It is the timescale kept by the National Institute of Standards and Technology in Boulder, Colorado, generated by an ensemble of caesium clocks and hydrogen masers and calibrated by caesium fountain primary standards. It is the reference for legal and commercial timekeeping across the United States.

Who decides what the correct time is worldwide?

No single body holds the clock. The International Bureau of Weights and Measures combines measurements from several hundred atomic clocks in around eighty national laboratories, producing International Atomic Time and Coordinated Universal Time as weighted averages published monthly rather than broadcast live.

What is the difference between UTC and UTC(NIST)?

Coordinated Universal Time is computed retrospectively and exists only as a published figure, so no equipment outputs it directly. UTC(NIST) is that laboratory's real-time realisation, available continuously and normally held within a few tens of nanoseconds of the value eventually published.

How can I get time directly from a national laboratory?

Point a computer at one of the public network time servers those institutes operate, or use a radio-controlled clock that receives their longwave broadcast. Satellite navigation receivers offer a third route, recovering time to nanoseconds from the atomic clocks carried aboard the satellites.

NIST Time and National Standards

Somebody has to be right about the time, and in every developed country that somebody is a national metrology institute. In the United States it is the National Institute of Standards and Technology, and NIST time is what the country’s computers, broadcasters, power grid and financial markets are ultimately set against. Similar institutions exist in Britain, Germany, Japan and dozens of other countries, all feeding the same international scale.

This article explains what these laboratories actually do, how their clocks are combined into International Atomic Time, why the time they distribute is not quite the same thing as the time they help compute, and how anyone can obtain it. The civil scale that results is shown on the current UTC clock.

What Is NIST Time and Who Keeps It?

NIST time is the timescale realised and distributed by the National Institute of Standards and Technology, whose time and frequency division sits in Boulder, Colorado. It is generated by an ensemble of caesium standards and hydrogen masers, and its rate is calibrated by the caesium fountain primary standards NIST-F1 and F2.

The distinction between the ensemble and the fountains matters. An ensemble of good clocks running continuously provides a smooth, uninterrupted timescale, but nothing in it defines the length of a second. The fountains do that: they realise the SI second directly from the caesium-133 hyperfine transition, and they are operated periodically rather than continuously, as evaluations rather than as clocks. NIST time is therefore the ensemble, steered by the fountains, which is the same arrangement every major laboratory uses.

What National Metrology Institutes Do

Timekeeping is only part of the job. A national metrology institute maintains a country’s realisation of the SI units generally, and time is simply the unit that is realised best and used most widely.

For time and frequency the work divides into four strands:

Realising the second: operating primary standards accurate enough to measure the SI second directly, and publishing their evaluated uncertainty.
Keeping a timescale: running an ensemble of clocks continuously, without gaps, so the country always has a live reference to point at.
Comparing internationally: measuring the local timescale against those of other laboratories, using satellite links and dedicated fibre connections.
Disseminating: making the result available to the public and to industry by radio, network servers, calibration services and legal traceability certificates.

That last strand is why NIST time appears in so many ordinary places. NIST also operates the shortwave stations WWV and WWVH, the longwave transmitter WWVB in Fort Collins, and a set of public network time servers handling an enormous number of queries every day.

How Do National Clocks Become International Atomic Time?

By being compared, weighted and averaged. Each laboratory reports its clock measurements to the BIPM outside Paris, which combines several hundred clocks from around eighty institutes into a single weighted average and corrects the rate of that average using the world’s primary standards.

The output is International Atomic Time, and applying the current leap second count to it gives Coordinated Universal Time. Neither is broadcast. Both are computed after the fact and published monthly in a document called Circular T, which lists, for each participating laboratory, how far its local realisation stood from UTC at five-day intervals. NIST and its counterparts therefore learn weeks after the event exactly how well they did, and steer their ensembles accordingly. The relationships between the resulting scales are set out in TAI vs UTC vs GPS time.

The Other Major Laboratories

Each large contributor plays a slightly different role, and several are worth knowing by name:

The United States Naval Observatory: operates one of the largest clock ensembles in the world, and its realisation is the reference to which GPS time is steered.
NPL: the National Physical Laboratory at Teddington keeps the United Kingdom’s timescale, operates a caesium fountain, and provides the signal broadcast as MSF from Anthorn in Cumbria.
PTB: the Physikalisch-Technische Bundesanstalt in Braunschweig runs Germany’s standards and supplies the time carried by the DCF77 transmitter near Frankfurt.
The BIPM: not a national body at all but the international bureau that combines everyone’s data, holds no clock ensemble of its own, and publishes the results.

These institutes cooperate more than they compete. Comparisons of primary standards across continents are how the uncertainty of the second itself is established, and no single laboratory could establish it alone.

Why UTC and a Laboratory Clock Are Not the Same

UTC exists only on paper. Because it is computed retrospectively from data supplied by many laboratories, there is no apparatus anywhere whose output is UTC, and nothing you can connect a cable to and receive it.

What exists in real time are local realisations, written UTC(NIST), UTC(NPL), UTC(PTB) and so on, one for each contributing institute. The best of them are held within a few tens of nanoseconds of the eventual published figure, which is why the distinction almost never matters outside metrology; it matters a great deal inside it. Legal time in most countries is defined by reference to a named national realisation rather than to UTC itself, so when a court or a market rule refers to the correct time, the practical answer is the national laboratory’s clock.

How to Get NIST Time and Its Equivalents

Three routes are open to anyone who wants NIST time or a national equivalent. Network servers deliver it over the internet to a few milliseconds, longwave radio broadcasts it to purpose-built clocks, and satellite navigation receivers recover it to nanoseconds from orbiting atomic clocks.

For ordinary purposes the network route is the sensible one: pointing a computer at a laboratory server or at a public pool gives more accuracy than daily life requires, and the practical steps appear in how to sync your computer clock. The radio route reaches clocks with no network connection at all, using WWVB in the United States, MSF in Britain and DCF77 in Germany, as described in radio controlled clocks and WWVB. Web pages showing NIST time and its national counterparts are convenient but limited by network delay, which is why any honest browser display should tell you how far your own device is out rather than pretend to be a reference itself. That is exactly what the live atomic time display does.

Conclusion

National standards laboratories realise the second, keep a continuous timescale, compare it with everyone else’s, and hand the result to the public. NIST time is the American instance of that pattern, alongside the Naval Observatory, NPL in Britain, PTB in Germany and dozens more, all reporting to the BIPM, which combines their clocks into International Atomic Time and publishes the outcome monthly. What reaches your devices is a national realisation of UTC, delivered by network, radio or satellite. Read the plain figure on the current UTC clock, or begin at the atomicclock.now homepage.

National Time Standards: NIST Time and Its Counterparts