How To list Top Authors of the month

Hi,

As the name of the trhead says, I would like to know how to list the top authors of the current month.

(So that I can give a prize to the top contributor of each month, and they are able to see witch one is winning)

So far I got this:

[Code moderated as per the Forum Rules. Please use the pastebin]

Its based on whats been said on this thread http://wordpress.org/support/topic/top-authors?replies=8#post-489916
but…this gives the list of all time.

I want it just for the current month.

Does any one knows how to do this?

thanks