VirtualBox Guest OSs gets the computer system date from the Host. Should you ever want the Guest to have a different system time than the Host, the following works for me on an Windows XP Guest hosted by a Windows 7 machine. Inside the Guest:
- Run services.msc in the command prompt (or open the systems dialog box via Control Panel)
- Find the VirtualBox Guest Additions Service
- Stop the service (and set its Startup type to manual)
The information was found here: https://forums.virtualbox.org/viewtopic.php?t=8535 -- provided by user ubshreenath.