Custom dates used in Analytics

Hi,

I'm a new user, and am trying to generate two reports, and am having different issues with both.  Any assistance will be greatly appreciated!

Report 1 - We created a custom date field named 'Installation Date'.  I'm trying to subtotal an opportunity report by the month and year.  We created a custom format for the report in mm/yy, figuring we'd simply subtotal based on this.  For some reason the 'sum' button generates a subtotal for every opportunity, it doesn't sum for each month.  It's like it's using the underlying date as the criteria for the sum.  How can I get subtotals by month on this custom date field?