Exact Time - Atomic Clock Time

What Is Atomic Clock Time?

Coordinated Universal Time (UTC) is the world's primary time standard, and it is defined directly by atomic clocks. Specifically, UTC is maintained by the Bureau International des Poids et Mesures (BIPM) in Paris, which continuously averages the output of over 400 atomic clocks operated by 80+ national metrology labs worldwide — including NIST in the United States, PTB in Germany, and NPL in the United Kingdom.

The workhorse of atomic timekeeping is the caesium-133 fountain clock. The SI second is defined as exactly 9,192,631,770 oscillations of the electromagnetic radiation corresponding to the transition between two hyperfine ground states of the caesium-133 atom at rest at 0 K. Modern caesium fountain clocks are accurate to about 1 second in 300 million years. Next-generation optical lattice clocks (strontium and ytterbium) are already 100× more accurate still, and will likely redefine the second within the next decade.

UTC is broadcast globally via GPS satellites (each carries multiple atomic clocks), NTP (Network Time Protocol) servers that synchronize every internet-connected computer, and dedicated radio time signals (WWVB in the US, DCF77 in Germany, MSF in the UK). When your computer or phone shows the correct time, it is because it has synchronized — directly or indirectly — with an atomic clock.

UTC differs from TAI (International Atomic Time) by an integer number of leap seconds, which are occasionally inserted to keep UTC within 0.9 seconds of UT1, the astronomical time scale based on Earth's rotation. As of 2024, UTC is exactly 37 seconds behind TAI.