Systems and methods are provided for monitoring or protecting an electric power distribution system using protective devices that may rely on accurate time signals from a common time source. A first protective device may receive a first time signal deriving from the common time source and a second protective device may receive a second time signal deriving from the common time source. The first time signal and the second time signal may be compared; when a difference between them exceeds a defined threshold, time signal manipulation may be detected. The protective devices may modify their operation in response to or to account for the time signal manipulation.