No they don't. Screensavers are launched by the system. So it's the system that keeps track of user activity.

I know this.... The screensaver doesn't know idle time, but the system that LAUNCHES the screensavers knows the idle time. Hence if you make your application into a screensaver (oh, the pain), you could be launched at a certain point in time... but man, that would be quite annoying work :)