Assign weekly date values to date list

[BRISBANEBOB]

I have a dump with several thousand lines. Column A has dates going back to 2009. I need to assign against each date in column A a weekly 'grouped' date in column B. So there may be five thousand lines with dates in column A ranging from 1/1/2010 to 12/31/2010 and I need to have assigned in B one of the 52 weeks into which they fall i.e. 1/1/2010 or 1/8/2010 or 1/15/2010 etc etc up to 12/31/2010.

I can't use nested IF statements because I will run out of nests, I can't use VBA because security settings, so I will need to do it by forumla.

Anyone any ideas?