Or... we could store Terrestrial Time and treat leap seconds as part of the timezone data. This isn't a complete panacea, there would still be clock drift and

Or... we could store Terrestrial Time and treat leap seconds as part of the timezone data. This isn't a complete panacea, there would still be clock drift and updates could still cause time to be non-monotonic. But a timestamp would have a consistent meaning, and the TT->UTC conversion could be taken care of as just another step in localization. Why on Earth was the decision taken in the first place to have NTP operate off UTC?