comparable week numbers over years
 

I want to compare weekly results 2006 vs. 2007; by using formula
weeknum(date,2) where date = 02jan06, I get directly week 2 (week 1 being one
day = 01jan06)
However, in 2007, week 1 begins on Monday 01jan07
I compare then apples and pears !
I could do it manually but a formula would be 'the' solution !
I suppose, this is a trivial question but any possible clue will greatly
help me !
Joël