How to accurately determine what was worked on when and for how long?

Have you solved this problem? Have you seen it solved?

So, the problem is tracking time.

It's annoying for users to keep track of what they worked on and for how long. Asking people to do it voluntarily don't work. We have a compliance rate of about 30%. How can we get that to 100%? There's talk of requiring it before paycheck time. And that might really work. But it seems a little medievil. There should be a way clever way to solve the tracking problem.

I work at a facility with sometimes up to 5 different projects going at full steam. There's a dozen or so departments and 200 or so max people working total at crunch time. Sometimes people rarely save the file they're working on, sometimes every 15 minutes. Sometimes people never log out. Sometimes people work in lots of files over different platforms. The only constant is the file system, all 3 Petabytes of it.

15 minute resolution would be great. 60 min rez would work too.