A timing circuit (24), in particular a real time clock circuit, in a lighting device (10) that is periodically (17) powered from a main power supply (15) in accordance with a power supply cycle having a power-on time and a power-off time, is automatically synchronized in case the power condition of an auxiliary power supply (23) is insufficient to assume correct operation of the timing circuit (24). At each power-on of the power supply cycle, a power condition of the auxiliary power supply (23) is monitored. If the determined power condition exceeds a set power threshold, it is assumed that the momentary timing value of the timing circuit (24) is correct and this value is stored in a non-volatile memory (29), If the determined power condition gives rise to assuming malfunctioning of the timing circuit (24), for example due to absence of power for a longer time, the timing circuit (24) is synchronised to the stored timing value.