Unix time is based on Coordinated Universal Time (UTC) and is subject to leap seconds, which could be negative though none has been yet; in contrast, PTP is based on International Atomic Time (TAI), which moves forward monotonically.
Unix time is based on Coordinated Universal Time (UTC). Unix time is subject to leap seconds. Seconds could be negative. None has been yet. PTP is based on International Atomic Time (TAI). This was in contrast. International Atomic Time (TAI) moves forward monotonically.