Track total time spent on forum

[HondaATC]

I'm not sure if this would even be feasible at all, but I think it would be neat to have a hour counter on a persons profile page that would keep track of the hours they have spent on the site/forum. It seems like since the board is able to keep track of how long it was since your last visit, maybe there is someway to easily integrate a way to count the number of total hours a person has spent on the forum? I got the idea from the timer in mIRC. How hard would it be to make something like this? Would anyone else have an interest in a mod like this for their forum?