1. CMOS Battery
This is the most likely scenario, especially if your computer is not brand new.
The CMOS battery sits on your computer’s motherboard and provides power to the Complementary Metal Oxide Semiconductor (CMOS) chip. The CMOS chip stores information about the system configuration, including the date and time. The CMOS battery makes sure the chip can store this data even while the computer is turned off and not hooked up to power. If the battery goes bad, the chip starts losing information and one of the symptoms is that your computer no longer maintains its time and date.
Replacing the CMOS battery is pretty easy. You just have to turn off your computer, ground yourself, open the case, find out which type of battery (step 3 in the linked guide) sits on your motherboard, go buy it, start over, and exchange the battery. Follow the links to the keywords for articles on the respective steps.
2. Time Zone
This is an easy to fix cause for when your computer clock loses time.
Your computer may simply be set to the wrong time zone and every time you fix the time, it resets itself to that time zone when you reboot. If the minutes are correct and only the hour is bad, that’s probably the issue you’re dealing with.
In Windows 7 you can easily fix the time zone. Right-click the system clock in your taskbar and select > Adjust date/time. Under the headline > Time Zone check whether the information is correct. If not, click the > Change time zone… button and set your time zone. Don’t forget to confirm your changes and you’re all set.
This is the least pleasant and most difficult to manage scenario.
Your computer may have been hijacked by a virus that messes with your computer time. To fix it, you need to gather a few tools. First, make sure your anti-virus program is up to date with the latest virus definitions. Then get a good malware scanner, for example Malwarebytes or Spybot Search & Destroy. For more tools and links to the respective download pages, check out our The Best Of: Windows Software page.
Once you have all these tools downloaded, updated and installed, start in Safe Mode and run them. It’s important to start in safe mode because the malware won’t launch and be active when you choose this boot mode. This means that it is less likely to escape detection and removal.