Everyone needs a knowledge of ‘the time’, even if the accuracy required is only sufficient to keep an appointment, catch a train or watch a particular TV programme. An isolated clock, started at random and ticking at an unknown rate, can only be used to compare intervals of time. Although this is often useful – for example, in cookery or athletics – the full value of a clock can be realized only if its reading provides a model of the status of a continuing process (such as the rotation of the Earth) or if the clock is a member of a group of clocks, all of which show the same time and may be expected to continue to do so. Particular applications may demand that some members of the group must be kept in closer agreement than others, or must all reliably remain within a stated tolerance for a longer period, but specifications of this kind arise only from economic necessity; in principle all clocks should agree at all times – subject, of course to the unavoidable restrictions imposed by the laws of physics and perhaps the complexities of daylight-saving time.