UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
GPS Time
gps time
Overview
GPS Time (GPST) is the internal time scale of the Global Positioning System. It began at 00:00:00 UTC on January 6, 1980 and runs continuously without leap-second adjustments. As of 2026, GPST is 18 seconds ahead of UTC because UTC has inserted 18 leap seconds since 1980.
Why No Leap Seconds
GPS positioning relies on measuring signal travel-time differences at nanosecond precision. Inserting a leap second would introduce a discontinuity in the time scale, disrupting real-time navigation calculations. By excluding leap seconds, GPST guarantees unbroken continuity. GPS receivers convert GPST to UTC using the UTC offset value broadcast in the navigation message.
Relationship to TAI
GPST has a fixed offset from International Atomic Time: GPST = TAI minus 19 seconds. The 19-second difference originates from the TAI-UTC gap at the time GPS operations began in 1980. Because both GPST and TAI are continuous scales without leap seconds, this 19-second offset remains constant indefinitely. In a 2022 resolution, the General Conference on Weights and Measures (CGPM) decided to widen the maximum permitted difference between UT1 and UTC by 2035. The new limit and its implementation date were still undecided as of September 2026, but once the limit is widened, leap second insertions will effectively stop and the GPST-UTC difference will stay at whatever value it holds at that point.
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Related Terms
Atomic Clock
atomic clock
An atomic clock measures time by using the quantum-mechanical transition frequency of atoms as its reference, achieving accuracies on the order of one second in hundreds of millions of years and forming the backbone of modern timekeeping.
Leap Second
leap-second
A one-second adjustment applied to UTC to keep it aligned with the Earth's irregular rotation.
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