nning oscillator inside a device generates a frequency signal close to its nominal frequency (assume within a few parts per million of 10 MHz). This makes it a known and somewhat stable source of frequency. Then, to make a timepiece from this frequency source, you need to add a counter, some device that can count those oscillations. For every 10,000,000 cycles of the (10 MHz) oscillator, this counter increments by one (a second). So now, the result is a device that can measure periods of time much like a stopwatch. But just as with a stopwatch, although it accurately measures time intervals, it has no concept of phase and is unaware of date and time. The specific instant (or cycle) that triggers the counter to increment the second represents the phase of the clock. So, if the counter is set to trigger that increment at the same instant as another source of phase/time, it becomes phase aligned to that source. Then it resumes counting for another 10,000,000 cycles and ticks over the second once more. Then you could add a circuit to the clock that can generate a signal (for example, a pulse on a cop- per wire) to indicate the exact instant when the counter increments the second counter. This pulse is called a 1 pulse per second (1PPS) signal. The device is now a source of phase. Of course, because the free-running 10-MHz oscillator is never doing exactly 10 MHz, it will start to diverge without either a source of frequency, or constant adjustment to correct any phase offset. Furthermore, if you calibrate the second counter to some timescale like International Atomic Time (TAI) (so that the second counter is some large number representing the seconds since an agreed epoch), the device can track the current time based on TAI. Then it is a simple conversion to UTC (by considering leap seconds) and to local time (by including information on time zone and daylight savings time changes). Hence, in its simplest form, a clock really is a device consisting of an oscillator and a counter. Clocks are normally characterized by 