That formula doesn't require CTRL+SHIFT+ENTER....

Your profile says Excel 2007 but I presume that's on your Mac, do you have Excel 2003 on your PC? That formula doesn't work in Excel 2003 because that version can't handle an "array" in EDATE function, i.e. this part

=EDATE(B1,{-1;-2;-3})

Is the formula actually doing what you want, because I notice you were talking about summing Jan to July if B1 is August? Using {-1;-2;-3} means it's only looking at the last 3 months (May, Jun and July if B 1= Aug).