Is it possible to use a date (created via an INDEX calculation) within a
SUMPRODUCT calculation?

I use a SUMPRODUCT calculation to generate headcount figures off monthly
master sheets as per the following:
=(SUMPRODUCT(--(Apr09!$O$1:$O$6000="XN01"),(Apr09!$CI$1:$CI$6000) )).

However, I would like to now change the date source of the data based on a
month name generated by a Spin Box so that when the month changes fropm Apr09
to May09, the SUMPRODUCT calculation refers to the sheet of May instead. So
how do you join the '$O$1:$O$6000' part to the cell containing the month date?

Thanks