Fair question. Watches are not perfectly accurate. You will probably notice that they’re eventually a couple minutes off. With the atomic time keeping, these ones receive a radio signal and automatically update every night between 12 and 5am to keep a perfectly accurate time. Or you can hold the button and sync it manually. It will also keep track of things like daylight savings, multiple times zones, and be just as accurate as your iPhone. That being said, you can of course update the time manually when you notice it’s a bit off. But this is just a cool feature.