You can't get an exact time, but you can get pretty darned close. The software looks at how often the page is refreshed, links clicked, etc. I do this for a living, and I have had people try to argue your point when caught wasting time. They always lose...
Yeah, well I guess that’s true. We analyzed the logs of someone who we suspected of moonlighting, and it was pretty clear that they had been attached to a remote development server for hours and hours a day.