Because of the way that timer devices in a virtual machine may fall behind real time and then catch up later, standard clock synchronization software such as the Windows Time Service (W32Time) or the Network Time Protocol (NTP) does not work well when run in a virtual machine. If the virtual machine is aware that it is behind real time and is already delivering timer interrupts at a higher rate so that the guest clock can catch up to real time, running non-VMware clock synchronization software inside the guest at the same time may also advance the virtual machine's clock, causing it to end up ahead of real time. Also, the widely varying timer interrupt
rate of a virtual machine, as compared with real time, is likely to confuse algorithms in non-VMware clock synchronization software that attempt to detect the machine's exact timer oscillator input frequency and correct for small variations from the specified frequency.
Instead of running non-VMware clock synchronization software in virtual machines, it is recommended that you run such software in the host operating system (for VMware hosted products) or the service console (for VMware ESX Server), then run VMware Tools in each guest operating system with the time synchronization option turned on. With this setup, your host receives the correct time from the network, and your virtual machines receive the correct time from the host operating system. Some customers may have requirements to run non-VMware clock synchronization software in guest environments, however.