Dennis,
You could capture the system time periodically in the app and compare it to the current time. Anytime there was a current time earlier than a previously recorded time you would know something had happened. This, however, would be app dependent and the user would still be able to change the system time prior to starting the app. Another way, which I don't think VFP can do, is to run a background app that starts when the machine starts and periodically records the system time into a log.