UTC, TAI, and GPS Time Scale for NI Synchronization Interfaces

Updated May 29, 2018

Reported In


  • PXI-6682
  • PXI-6683


  • NI-Sync
  • NI-TimeSync

Issue Details

I want to know the difference between the UTC, TAI, and GPS timebases, their relationships to each other, as well as how to convert them from one to another.


International Atomic Time (TAI)
TAI time is maintained by the Bureau International des Poids et Mesures. TAI time is not adjusted for leap seconds and is always ahead of GPS time by 19 seconds.

Coordinated Universal Time (UTC)
UTC time is the same as TAI time, except it contains a certain number of leap seconds. The total number of leap seconds is maintained by the International Earth Rotation Service (IERS). The schedule of leap seconds and the conversion between TAI and UTC time can be found at http://maia.usno.navy.mil/ser7/tai-utc.dat.
Note that this schedule may change in the future. While the TAI and GPS timescales constantly progress, the UTC timescale jumps to account for leap seconds. This can cause issues with future time events since a second can be entirely skipped. The offset from GPS to UTC can be obtained from the GPS data packet and can be obtained programmatically. 

Global Positioning System (GPS)
GPS time is synchronized with UTC and TAI time, but it is not adjusted for leap seconds. GPS time uses the same scale to measure time as TAI and UTC, but there is an integer number of seconds of offset from each.

Conversion between TAI, UTC, and GPS

* These times are subject to change with the addition of leap seconds. This table was updated on 28th May 2018. The last leap second accounted for was on Jan 1, 2017.  Please refer to the related links section to find more information.


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