Entered with scepticism but left convinced. Looks very thoughtfully designed, nice!
I admit i do feel a bit uneasy about the rational numbers used in the definition, but i guess it doesn't matter for the purposes of this? And you do seem to have considered it. I still wonder if a less neat but integer-second or -millisecond definition might be useful, as round tripping could then be well defined i think? (If, as another poster hinted, you consider the interaction with leap seconds too)
I admit i do feel a bit uneasy about the rational numbers used in the definition, but i guess it doesn't matter for the purposes of this? And you do seem to have considered it. I still wonder if a less neat but integer-second or -millisecond definition might be useful, as round tripping could then be well defined i think? (If, as another poster hinted, you consider the interaction with leap seconds too)