We've made our way to the brains of the Apple Watch—the Apple S1 microcomputer, including:

Ultra low-power microcontroller

Quartz crystal, operating at 32,768 Hz

When power is applied to the crystal, it oscillates 32,768 times a second, filling a counter register in the processor. When the register fills, it overflows back to 0, triggering a pulse of the motor—and one second on the second hand.

This incredible processing power is capable of maintaining time accurate to within 1 second per day!