Clock/ time problems in Vista x64. Strange and I'm confused?

[Diablo13]

I wonder if anyone else has ever had this problem.
Here I am Sat sitting at my computer as normal. I might be browsing or checking email or any other random thing. Suddenly I notice that its yesterday, or its 10:00am when I know it's 01:00 am, according to my Windows Vista x64 clock. This has happened to me 3 times now and I am confused by it??
I reset the clock and was going to sync the internet time to nist.gov as I usually do but apparently I am not pinging the server, it sync'ed fine on another time server, although my net access is fine. I have never known an online clock to go wrong before, it's usually about the most accurate time check you can get.
Has anyone else had this strange problem, or have an idea what could cause it?
I am running a watercooled system on an Asus P5W DH deluxe MB, with a Core 2 duo 6700 cpu and 4gig of OCZ gold DDR 2 6400 memory. Operating system is legal Vista Home Premium x64 bit. I doubt any of this matters but I thought I would give as much info as possible, instead of asking a question and giving no background to what I'm running.