In general, real-time games are sensitive to changes in the system clock, especially moving the clock backwards in time. Different games will respond differently under those circumstances, ranging from statistics anomalies to killing whatever is living in the game. Virtual Villagers has a much more complex real-time engine than Fish Tycoon, and you're much more likely to see problems when adjusting the system clock. Once you stop messing with the system clock, the game should return to normal operation at some point.
While many people do adjust their system clocks to speed up play, doing so affects far more than just the games. Every application in your computer is affected, including the file system and Windows system applications.
_________________________
Barbara
Unicorn
Last Day of Work