NIST Time & Official
Atomic Time Standards

How UTC, TAI, UT1, and Terrestrial Time relate to each other, the role of NIST-F2 and NIST-F3 cesium fountain clocks, what leap seconds are and why they exist, and how the WWVB radio signal delivers official atomic clock time to your devices.

time.gov — Official US Time
time.nist.gov
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Time Standards Explained

Five key standards underpin all atomic timekeeping worldwide.

TAI

International Atomic Time

UTC − 37s

TAI (Temps Atomique International) is the uninterrupted count of SI seconds maintained by the BIPM using a weighted average of ~450 atomic clocks in over 80 national laboratories worldwide. TAI never has leap seconds and has been running continuously since 1958.

Managed by: BIPM, ParisPrimary use: Scientific baseline, space navigation
UTC

Coordinated Universal Time

TAI − 37s

UTC is the primary civil time standard. It is TAI adjusted by integer leap seconds to stay within 0.9 seconds of UT1 (Earth's actual rotation). UTC is the basis for all civil time zones, the internet (NTP), and GPS time corrections.

Managed by: BIPM / ITU-RPrimary use: Internet, civil timekeeping, GPS
UT1

Universal Time 1

UTC ±0.9s max

UT1 tracks Earth's actual rotation relative to distant quasars. Because Earth's rotation is irregular and gradually slowing, UT1 drifts from atomic time. Leap seconds are added to UTC when UT1−UTC approaches ±0.9 seconds.

Managed by: IERS, ParisPrimary use: Celestial navigation, VLBI
TT

Terrestrial Time

TAI + 32.184s

TT is the theoretical time standard for clocks on the surface of the geoid (mean sea level). It is used in astronomical calculations. TT = TAI + 32.184 seconds, accounting for the historical offset when the ephemeris second was defined.

Managed by: IAU / BIPMPrimary use: Orbital mechanics, ephemeris
GPS Time

GPS System Time

UTC + 18s (no leap seconds)

GPS Time started on January 6, 1980 when it was synchronized to UTC. GPS Time does not use leap seconds, so it is now 18 seconds ahead of UTC (at 2025). GPS receivers apply a UTC offset correction broadcast in the navigation message.

Managed by: USNO / US Air ForcePrimary use: GPS navigation, timing

How UTC, TAI, and UT1 Relate

TAI (atomic, continuous)
UTC (TAI − leap seconds)
UT1 (Earth rotation)
Atomic Clocks → TAI
~450 clocks at 80+ national labs → BIPM computes weighted TAI
TAI → UTC
UTC = TAI − 37 seconds (27 leap seconds + initial 10s offset in 1972)
UTC → Civil Time Zones
Each zone adds its offset (e.g., EST = UTC−5, JST = UTC+9)
Earth Rotation → UT1
Measured by VLBI/IERS. Drifts from UTC by up to ±0.9s
UT1−UTC → Leap Seconds
When |UT1−UTC| ≥ 0.9s, IERS announces a leap second in UTC

NIST Atomic Clocks

NIST-F2

Cesium fountain clock · Boulder, Colorado

NIST-F2 is the primary US time standard. Laser-cooled cesium atoms are tossed upward in a vacuum tube and interrogated at the peak of their arc. It would neither gain nor lose 1 second over 300 million years.

Accuracy: 8 × 10⁻¹⁶Since: 2014 (US primary standard)

NIST-F3

Cesium fountain clock · Boulder, Colorado

NIST-F3 is the latest cesium fountain clock deployed by NIST, providing backup for NIST-F2 and contributing to TAI. It features improved magnetic shielding and cryogenic microwave cavity for higher performance.

Accuracy: 5 × 10⁻¹⁶Since: 2023 (backup/secondary)

NIST Strontium Optical Clock

Optical lattice clock · Boulder, Colorado

NIST's strontium optical lattice clock holds the record for absolute accuracy. It traps ~1,000 strontium atoms in a laser lattice and interrogates them at 429 THz (visible light). It will not lose a second in 15 billion years.

Accuracy: 2 × 10⁻¹⁸Since: 2013 (research)

WWVB Transmitter

60 kHz radio time signal · Fort Collins, Colorado

WWVB broadcasts UTC time encoded as phase modulation on 60 kHz. It covers all 48 contiguous US states and much of Canada/Mexico, synchronizing millions of radio-controlled "atomic clocks" in consumer devices.

Accuracy: ±0.1 ms (received)Since: 1962

Leap Seconds

A leap second is an occasional one-second adjustment added to UTC to keep it aligned with Earth's actual rotation (UT1). Earth's rotation is not perfectly uniform — it is gradually slowing due to tidal friction from the Moon, and it fluctuates unpredictably due to atmospheric and oceanic angular momentum.

Leap seconds are announced by the IERS (International Earth Rotation and Reference Systems Service) in Paris about 6 months in advance. They are always added on June 30 or December 31, at 23:59:59 UTC. The clock reads 23:59:60 before rolling to 00:00:00 — a second that does not exist in normal timekeeping.

27 leap seconds have been added since 1972. The last was on December 31, 2016. No leap seconds have been added since 2017 because Earth's rotation has been slightly speeding up in recent decades.

In 2022, the ITU voted to abolish leap seconds by 2035. After that, UTC will be allowed to drift from UT1 without correction, accumulating several minutes of difference over centuries before a large "mega leap second" correction would eventually be applied.

Leap Second History (1972–2016)

YearAddedTotal TAI−UTC
1993+1s+28s
1994+1s+29s
1996+1s+30s
1997+1s+31s
1999+1s+32s
2006+1s+33s
2009+1s+34s
2012+1s+35s
2015+1s+36s
2016+1s+37s
2025+0s (none)+37s

Current TAI offset: UTC + 37 seconds. GPS Time offset: UTC + 18 seconds.

Related Atomic Time Resources

Atomic Clock Guide
Overview, types & history
Live Atomic Clock
Current time with ms
Clock Technology
Cesium, optical, nuclear

Frequently Asked Questions