...unless it's running software that uses signed 32-bit timestamps, or stores data using that format.
The point about the "millennium bug" was that it was a category of problems that required (hundreds of) thousands of fixes. It didn't matter if your OS was immune, because the OS isn't where the value is.
Edit: Oh damn, I never noticed that the timestamp is indeed signed. For anyone curious, it is mostly historical as early C didn't really have a concept of unsigned