Complex sentence: 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.
Split into simpler sentences: Unix time is based on Coordinated Universal Time (UTC) and is subject to leap seconds. In contrast, PTP is based on International Atomic Time (TAI), which moves forward monotonically.