You need to see what is the offset of the time. If it is a nice round number - i.e. multiples of half-hour or full-hour differences, it could be a combination of two problems: first, your machine is synchronising its time with a remote network server; and second, you have your time-zone information set incorrectly or you have set your hardware time and system time to the same thing.
You can easily block all off-network NTP synchronisations at the router level by changing the firewall settings. For the second and third, you may need to change your BIOS settings and/or your OS settings.