If this is still unsolved, here is what worked for me:
If your guest operating system is Windows (Any ver), Time Sync will happen only when you have installed the Guest Additions.
So method 1 would be to uninstall the Guest Additions - but then you lose mouse integration and display resoultion adaptation along with other benefits.
Other option would be to go to the Services mmc (services.msc) and disable the VirtualBox Guest Additions service and stop it. This will stop the time sync as it did for me while the mouse pointer and display resoultion did not get affected.
There were command line options for the VirtualBoxService.exe (--disable-timesync) but that did not work for me. If that worked for anyone, please let me know how you achieved it. Also got to know of running the VBoxManage.exe from the host machine and setting a paramter for the specific Virtual Image but I didn't try that either since disabling and stopping the service was easier for me! Got this info from the Virtual Box Help so you can use that as reference to do your own thing.