Atomic Clocks and Radio Controlled Clocks: Buy an atomic clock.
How does an atomic clock work? NIST distributes time-of-day and frequency information to the public using radio broadcasts. The low frequency (LF) station WWVB is located in Fort Collins, Colorado and broadcasts on 60 kHz. The high frequency (HF) stations WWV and WWVH are located in Colorado and Hawaii respectively, and broadcast on five different frequencies ranging from 2.5 to 20 MHz. At least one HF signal should always be usable at any given time anywhere in the United States.
The method used to broadcast the time is similar at all three stations, and is outlined below. You can click any of the images.