The invention relates to a communications system comprising means (12) for generating a succession of time message signals; a central station (2) having means for transmitting said time message signals; at least a first receiver (5) for receiving said time message signals; and timing means (9,10,11) for determining the real time of receipt of each time message signal. Each said time message signal is representative of the real time of receipt by said first receiver of a preceding time message signal. The first receiver comprises clock means (40,41) for providing said first receiver with an internal time signal; means (7,7a) for resetting said clock means upon the receipt of a first time message signal; and means (7,7b,42) for advancing said clock means, upon receipt of a subsequent time message signal, by the real time indicated by said subsequent time message signal.