I recently worked on a project that involved precisely synchronizing multiple devices over a network. This included an NI PXIe, an NI cRIO, and a Rockwell PLC, all connected over a network switch to a Microchip SyncServer. Our team was able to successfully set up all three devices to receive time from the SyncServer over PTP, but were confused why one of our devices, the PXIe, was reporting a time exactly 37 seconds offset from the others. This experience opened my eyes to the complexities of leap seconds and various timing standards.
Various Timing Standards
UT1 time, or Universal Time, is a timing standard used by astronomers and physicists, directly based on the physical position of the Earth in Space. Any irregularities in the Earth’s rotation affect UT1 time immediately.
TAI time, or Atomic Time, is a precise scientific time, maintained by averaging the readings of over 400 atomic clocks around the world. These clocks are designed to count in precise seconds and have extremely little drift. Due to irregularities and slight slowdown in the earth’s rotation, UT1 time and TAI time have diverged by approximately 37 seconds at the time of this article.
UTC time is the most commonly used global time standard. Since 1972, UTC uses the same seconds (called “SI seconds”) as TAI time. Thus, without adjustment, UTC time would slowly diverge from UT1. To adjust for this, additional seconds are periodically added to UTC time, called “Leap Seconds.” As of 2024, there have been 37 leap seconds, with the most recent one added in 2016.
The addition of leap seconds is overseen by the International Earth Rotation and Reference Systems Service. The IERS monitors the deviation between UTC and UT1 time, and adds leap seconds as necessary. A leap second may be added up to twice yearly, on December 31 and June 30. A leap second is added onto the very end of each of these days, at 23:59:60 UTC.
Future of Leap Seconds
Leap seconds have long been controversial due to their disruptive effects on telecommunications, satellite systems, and more. At the 2022 General Conference on Weights and Measures, it was announced that by 2035, a new maximum acceptable difference between UTC and UT1 will be defined that will ensure UTC remains unchanged for at least a century.
Leap Second Pitfalls
In my project mentioned earlier, we found that our PXIe was reporting timestamps in TAI time, whereas our SyncServer, cRIO, and PLC all reported timestamps in UTC.
Beyond this small programming setback, leap seconds have the potential to cause serious issues when not handled intentionally. As leap seconds are added rather infrequently, and many people don’t even know they exist, many systems are not designed to handle a minute with 61 seconds. In the case of high-speed controls and data acquisition systems, the best solution may be to simply not run these systems during the 0-2 minutes per year that have 61 seconds. Otherwise, it is important to understand how your system will behave during these instances and design accordingly.
Read more about how Google applies a “Leap Smear” to reduce the impact of leap second additions.
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