In some cases, when a lidar is PTP synchronized with a Grand Master Clock (GMC), the lidar data timestamp can show an offset compares to the time displayed by the GMC. That offset is +/- 37 seconds.
1. TAI vs UTC
Nowadays (February 2025), 37 seconds is the difference between the International Atomic Time (TAI) and the Coordinated Universal Time (UTC).
Indeed, UTC is based on Earth’s rotation and as the Earth’s rotation is not perfectly regular, leap seconds are added or subtracted from time to time to fit with the solar time.
This is not the case for TAI, because it is not linked to Earth’s rotation and therefore there is no need to add leap seconds.
2. Lidar Time Synchronization
The lidar takes the time provided by the GMC whatever the timebase is. If the GMC provides TAI time, the lidar will timestamp data with TAI time. The same if the GMC provides UTC time.
ThePTP_TIMESCALE flag in the GMC settings indicates to the PTP domain whether TAI is used (True) or not (False).
Therefore, when a 37 seconds offset is observed between the lidar and the GMC, it probably means the GMC converts time from TAI to UTC when the user performs a Get Time request.
Then, GMC can provide the offset and set the currentUtcOffsetValid and currentUtcOffset in the ANNOUNCE packets. This is the preferred way to get UTC from TAI from a correctly functioning PTP master clock.