1. A method of operating a computing device, the method comprising: operating at least one physical processor of the computing device to: execute a first hypervisor, the first hypervisor executing a virtual machine including an emulated processor having a time stamp counter, the first hypervisor executing the virtual machine from execution state of the virtual machine that was saved by a second hypervisor, the execution state of the virtual machine including a flag with an initial value set by the second hypervisor, wherein when the first hypervisor executes the virtual machine, the first hypervisor sets the flag to an updated value that is different than the initial value, the first hypervisor making the updated flag value available via an interface available to the virtual machine, the updated flag value indicating either that the virtual machine is no longer being executed by the second hypervisor; execute a guest computer operating system residing and executing in the virtual machine being executed by the first hypervisor, the guest operating system using the emulated processor to perform an operation of computing a normalized reference time by reading the updated flag value through the interface, and based on the updated flag value determining whether to use the interface to compute a reference time, and when determined from the updated flag, performing the steps of: reading a timer value from the time stamp counter from the emulated processor, and obtaining through the interface a scale factor and an offset; and calculating the normalized reference time based on the updated flag value read through the interface by applying the offset and the scale factor to the value obtained from the time stamp counter.