Abstract: A quad oscillator fault-tolerant clock system for a computer complex comprises two clock sources at each of two computer locations, which are coupled by two duplex links. Each clock source supplies its own clock signal to the other clock source at the same location as well as to the clock sources at the other location over one of the duplex links coupling the two locations. Each clock source continually measures the phase difference between its clock signal and each of the other three clock signals. Periodically, the propagation delay for each link is calculated by taking the average of the phase differences measured by the clock sources driving the two ends of that link. These calculated propagation delays are supplied to each individual clock source, which corrects the phase differences measured by it for the propagation delays.