The true nature of time has no rational explanation; we simply feel that time never stops or reverses. But, apart from philosophical considerations, experience indicates that any event can be localised by specifying three space coordinates and one temporal coordinate, generally denoted (x, y, z, t). So far, no experiment has ever called for more independent parameters. Intuitively, a time scale is thus defined as the time axis of a system of coordinates. Recommendation ITU-R TF.686 defines a time scale as an ordered set of scale markers with an associated numbering.