I'm not sure if this would apply to your case, but the since exact same thing was happening to me, I thought I'd share what I found out in my own case.
It turned out that both of my daughters had discovered that if they changed the system time, they could get more time on the PC even though they'd really gone over their allotments and/or into curfew periods.
Now when setting up their PCs, I had created "normal" (i.e. non-admin) Windows 7 accounts for each of them, and I assumed that having them use non-admin accounts would prevent something like this from happening. But when I noticed that they seemed to be spending a lot more time on their computers than what we'd allowed, and when I checked their NOF time reports and saw really weird hours reported for some days (like 36 hours of use on a Tuesday...a good trick!) I suspected something fishy was going on.
I decided to take the time to carefully peruse the (very long) log reports and discovered that they both had been changing the system time!
Because I didn't think that was possible using non-admin accounts, I decided to try it myself - I logged on to one of their accounts and was very surprised to discover that I did indeed have the ability to change the system time.
After some research I learned that by editing the local security policies database I could disallow this. That's what I did and their time reporting has looked reasonable ever since.
HTH
