VirtualBox doesn't have great support for changing the BIOS date, but you can set a time offset. While the clock in VirtualBox still synchronises with the host system clock, the clock will be delayed by a certain number of milliseconds. You can use this to set the BIOS clock back a few years, so you can install betas. I tested this with a timebombed Whistler build and it worked fine. 

To do this, first open up Calculator. Find out how many days it is between now, and the BIOS date you want. For example if you wanted a Whistler build, a date of 1/1/2001 would be okay. Using a calendar or by using this site you can get the number of days. Between today and 1/1/2001, it's 2386 days. 

Now, multiply the number of days by 86400000 (which is 24*60*60*1000) to get the number of millseconds between now and the target date. 

Fire up Command Prompt, and CD to the folder containing VirtualBox (most likely c:\program files\innotek virtualbox). Now type: 

VBoxManage modifyvm <name> --biossystemtimeoffset -<milliseconds>

Where <name> is the name of the VM, and <milliseconds> is the number of milliseconds.

VBoxManage modifyvm "Windows Whistler 2419" --biossystemtimeoffset -206150400000

When you boot the VM, you'll find that your date has been wound back, so you can install your timebomed betas easily.