I'm not sure.
Maybe it depends on how one frames the problem.
The posting software as it exists might not provide a mechanism for getting this information.
However, I notice that some of the software at least is written in perl. I don't imagine it would be difficult to add a kluge to the login script that would record only the most recent login time of a user. Thereafter, at some normally scheduled maintainance time, another script (perhaps executed by the administrator and not necessarily by a user's actions) might go through the several thousands of users and tally only those that have been on in the last X days. This number is then stored in another file to be recalled when a user views the statistics.
There are a few minor decisions that would have to be made, but it's doable. OTOH, there is a good reason not to go indiscriminately making changes to your software if it already works very well.
k
