Compund returns for certain date

[hedgie]

I usually use the following formula to get the compounded returns for a given period:
Assume column A1:A12 has dates (1/31/2011, 2/28/2011....to 12/31/2011) Column B1:B12 has a return stream

={Product(b1:b12+1)-1} will give the return for the year, or portion of the year if there are blanks.

If however I have a15 have say 6/30/11 and only wish to see the return through to that date, I am unclear how to write the proper formula - I have been trying to use Index and lookup the date in column A, but I am getting errors.

Any ideas?

 

[VoG]

Maybe

=SUMPRODUCT(--(A1:A10=A15),B1:B10)

 

[hedgie]

That only finds the return for the date in question (6/30/11). I also need to be adding 1 to each return before multiplying them.