The Julian Calendar was good enough for centuries after the empire that invented it had fallen, which I think counts as good enough in general. That said, I'd love to read a story about tracking down some time bug a million layers of abstraction down from the state of tech in 9000AD.

The thing is, we've remade calendars many times in the past millennium and our current calendar is not gospel. Sidereal drift can and should be addressed with a calendar fix, not a clock fix. The clock's a clock, the calendar's a calendar. It doesn't matter if we can make our clocks calendar-like because they shouldn't be anyway.