Hi

I am trying to create a formula to Match values in row 8 all of which are dates in this format - Jul-14,
I have worksheets that are named by these dates too eg Jul-14,

I want excel to see if there is a worksheet named what is in cell AA$8 and then if so,
perform a SUMIF on that worksheet to return a value. (if col A:A in the Jul-14 worksheet = the centre number e.g 58, then sum column AH:AH).

also

if there is not a worksheet by this name I want the value to just equal what was in the previous month.

So: if there is a workshete by the name of Ju-14, then SUMIF that worksheet and return the value,
IF NOT, equals Jun-14's value

Hope you can help