As Windows administrators seem to have an increasing percentage of virtual machines, the concept of system time needs detailed focus. This becomes more important as the virtual systems contain more and more elements of the core workload.

A virtual machine can get its time from the host system, which in many situations is convenient and a good enough solution. Windows virtual machines that are members of a domain are also instructed to get their time from the domain. As we can see where this is going, it is important to determine one authoritative time source for virtual machines. I prefer to have the domain manage the time and have the virtual machine not sync to the host. This configuration can make multiple time zone configurations on the same virtualization host a little more transparent.

For VMware platforms, the VMware Tools Properties dialog box has an option to synchronize the time with the host to the guest. Figure A shows this option in VMware tools.
Figure A

Current versions of VMware tools do not enable this configuration by default, but the option is available and may be configured for older virtual machines. Other virtualization platforms, such as Sun xVM VirtualBox with the Guest Additions installation, have a default option for time being synchronized to the host, and it is less than intuitive to change that configuration. This becomes extremely important if a domain controller is run as a virtual machine. A common practice is to not synchronize the time to the host system, but to synchronize it to a trusted NTP time server.

Some advance planning in regards to the time configuration of a virtual environment will prevent surprises as environments grow.

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