The GPS Time Epoch: Precision Without Leaps

Global Positioning System (GPS) technology relies on incredibly precise timekeeping to function. While most of the world operates on Coordinated Universal Time (UTC), the GPS constellation uses its own internal time scale, known as GPS Time, which began its epoch on January 6, 1980.

The Zero Point: Jan 6, 1980

The GPS epoch was set at 00:00:00 UTC on January 6, 1980. At that exact moment, GPS Time was perfectly synchronized with UTC. However, since then, the two time scales have diverged significantly.

The Leap Second Divergence

The most critical difference between GPS Time and UTC is how they handle leap seconds. UTC is periodically adjusted by adding a "leap second" to keep it synchronized with the Earth's slowing rotation. GPS Time, however, is a continuous time scale. It does not include leap seconds.

Every time a leap second is added to UTC, GPS Time moves one second further ahead. As of mid-2026, GPS Time is exactly 18 seconds ahead of UTC (this offset is broadcast as part of the GPS navigation message so receivers can calculate local UTC).

Why Continuous Time?

For a satellite navigation system, time is distance. Light travels about 300,000 kilometers per second. A timing error of just one microsecond translates to a positioning error of 300 meters.

If the GPS satellites were to suddenly "jump" or pause for a leap second, the navigation algorithms used in cars, planes, and ships would suffer catastrophic failures in that instant. By maintaining a perfectly continuous count of seconds since 1980, GPS ensures that the velocity and position calculations remain stable and predictable.

The Week Number Rollover

Because of how the GPS signal was originally designed, the "week number" was stored in a 10-bit field. This meant that every 1,024 weeks (roughly 19.7 years), the week number would reset to zero. This "Rollover" event has occurred twice since 1980: once in 1999 and again in April 2019. Modern receivers are designed to handle this, but older hardware sometimes requires firmware updates to avoid showing the date as 1980 or 1999.