date cost $ sales $
01/21/09 $4.00 $8.00
01/21/09 $7.00
02/21/09 $5.00 $8.00
02/28/09 $3.00

what formula can I use to look at the "date" column, select a time period
(i.e 01/01/09-01/31/09) and sum the the corresponding "Sales $" only if the
"cost $" is <"" ?

thanks