Not even that. UTC has leap seconds, which this code doesn’t handle (FTA: “The calendrical calculations always assume a Gregorian calendar, with no leap seconds”)

It copies that from the golang time package, which makes the same claim (https://pkg.go.dev/time)

That makes life a lot simpler for the implementer, but doesn’t that mean you can only reliably use these two libraries for computing with durations, not with moments in time or vice versa? The moment you start mapping these times to real world clocks and adding durations to them, you run the risk of getting small (up to about half a minute, at the moment) inconsistencies.