  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.
