How to create an excel report by month

Using the SUMIFS function, you can specify multiple criteria. So

=SUMIFS(data!B:B, data!A:A, ">="&startDate, data!A:A, "<="&endDate)

where column B in the data worksheet contains the data you want to sum, and column A contains the dates. startDate and endDate are named ranges with your start and end dates (in this example they would contain 1 Apr 2008 and 30 April 2008 respectively). Obviously you don't have to use named ranges; I've just put that in for clarity.

Alternatively, use a 'helper' column as you have, but put a formula in it instead of manually typing "April08" etc. Something like =DATE(YEAR(A1),MONTH(A1),1) would do the job.