Only one clock is accurate to billionths of a second.... the UNITED STATES ATOMIC CLOCK. Continuous time signals from the Atomic Clock are transmitted from a command center in the Colorado Mountains to the entire United States. Your Atomic Clock will lock on that master signal and keep itself synchronized with the Colorado Command Center. Set the clock to your time zone and the clocks computer does the rest; leap seconds to changes between standard and daylight savings time is automatic. Digital display with month/day, dual alarms, back-light for night viewing, GMT reference. Low profile 5" X 4" X 2 1/2" Black Arch design.