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The following command will disable the date/time synchronization for a Guest operating system.

Assuming the host is running Windows OS.

Open the command prompt. (Run -> cmd)

Enter command:
C:\Users\User>”c:\Program Files\Oracle\VirtualBox\VBoxManage.exe” setextradata ENTER_MACHINE_NAME_HERE “VBoxInternal/Devices/VMMDev/0/Config/GetHostTimeDisabled” “1″

* Replace ENTER_MACHINE_NAME_HERE with Guest system name
* This command will add an extra line to the “C:\Users\User\VirtualBox VMs\MACHINE_NAME\MACHINE_NAME.vbox” file.
* To enable the sync again, edit the file above

UPDATE:

If GuestAdditions have been installed, they will automatically sync the clock. Disable GuestAdditions.

# /etc/init.d/vboxadd-service stop