Easy.
Click on the link for your username: [link|/forums/render/user?username=Nightowl|Nightowl]
It lists all of your posts in reverse chronological order. You can note the dates and keep clicking "Next 20" until you reach the end period of interest. (You can also manually edit the URL to get the posts in larger increments. How is left as an exercise for the reader.) Do the subtraction of the post numbers and you'll have your answer.
Oh, you want something automated? Well, you'll have to rely on the kindness of strangers for that...
HTH.
Cheers,
Scott.
