I'm using the following formula to summarize data from one of my other
worksheets. Right now it's set up to sum the data when the month/year is
equal to my summary tab cell A11. What I want to do is have it sum up the
total when the values are < the month/year in cell A11. How come when I just
change the "=" sign to a "<" it doesn't work? Is it not that easy?

=SUMPRODUCT((TEXT(OFFSET('PC-01'!$O$2,,,COUNT('PC-01'!AB:AB)),"mmmyy")=TEXT($A11,"mmmyy"))*OFFSET('P C-01'!$AB$2,,,COUNT('PC-01'!AB:AB)))