Here are a couple of tips for manipulating the clock in Oracle's VirtualBox.
Sometimes you need to tweak a virtual machine's clock for some kind of test or another. With the VirtualBox Guest Additions installed, you'll have trouble because the service will keep the guest's clock synchronized with the host's clock. You'll change the guest's clock only to see it snap right back to the host's date and time. One way to defeat that is to simply kill the Guest Additions service.
A more persistent way is to set the GetHostTimeDisabled parameter. With the VM off, you can do this by running this command on the host (VBoxManage is in the VirtualBox install directory):
VBoxManage setextradata "VM name" "VBoxInternal/Devices/VMMDev/0/Config/GetHostTimeDisabled" 1
Another interesting capability of VirtualBox is to speed up or slow down the guest's clock to some percentage of real time. To do this, you need to first turn off time synchronization with GetHostTimeDisabled. After that, again with the VM off, run this command:
VBoxManage setextradata "VM name" "VBoxInternal/TM/WarpDrivePercentage" 200
The 200 here means 200% of normal, or twice as fast. This could be interesting for testing processes that naturally take a long time -- such as a series of scheduled events. Of course it doesn't make your CPU do things twice as fast, the clock just runs faster. If you bring up a clock widget on the guest, you'll see the seconds tick by at two per (real) second.
The range of values for WarpDrivePercentage are 2 to 20000, but there are practical limits to how high you can go. As just a single example, consider the mouse double-click. The OS decides between a double click and two separate clicks by timing the interval between them. With the clock accelerated, it can get pretty tricky to manage click's quickly enough.
The documentation warns that other abnormalities in the guest are likely, such as failures due to shortened timeouts for devices that are still running in the "real" world.