I've posted a couple of notes about encountering this problem. Based on some 
suggestions from:

Mark Aitchison, University of Canterbury, New Zealand.
	and
Chris A. Larrieu @cs.wm.edu

I think that my problem is a screen saver that also outputs sound (to my 
PC speaker). I'm still looking at some of the other screen savers that I 
use (with a randomizer), but this one definately caused the loss of several 
minutes over night (but not the date this time). 