If you’re running a newer machine, you may have your timezone set wrong or there may be a setting in the BIOS that’s telling your computer to resync to the wrong time. First off, go into your regional settings in the control panel, make sure you’re set to the correct timezone, and that you don’t have the computer set to automatically sync with a time server that’s not your actual timezone. You may want to restart the machine and go into setup to make sure the time is set correctly in your BIOS.

If it’s an older PC it’s also possible that your motherboard battery needs replacing. In most cases that’s a simple job you can do yourself although some motherboards have batteries that can’t easily replaced. If it’s a newer machine, the clock crystal may be defective. That happens from time to time, but you can use software to automatically correct it (since the time shift will be consistent) or just contact the atomic clock periodically to set the time. There’s a list of the latter programs at http://tf.nist.gov/general/softwarelist.htm.