Why does Windows Genuine Advantage care what time is it?

I have a virtual machine that I use for nothing other than to VPN into work.  My PC is a dual core Athlon that can apparently run the two cores at different speeds, which leads VMWare to incorrectly calculate the time. 

The solution to this problem, according to VMWare's Knowledge Base, is to disable power management.  Maybe there's a better solution now; I ran into this problem months ago, and just decided I was ok with the time being off in the VM since I don't do anything in that VM anyway.

But WGA isn't happy with this:

The Windows validation process could not be completed because the computer’s system date and time may not be set correctly.

Your computer’s date and time must be set correctly in order for the validation process to complete. Check your computer’s date setting now or go to the Microsoft Genuine Advantage Diagnostic Site where you will be guided through the process of correcting your settings and running validation again.

I've already "validated" this machine, so this is the ongoing validation that it does, after the initial check, that's failing because my clock is wrong.

I don't remember agreeing to keep my clock set when I bought Windows, but apparently that's a requirement now.

So my question is, why does WGA care what time it is?  Is there some timebomb code or expiry date in there that I'm not aware of?