I have a list of revenue broken down by transaction for the past four years.
I want to see the total amount of revenue generated during the month of
December 2006 v. December 2008. Right now, my dates on the spreadsheet are
in the form 12/31/2006. What type of formula should I use to do this?

I have tried this but it is coming up with nothing and I can see that there
were sales during the month of December 2006.

=SUMIFS(M:M,I:I,"<=(12/31/2006)",I:I,"=(12/01/2006)")

Thank you for all of your help offered.