Editing Global Positioning System (section)

==== Leap seconds ====
While most clocks derive their time from [[Coordinated Universal Time]] (UTC), the atomic clocks on the satellites are set to ''GPS time''. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain new [[leap second]]s or other corrections that are periodically added to UTC. GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with [[International Atomic Time]] (TAI) (TAI – GPS = 19&nbsp;seconds). Periodic corrections are performed to the on-board clocks to keep them synchronized with ground clocks.<ref name="NAVGPS" />{{rp|at=Section 1.2.2}}

The GPS navigation message includes the difference between GPS time and UTC. {{As of|2017|1|post=,}} GPS time is 18 seconds ahead of UTC because of the leap second added to UTC on December 31, 2016.<ref>{{cite web|title=Notice Advisory to Navstar Users (NANU) 2016069 |access-date=June 25, 2017 |url=https://www.navcen.uscg.gov/?Do=gpsArchives&path=nanu&year=2016&file=25665&type=messageBody--nanuId--NANUS&name=2016069.txt |archive-url=https://web.archive.org/web/20170525063405/https://www.navcen.uscg.gov/?Do=gpsArchives&path=nanu&year=2016&file=25665&type=messageBody--nanuId--NANUS&name=2016069.txt |archive-date=May 25, 2017 |publisher=GPS Operations Center}}</ref> Receivers subtract this offset from GPS time to calculate UTC and specific time zone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits).