Excel 2007 : Brackets being inserted into sum formulas

So this is bizarre, I have never seen anything like it, hopefully someone can help me. Excel is randomly inserting brackets into my sum formulas. My formula is below, when I go to change the date to anything, in this case 04-01-2011, it randomly inserts the brackets before it. Nothing I can do will change this. Does anyone have any ideas?


=SUM('2007'!E5+'2008'!E5+'2009'!E5+'2010'!E5)+SUM('01-07-2011:[04-01-2011]04-01-2011'!E5)

Thanks for your help!!!!