Unix time is based on Coordinated Universal Time (UTC) and is subject to leap seconds, which may 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 may 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.