How the timer works

The device is always measuring the heartrate(bpm) from the IR LED and phototransistor as it receives pulses the chip calculates the time since the last pulse which results in the heart rate.

The chip counts real-time milliseconds since the last displayed second, while the bpm reading affects the threshold of the millisecond timer, so if the heart rate was at 120 the threshold might be about 500 (1000 is standard time). this means the clock ticks a second every 500 milliseconds, where as if the heart rate was low (50 for example) the threshold could be about 1300, which results in time moving slower.