Aside from the AM / PM change, does the time also become wrong? I'm thinking that you mobo's clock / BIOS battery is going dead and should be replaced. It usually looks like a quarter and is mounted in a holder on the mobo. The usually last a couple years or so.
I have the Asus Rampage Formula and it has only been out for about a 1 1/2 years or so. The time is always wrong by 12hrs and only changes about every few days. Do you think it could be my anti virus blocking the auto time update from the internet?
Antivirus programs do not block time sync requests by the OS... your firewall could, but this is usually averted by the firewall allowing (someone else correct me if i'm wrong here) the svchost.exe process access to the 'net.
The CMOS battery dying is much more likely. If you have a multimeter handy, pull the CMOS battery off of the motherboard (after unplugging the system of course) and test it with the multimeter. If you're not getting a constant reading of 3V DC +- 5%, you're battery should be replaced.
If you need to replace it, just about any consumer electronics store should have them. Just ask for a "CR 2032" and they should be able to find it for you.
Turn the big dial to 20 in the DC Volts area. Connect the red lead to the hole marked "VΩmA" and the black to "COM" (for Common). Normally you'd use the red lead for the positive (+) side of the battery. On most meters like this, though, if you get it backwards it will still tell you the voltage and put a "-" sign in front. When you put the battery back in (or a new replacement) make sure it's turned the right way. Afterward, of course, you will need to reset your clock.