workflow & field update problem, relating a date period to a week number

Hi Guys

Been stuck on this for around a week wondering if anyone has any suggestions or tips for me, what I am trying to achieve is

If a date is submitted in to a date field I need a number to populate another custom field via a workflow and field update with the financial week for our business, I have tried numerous things but cannot get it to work, I think am close but missing something that could be simple that is vital or am miles off haha :S

Some example I have tried are below for the field update part

IF(Last_Modified_Date__c = >01/07/2011 < 07/01/2011, Week__c,1)

This is possibly miles off, what I am trying to get the formula to do is, if the dates range between the dates show then in the field week put a 1(if only it was that easy)

Another path I tried was the below, I think this may not be far off, but will be a long formula as I would have to define all the dates but seems easiest way is this,

CASE(Last_Modified_Date__c,

"01/07/2011", "1",

"02/07/2011", "1",

"03/07/2011", "1",

"")

I get this error from this formula so am guessing I am putting the dates in, in the wrong format L

Error: Incorrect parameter for function 'CASE()'. Expected Date, received Text

Any help with this would be greatly appreciated J

Thanks

James