The basis for measuring time is a steady beat, i.e. an oscillation or frequency - generated by a pendulum or balance wheel in mechanical watches.
Time information is modulated to this frequency. The precision of the "on-time" marker is important here, i.e. the exact marking of the start of the next unit of time, for example the next second.
A very precise clock has its own constant beat and an exact original time. In addition, the stability of each watch is checked and corrected by at least one reference.
