A system for time validation comprises a terminal (1) with means (7,7') for tuning in to a number of different carrier frequencies (a-e), an authorisation device (6), e.g. a smart card, capable of communicating with the terminal (1) and means (2,5) to transmit time stamps, using a modulated signal having a carrier frequency, to the terminal (1). The authorisation device (6) comprises means (8) for selecting a carrier frequency to tune in to for retrieving a time stamp. A terminal (1) and authorisation device (6) are provided for use in the system. A computer program is suitable for loading into a programmable device, e.g. a smart card, to use it as an authorisation device (6) for use in such a system.