Has actual atomic clock hardware inside
The big and bulky wristwatch you see below may look like some sort of joke, but it's actually one of the most accurate wristwatches ever made. The watch is made by a company called Bathys Hawaii and the watch is called the Cesium 133. The reason watch is a big is because it has all the components needed for a real atomic clock stuck inside the rectangular case.
The watch has a laser, heater, a sealed container of cesium gas, a microwave filter, and a photodiode detector inside. The Cesium 133 wristwatch uses the same sort of hardware used by the atomic clock at the National Institute of Standards and Technology.
That technology means that the watch is accurate to the level of 1 second per thousand years. The people behind the watch hope to make a new version that is smaller and has better battery life future. The current watch can only last a couple of hours per charge the internal rechargeable battery. The price for watch like this would be somewhere around $12,000.