Time indicating signals are transmitted from a master clock at a central station to a slave clock at a peripheral station via a two-wire line as binary values represented by pulsed duration modulated signals. Clock, coding and transmitting circuitry at the central station are normally energized by an a.c. power line source. In the event of failure of the a.c. power source, the clock is energized by a battery and transmission from the central station to the remote station ceases. On resumption of power, the first transmission from the central station to the peripheral station enables the slave clock to provide a correct time indication. Transmission can be over any existing lines interconnecting the central and peripheral stations, such as a 60-cycle power line, television cable, or a telephone link. If television cable or 60-cycle line is employed, the presence and absence of voltage is indicated by frequency shift keying transmission. Transmission of the time indication requires only a fraction of the total transmission time, and is constant for all time indications, whereby other data signals can be transmitted between the central and peripheral stations.