 4 January,
so it's possible
for the 31st of December to be part of
week 1 of the new year, or for 1 January
to be part of the last week of the old
year. You won't notice this with 2000-1,
but 2001-2 doesn't divide as neatly.