This is probably very simple but I cannot seem to find a solution that works.

I need to refernce information from a specific tabs of a file in formulas. I know the tab titles, but the user can name the file anything they want. Can someone help me write the code that will allow them select which the information is to come from and then how to reference the cells I need within that file?
Note: the formula is an array.

Below is the formula.
{=SUM(('[UNKOWN]Position1'!$H$17:$U$17>=1/1/2010)*('[UNKOWN]Position1'!$H$18:$U$18<=6/30/2010)*('[UNKOWN]Position1!$H20:$U20))}
Where "UNKOWN" is the filename of the excel file. And Position1 is the known Tab name.

Any help on this would be greatly appreciated. Thanks!