“The time that ends up on your smartphone—and that synchronizes GPS, military operations, financial transactions, and internet communications—originates in a set of atomic clocks on the grounds of the U.S. Naval Observatory. Dr. Demetrios Matsakis, Chief Scientist for USNO’s Time Services, gives a tour.”
It’s nice to know the clock on my iPhone is this exact. Although, I was gutted the day my vintage Timex stopped working – it never actually told the correct time, and had to be wound up every few hours, but it had charm.