[imedic]

Suggestions:
1. I think you miss in template: iwt_timespentonline_leaderboard the footer: $footer
Works much better with this addition.
In rest is working great, just as it was said before I wish I have it 3 years ago

2. I have also modified phrase Cumulative User Time Online from July 2010 as the calculation begins when you install it.

3. You could add a math script and approximate form user posts an average previous time (before installation of this mod) because it is obvious that many have already forums and users for some time.
Like posts X 2 min or also evaluate form total number of posts (if many) to add additional time as more posts are more you need to read. Just an approximation but can be something.
Good job

4. I don't know if this hides the statistical data of being online if the users is hidden? I suggest to have it hidden (all information) as is obvious that a hidden user don't want other to know how much and when he is online.