adding formula to pivot table

[bskbal]

I have a data set that has a row per each of 4000 stock symbols per day, which contains columns of values for different categories. I want to average the Pivot table for each month. A month has about 80000 rows. I added a column in each row that uses the date and loads the correct value for number of days into each row for a month. When I try to create a formula, I can't get the calculation to use the average or max. How can I do this. Also is there an easier to summarize by month without adding a column to the data?