A determination of the duration between two instants of time (epochs). Time intervals are measured with high precision with a digital display counter. An electronic oscillator generates pulses; the count begins with a start signal and ends with a second signal. For an oscillator frequency of 100 MHz, for example, a direct reading is correct to 10 nanoseconds (1 ns = 10⁻⁹ s). Two atomic clocks, however, can be compared in epoch to 1 picosecond (1 ps = 10⁻¹² s) by electronic interpolation.