Operative Time Definition and Principal Uncertainity

Arguments are given that time must be defined in an operative manner,i.e., by constructing devices which can serve as clocks.The investigation of such devices leads to the conclusion that there is a principal uncertainity of time if one considers periods which are not large compared with the Planck time. Thus,according to the old (classical) concept,time cannot be well-defined at this scale.The uncertainity of time leads to a breakdown of Special and General relativity in the Planck regime;the same happens with causality. We present arguments that the classical concept of time,which treats t simply as a real parameter,must be replaced by a new one.