1. In a host computer having virtual machines with guest operating systems instantiated therein, a method of supplying reference time to the guest operating systems, comprising: receiving at a virtual reference clock a reference time request from a first guest operating system; and in response to the reference time request from said first guest operating system: sending, by the virtual reference clock, a request to a virtual clock switch to generate a first reference time, the virtual reference clock saving a time at which the request to the virtual clock switch is sent; receiving at the virtual reference clock a first reference time, the first reference time being generated by the virtual clock switch from multiple reference times received from different timing sources, the virtual reference clock saving a time at which the first reference time is received from the virtual clock switch; computing at the virtual reference clock a first latency value based on the times that the virtual reference clock requests and receives the first reference time; transforming at the virtual reference clock the first reference time from a format that is incompatible with a clock driver of the first guest operating system into a format that is compatible with said clock driver of the first guest operating system; and supplying, by the virtual reference clock, the transformed first reference time and the first latency value to the first guest operating system.