How do I find my 3 top posters within the last month?
 

I need to find my 3 top posters for the last month so that I can run a competition so that the top posters on Christmas Day receive a gift voucher from a town centre store.

Is there any way to find the people who have posted the most during the last month (not overall)? I don't want to alienate new members who have a low post count from trying to become Decembers top poster.