So wouldn't you just want the previous weeks numbered 1-13? Thus for this week:

Week 1: 3/16 - 3/22
Week 2: 3/9 - 3/15
Week 3: 3/2 - 3/8
Week 4: 2/23 - 3/1
Week 5: 2/16 - 2/22
Week 6: 2/9 - 2/15
Week 7: 2/2 - 2/8
Week 8: 1/26 - 2/1
Week 9: 1/19 - 1/25
Week 10: 1/12 - 1/18
Week 11: 1/5 - 1/11
Week 12: 12/29 - 1/4
Week 13: 12/22 - 12/28

Yep, that would work, what do you suggest?

Edit: Actually, I just realized that it wouldn't work. The database use the week number 1 to 52 which needs to be associated to my formulas...