Where does this stat come from?

[Value Of Pi]

Total Time Spent Online. We each have a number for that, but how does the software calculate this?

The reason I ask is that, in my case, it is grossly exaggerated. There is no way in hell I personally have spent over 42 days on this site (the real amount of time would be bad enough). Sure, I may be logged in with an open window in the background for hours and hours, but this stat seems to imply that one is actually reading, posting or somehow paying attention to the site.

Also, what would be the point of measuring whatever this stat is measuring if not to pump up the numbers for ad revenue purposes?