Assign weekly date values to date list

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?

I am unable to understand your requirement which you described and requesting you to attach a sample file with expected output for my better understanding to get solution for your query.

ok - what do you actually want to return.

for example, this formula will return 2012_03 for 18 January 2012 - this will create a sortable column based on year and week

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this one should do the same but with fortnights, not weeks:

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