trying to use a formula to specify a spend based on date criteria to give a
sum of others cells

A B C
1/01/07 $40 sum of all transactions in
january
20/01/07 $35 sum of all transactions in
February
23/01/07 $40 sum of transactions in March
2/02/07 $35
5/02/07 $40
6/03/07 $35
21/03/07 $40

I haven't been able to sort out a formula as yet and I have a lot of
worksheets to do this for so any help with this would be great