http://www.howtogeek.com/howto/windows-vista/dealing-with-windows-vista-time-sync-problems/
If you are having problems with your Time while the Computer is running normally, read the info at the above link on how to synchronize your time to a Time Server, to maintain correct time.
If, on the other hand, you set your time correctly, turn off your Computer, turn it back on, and the time has changed back to an earlier Date/time/year, it is somethimg else.
There is a coin-sized, trickle-feed Battery in your Motherboard that maintains time and settings while your Computer is turned off.
It can go flat.
If that is the problem, it costs about $5 to replace.
Cheers.
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Mick Murphy - Microsoft Partner